GB2159753A - Method and apparatus for cleaning lead pins and the like before soldering operations - Google Patents

Abstract

A method and apparatus are provided for use in for manufacturing dual interline semiconductor packages including lead frame assemblies, for later subjection to soldering operations, in the making of printed circuit boards, and the like, wherein the surface contaminants and impurities including plastics residue, dust, dirt, oil, grease, products of oxidation such as oxides and sulfides, and other impurities such as CF4, and the like are removed or cleaned, particularly from lead pins 87, through a highly efficient, low cost, dry reverse sputtering operation. Substantially all contaminants are removed resulting in increased process efficiency, a higher reliability in product, and a much more effective and high integrity solder bond than was possible with prior mechanical and chemical cleaning techniques. The method is carried out by enclosing the package, including encapsulation 86 And lead pins 87 within an evacuable container 53 and connecting the package to a support 68 electrically connected to be the cathode, introducing an ionizable gas after evacuation of the chamber 53 and applying a voltage between the anode and the cathode to cause a discharge or plasma between the electrodes and to cause a flow of positive ions towards the parts to be cleaned 87. An alternative apparatus includes a cathodic mask (135; Fig. 4) and a non-conductive mask (132). <IMAGE>

Description

SPECIFICATION Method and apparatus for cleaning lead pins and the like before soldering operations BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to an improved system for cleaning selected portions of a semiconductor package before soldering and, more particularly, to an improved method and apparatus for cleaning at least selected surface areas of a dual in-line semiconductor package including the electrically conductive metal lead pins thereof by reverse sputtering prior to soldering operations.

2. Description of the Prior Art A A typical prior art method of making dual in-line semiconductor packages includes preparing a lead frame having an input/output lead portion and a plurality of lead pins or connectors extending therefrom. An integrated circuit is mechanically secured and electrically coupled to the leads on the lead frame, and both the integrated circuit portion and the input/output leads are encapsulated in a plastic-like material during a molding operation.

After the molds are separated, bits of plastic material and plastic residue often remain on the surfaces of the lead frame apparatus along with additional surface contaminates or impurities such as dirt, dust, oil, grease, products of oxidation such as oxides, sulphides and the like, and additional process products such as CF4.

It has been found that all of these impurities must be removed prior to subsequent soldering operations since the presence of the impurities destroys the integrity of the electrical and mechanical contact made when the lead pins are inserted within the pin-receiving apertures, holes, or sockets of a printed circuit board and soldered therein.

Additional processes provide even further complications since the leads are cleaned before or after pin bending and before or after separation. Bending of the pins takes place after the individual encapsulated lead frame assemblies, which are to become the dual inline semiconductor packages, or the like, are removed from the lead frame assembly strips in which they are manufactured.

The prior art has utilized many different methods and systems for cleaning selected areas of a semiconductor package and particularly for cleaning the individual lead pins prior to insertion in the sockets of a printed circuit board for dip soldering, wave soldering, hand soldering, or any conventional soldering-type process.

The cleaning techniques normally used in the prior art are typical of various other applications for cleaning metal surfaces to achieve a high degree of purity. The cleaning generally requires the removal of undesirable materials such as plastic residue, oxides and the like lying on the surface of the semi-conductor device or at least portions of the surface such as on the surfaces of the lead pins themselves. The cleaning must include not only the removal of visible dirt, oil, grease, dust, and the like from the surfaces, but also the subsequent removal of any other physical contaminates stuck to the surface and those resulting from a chemical reaction such as oxides, sulphides, and the like. These are generally removed by a mechanical and/or chemical process or method such as abrasive blasting, wire brushing, pickling and etching.

The cleaning of oils and greases depends upon their individual natures. If the contaminates are soap-forming oils and greases such as those of animal or vegetable origin, they can be removed by transforming the contaminates by hydrolysis into fatty acids and by reacting these acids with alkaline solutions to obtain water soluble soaps. The removal of mineral oils is obtained by dissolving them in organic solvents and they may, in certain cases at least, be washed with alkaline solutions containing detergents for complete removal. However, since the nature of the contaminates is generally unknown, a reliable cleaning method must include at last two successive steps including (1) degreasing with organic solvents, and (2) using an alkaline degreasing process.

The sequence of cleaning operations normally used in the prior art for cleaning metal surfaces, or the like, begins with mechanical cleaning, and is then followed by pickling, detergent cleaning and degreasing.

Mechanical cleaning methods are not specific to any particular prior art and are wellknown in many technologies. For the purpose of cleaning or removing many surface contam- inates, abrasive blasting, wire brushing, or the like is often utilized.

Pickling is the chemical removal of oxides and other surface layers and generally leaves the cleaned part with a bright shiny metallic appearance and either a smooth or rough finish depending upon the concentration of the solution and the pickling time used. The pickling solutions used are well-known in the art and can be chosen depending on the particular metal to be cleaned.

Electrolytic etching and polishing involves the anodic (or cathodic) treatment of metal surfaces in appropriate etching solutions.

Again, electrolytic etching techniques are well known in the art and will not be described in detail herein.

Alkaline detergent cleaning is performed either by emersion or as an electro-cleaning process. Emersion cleaning is usually done with hot solutions and ferrous metals are usually cleaned with relatively strong cleaners using sodium hydroxide, soaps and wetting agents in relatively high concentrations. Electro-cleaning techniques in alkaline'solutions can be used by coupling the particular metal portion to be cleaned as the cathode or as the anode, with the tank itself being the second electrode. With anodic cleaning, oxygen is liberated on the surface of the metal being cleaned and with cathodic cleaning, hydrogen is liberated on the clean surface. Anodic cleaning is normally recommended for ferrous metals although cathodic cleaning could also be used. Either, however, are expensive, slow, unreliable, and not conducive to continuous in-line operations.

Solvent cleaning is done in the prior art by using the solvent in either a liquid or in a vapor state. Liquid cleaning can use such materials as benzene, xylene, or inflammable solvents such as dichloroethylene, carbon tetrachloride, trichlorethylene, perchlorethylene, or the like.

Vapor degreasing is much more effective than liquid solvent cleaning and the solvent is normally heated to boiling with the parts to be cleaned being hung or suspended within the chamber until the hot vapor which condenses on the metal surfaces, dissolves the oiling grease thereon, and flows back into the solvent container. Some more advanced techniques, teach the use of high pressure liquid solvent spraying.

All of these prior art cleaning techniques provide many disadvantages particularly when semiconductor circuits and relatively small areas to be cleaned are involved. The harsh physical cleaning often results in abrading the encapsulated portion of the semiconductor package, in breaking or weakening the mechanical and/or electrical connection between the encapsulated portion and the lead pins extending therefrom, and often bends, mars, scratches, or otherwise affects the surface of the electrically conductive metal lead pins or connectors being cleaned.

Similarly, harsh chemical agents in cleaners can dissolve not only the plastic residue to be removed from the lead pins, but the plastic from portions of the encapsulated integrated circuit portion itself. This can destroy the circuit integrity and reliability, and can be quite costly, particularly when complex integrated circuits are used. Further, the various prior art cleaning techniques often take far too much time, at far too much expense, and require far too much maintenance to justify their use in a continuous in-line system for manufacturing semi-conductor components and soldering them in printed circuit boards for subsequent circuit utilization.

The phenomenon of cathodic sputtering refers to the dislocation of atoms or molecules from the surface of a material by the impact energy of gas ions which are accelerated in an electric field. Cathodic sputtering is established by the creation of a glow discharge, arc discharge, or plasma between the anode and cathode electrodes, wherein the current therebetween is composed of electron flow to the anode and positive ion flow to the cathode.

The ions are created by the ionization of the inert gas molecules existing within the glow discharge region between the anode and cathode, and the ionization results in collisions of the gas particles with the electron flow from the cathode to the anode.

The removal of surface contaminants by cathodic sputtering is known in the art and may be referred to generally as "reverse sputtering" since it is the opposite of the process of cathodic sputtering wherein substances are deposited onto the surface of a material. For example, reverse sputtering has been observed in electric-arc-inert-gas welding applications wherein contaminants are removed from the surfaces of the materials to be welded prior to the actual weldment.

Reverse sputtering has been used, at least on a laboratory scale bais, to clean relatively large surface areas of a non-metallic semiconductor wafer as a preliminary step to the manufacturer of a semiconductor device such as a photo-electric cell. Similarly, reverse sputtering has been used with limited success to clean various metallic components on a small scale such as accelerators, storage rings and plasma machines.

The prior art also teaches the use of various masking devices which attempt to shield portions of the surface which are not to receive undesired ion bombardment. However, most prior art masking techniques are designed for extremely small semiconductor areas and not for larger metal surfaces or the the like.

The problems and disadvantages described above are substantially solved by the method and apparatus of the present invention for cleaning lead frame assemblies and, more particularly, for cleaning the electrically conductive metal lead pins or connectors of dual in-line semiconductor packages, or the like, by a reverse sputtering operation prior to soldering the lead pins into pin-receiving sockets operably disposed on a printed circuit board, or the like. Relatively all of these problems are solved or at least minimized utilizing the techniques of the present invention.

SUMMARY OF THE INVENTION The present invention provides an unique method and apparatus for cleaning at least selected portions of a lead frame assembly or a semiconductor package prior to subsequent operations such as soldering by the use of reverse sputtering. The invention provides a method and apparatus for cleaning, by reverse sputtering techniques. the lead pins of a multiple lead dual in-line encapsulated semiconductor package prior to soldering the leads into pin-receiving sockets in a printed circuit board.Further, the invention relates to an improved method of incapsulating lead frame assemblies wherein the finished assemblies are cleaned by reverse sputtering prior to utilization; and an improved method for soldering lead pins into the sockets of a printed circuit board which includes the step of cleaning the lead pins by reverse sputtering prior to the soldering operation.

Further, the present invention contemplates an apparatus for cleaning at least portions of the metal surfaces of a lead pin apparatus or semiconductor package such as the lead pins thereof prior to subsequent utilization; and an improved system for manufacturing printed circuit boards wherein the previously manufactured lead frames are cleaned by reverse sputtering prior to inserting the lead pins into sockets of the printed circuit board for soldering.

A method of cleaning the lead pins of a multiple dual in-line semiconductor package prior to the soldering of the lead pins into pinreceiving sockets in a printed circuit board include the steps of providing a vacuum chamber; operable disposing a dual in-line semiconductor package in the vacuum chamber; cleaning contaminants off of the surface of the lead pins of the dual in-line semiconductor package by reverse sputtering; and removing the cleaned semiconductor package from the vacuum chamber for subsequent soldering operations.

The invention also contemplates the cleaning step as including the steps of providing an anode electrode; operably coupling at least the conductive leads of the dual in-line semiconductor package as a cathode electrode; evacuating the chamber; introducing an inert ionizable gas into the vacuum chamber; apply- ing an electrical potential between the anode and cathode electrodes to establish an electrical field therebetween for creating a glow discharge for ionizing said inert gas; and accelerating positive ions produced by the ionization of the inert gas through the electric field to bombard at least selected surfaces of the lead pins for cleaning purposes.

The system also contemplates a lead bending operation at some point either before or after the cleaning, and preferably before the cleaning operation; and the step of separating individual dual in-line semiconductor packages from the strips of lead frame assemblies which were encapsulated with plastic material during the encapsulation process. Further, the invention contemplates the step of masking the encapsulated portion of the dual in-line semiconductor package for shielding it from undesirable ion bombardment and the final steps of restoring atmospheric pressure to the vacuum chamber, opening the chamber, and transporting the cleaned dual in-line semiconductor package for insertion into the pinreceiving sockets of a printed circuit board prior to a soldering operation.Yet further, any type of soldering operation may be used such as dip soldering, wave soldering, hand soldering, automatic soldering, or the like. Different masking techniques or shielding technique are also contemplated.

The present invention also includes a method for cleaning at least selected parts of the lead frame assembly having an encapsulated semiconductor portion and a plurality of conductive lead pins extending from the encapsulated semiconductor portion comprising the steps of providing a vacuum chamber, providing an anode electrode, inserting a lead frame assembly to be cleaned into the vacuum chamber, establishing a cathode electrode, evacuating the vacuum chamber, introducing an ionizable inert gas into the vacuum chamber, applying a DC electrical potential between the anode and cathode electrodes to establish an electrical field therebetween, initiating and sustaining a glow discharge within said electrical field for ionizing said inert gas, excelerating positive ions towards at least selective surfaces of the lead frame assembly to be cleaned for bombarding same to remove surface contaminates and impurities therefrom, restoring normal pressure to the vacuum chamber, removing the cleaned lead frame assembly for subsequent processing, and the like.

The method includes utilizing the conductive portion of the lead frame assembly as the cathode itself or using the lead frame assembly in conjunction with the cathode in masking devices to shield the encapsulated portion from undesirable ion bombardment.

Invention also contemplates an improved method for encapsulating a lead frame assembly wherein the improvement resides in the step of cleaning at least the lead pins by reverse sputtering prior to subsequent processing. The actual method steps of cleaning are similar to those described above and the contemplated subsequent processing is that of soldering, either dip, wave, hand, or the like.

Lastly, the invention contemplates a method for soldering the electrically conductive metal lead pins of a dual in-line semiconductor package into the lead pin receiving sockets of a printed circuit board prior to soldering the pins within the sockets and subsequent use of the printed circuit with the improvement comprising the step of cleaning the surface contaminants off of at least the lead pin portions of the dual in-line semiconductor package prior to insertion of the pins into the printed circuit sockets. The steps of the cleaning process are similar to those discussed previously and will not be repeated herein.

The present invention also contemplates systems or apparatus for enabling the utilization of the system of the methods described hereinabove.

Other advantages and meritorious features of the present invention will be more fully understood from the following description of the drawings and the preferred embodiment, the appended claims and the drawings which are described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a process flow diagram, in block form, of a portion of the overall process of making a semiconductor package and utilizing it in a printed circuit embodiment; Figure 2 is a process flow diagram, in block form, of the reverse sputtering operation of blocks 14 and 22 of Fig. 1; Figure 3 is a diagrammatic representation of a simplified form of apparatus for practicing the present invention and illustrating the environment in which the process is carried out; Figure 4 is a partial diagrammatic representation of a simplified form of a modified cathode assembly and masking apparatus useable in the vacuum chamber apparatus of Fig. 3; Figure 5 is a perspective view, partially broken away, of a portion of a strip of encapsulated lead frame assemblies to be cleaned by the method and apparatus of the present invention;; Figure 6 is a perspective view, partially broken away, of a single dual-in-line semiconductor package having lead pins extending therefrom which are to be cleaned by the method and apparatus of the present invention; and Figure 7 is a perspective view of a finished dual-in-line semiconductor package having the lead pins bent at the desired angle for insertion into a printed circuit board for subsequent soldering.

DESCRIPTION OF THE PREFERRED EMBODI MENT The process flow diagram of Fig. 1 illustrates a portion of the overall semiconductor package manufacturing and utilization operation from the step of removing the encapsulated lead frame assembly from the bottom mold portion as represented by block 12. The process arrow 11 may be taken, for example, from the process for encapsulating or molding lead frame assemblies described in my copending patent application entitled "Method and Apparatus for Mold Cleaning By Reverse Sputtering" which is filed on even date herewith, and which is incorporated by reference herein.

When the lead frame assemblies are removed from the molds after the encapsulation process, residue plastic material and plastic contaminants remain on the surface of the lead frame and the individual electrically conductive metal lead pins. Further, additional contaminants and impurities may exist on the lead pin surfaces such as oils, greases, dust, oxides, sulfides, other products of oxidation, CF4 , etc. Any and all of these are detrimental to the utilization of the semiconductor device for its intended purpose and cleaning is absolutely essential prior to soldering operations.

The first leg of the process is represented by the encircled letter "A", designated by reference number 13, which proceeds to the process step of block 14 wherein the lead frame assembly strips are separated into individual encapsulated lead frame assemblies.

Subsequently, the reversed sputtering process is used to clean the lead pins to remove the plastic residue and other surface contaminants as indicated by process block 15. The process then proceeds to subsequent processing steps as represented by block 16. Block 17 represents the bending of the lead pins to a desired angle and, this step may be taken or performed before the sputtering operation, as indicated by arrow 18, or preferably after the sputtering operation, as indicated by arrow 19.

The second process path proceeds along the encircled "B" route, represented by reference numeral 21, to the step of reverse sputtering to clean the lead pins to remove plastic residue and other surface contaminants as indicated by block 22 prior to any separation step; and the output of the process step of block 22 proceeds directly to subsequent processing as indicated by block 16. The step of separating the lead frame assembly into individual encapsulated lead frame assemblies is represented by block 25 and dotted arrow 26 indicates that the separation step may be performed after the cleaning step of block 22 or before the bending step of block 17.

The various subsequent processing steps represented by block 16 includes the first subsequent processing path represented by the encircled "C", designated by reference numeral 29. In the process of path 29, the step of inserting the individual lead pins into the pin-receiving sockets of a printed circuit board is represented by block 31. Then, the pins in the printed circuit board sockets are soldered as by dip soldering, wave soldering, hand and/or robotic soldering, etc. as represented by process block or step 32. Lastly, block 33 represents subsequently utilizing the soldered PC board in any number of utilization devices or circuits, as known in the prior art.

Process path 34, represented by the encircled "D", leads to block 35 which represents the storage of the cleaned semiconductor device. As indicated by the third process path 37, shown by the encircled "E" path, the subsequent processing of block 16 may include the packaging step of block 38 followed by shipping and sale steps represented by block 39.

The storage block 35 shows a bidirectional path 34 which indicates that parts may be placed in storage and removed from return to the above-described processes so that entire lead frames or lead frame assemblies may be stored, removed from storage, separated, bent and cleaned prior to insertion in printed circuit boards, or the like. Similarly, stored semiconductor packages can be supplied as indicated by dotted arrow 36 to the insertion step of block 31 with or without additional cleaning.

The dotted process path 42 from the storage step of block 35 to the shipping step of block 39 indicates that semiconductor packages may be removed from storage and shipped for sale, or the like. Lastly, the dotted process path 41 indicates that packaged devices may be returned to storage 35 prior to use or sale.

Fig. 2 shows considerable more detail of the various steps included within the reverse sputtering or cleaning steps of blocks 15 and 22 of Fig. 1. The reverse sputtering process of Fig. 2 shows the step of providing a vacuum chamber as indicated by block 101 and providing an anode electrode within the vacuum chamber as indicated by block 102.

Two process paths diverge from the step of providing an anode block 102 and the first path is represented by the encircled "A", designated path 103. Path 103 proceeds to the step of coupling a lead frame assembly to be cleaned as a cathode within the vacuum chamber, as indicated by block 104, and block 105 indicates the step of evacuating the chamber. Step 106 represents introducing an ionizable inert gas such as argon into the vacuum chamber, while block 107 represents establishing an electrical field between the anode and cathode electrodes. Block 108 illustrates the step of accelerating positive ions toward surfaces of the lead frame assembly to be cleaned, including the lead pins in order to remove surface contaminants therefrom by ion bombardment.Block 109 represents the step transporting the cleaned lead frame assembly for subsequent processing and the process path 111 indicates the process path to additional operations as shown in Fig. 1 which may include bending, separating, packaging, shipping, storing, and/or soldering operations, and the like.

The second flow path from block 102 is represented by the encircled "B" and is designated by reference numeral 112. Path 112 goes to the step of providing a cathode electrode of block 113 and the step of coupling a lead frame assembly to be cleaned at a desired location between the electrodes as represented by block 114 which then returns to the original process path at the evacuation step of block 105. The process may include the step of transporting a lead frame assembly to and inserting it in the vacuum chamber as represented by block 15 and the dotted arrow 116.

Additionally, the process may also include the step of masking selected areas that the lead frame assembly to be cleaned to shield them from undesired ion bombardment. This step is represented by block 117 and is shown as being connected by a dotted arrow 118 back to the process following the step providing a vacuum chamber of block 101.

The alternate steps which may be inserted between the step of accelerating of block 108 and transporting of block 109 include the step of restoring normal pressure within the vacuum chamber as represented by block 119; opening the chamber as represented by block 122; and removing the cleaned encapsulated lead frame assembly from the vacuum chamber as represented by block 122. Dotted arrow 123 indicates the inclusion of the steps of blocks 119, 121, and 122 into the process prior to the transporting step of 109.

Fig. 3 shows a reverse sputtering apparatus 52 which includes a vacuum chamber, vacuum enclosure, or bell jar 53. The vacuum chamber 53 includes a support base 54 and pressure-tight seals 55 about the base of the chamber 53 to prevent the leakage of air therethrough. The base support 54 includes a vacuum port 56 which communicates through a conduit 57 to a vacuum pump 58. The vacuum pressure is measured and controlled by a vacuum pressure valve and meter 59, as conventionally known in the art. The base 54 also includes an inert gas input port or inlet 61 which is operatively coupled through a conduit 62 to an inert gas reservoir 63. In the preferred embodiment, the inert gas reservoir 63 may house any ionizable inert gas although a "heavy" inert gas such as argon is preferred since it generates more ions for greater cleaning efficiency during the ionization process.The amount of inert gas admitted to the vacuum chamber 53 is controlled by a conventional inert gas valve and meter 64 operatively disposed in the conduit 62 between the inlet port 61 and the gas reservoir 63. Any type of well-known micrometer needle valve, such as a Whitney micrometer needle valve, may be used so long as it is capable of measuring pressure in very small units of mercury.

The cathode apparatus 65 is shown as including a high voltage feedthrough electrode 66, a seal 67, and a hollow nonconductive cathode support member or frame 68.

The frame 68 physically supports and electrically connects the cathode 69 to the feedthrough terminal 66 through the hollow interior of the cathode support frame 68. The cathode 69, in the preferred embodiment of the present invention, is electrically coupled so that the metal surfaces of the lead frame assembly or individual lead pins which are to be cleaned become the acutal cathodes. In Fig.

3, for example, the cathode 69 would not actually include the encapsulated portion of 86 of the semiconductor package but would include at least the conductive lead pins 87 thereof.

The anode assembly 75 includes a high voltage feedthrough electrode 76, a seal 77, and a generally hollow anode support or frame member 78. An anode 78 is physically secured to the support 78 and electrically connected to the high voltage pin 76 through the hollow interior of the support 78.

A power supply 84 such as a source of high voltage DC power is connected with the positive terminal to the anode feedthrough electrode 76 and the negative terminal to the cathode feedthrough electrode 66. This establishes an electrical field between anode 79 and the cathode assembly 69. The electric field causes a glow discharge, arc discharge, or plasma 82 to form somewhere between the electrode 69, 79 and this ionizes the inert gas 70 within the vacuum chamber 53 to create positive ions 81 as well as electrons (not shown). The positive ions are then accelerated by the electric field between the anode 79 and cathode 69 toward the cathode and bombard the surfaces of the electrically conductive metal lead pins to clean or remove any surface contaminates such as plastic residue, oxides, and the like therefrom.

Fig. 4 shows one alternate method of masking a lead frame assembly or dual in-line semiconductor package so that the ion bombardment is masked or shielded from striking undesired surface portions such as the encapsulated plastic portion to prevent deterioration or damage thereto. In Fig. 4, like reference numerals designate like parts to those of Fig.

3 and the vacuum chamber 53 is shown as including an additional masking assembly 135. The masking assembly 135 includes a feedthrough electrode 136, seal 137, and hollow support 138. The hollow support 138 has attached thereto a conductive mask 139 which is connected electrically to the feedthrough terminal 136 through the hollow interior of support 138. The conductive mask 139 includes a plurality of hollow portions or apertures 141 through which the positive ions may pass to strike and clean selected surfaces of the lead frame assembly being cleaned. In solid portions 142 which prevent the ions from passing through to bombard portions of the lead frame assembly which could be harmed by the bombardment.

The circuit of Fig. 4 shows a lead frame assembly 131 operatively and electrically coupled to the hollow cathode support 68 and a relatively thin nonconductive insulating mask 132 operably mounted over and proximate to the lead frame assembly 131. The mask 132 may include apertures 133 through which the ions may pass to clean selected portions of the lead frame assembly such as the electrically conducted metal lead pins while solid portions 134 shield the encapsulated plastic portions of the lead frame assembly from ion bombardment.

Fig. 5 shows a portion of a strip of lead frame assemblies wherein the lead frames have the integrated circuit portions operably exposed therein and have gone through a molding or encapsulating process wherein the input/output lead portions and the integrated circuit portions have been encapsulated as represented by the plastic portions 153, 154.

The metal side frames 152 continue to hold the strips together and each of the encapsulated semiconductor packages 153 and 1 54 include a plurality of electrically conductive lead pins 155, 157, respectively, which are alternately formed in a staggered arrangement between adjacent pins. Pins 156 on the oppo- site side of the encapsulated circuit 153 and pins 158 on the opposite side of encapsulated portion 154 are associated with still other encapsulated portions along the strip, not shown.

After the encapsulation or molding operation is complete, plastic material and plastic residue remains on the surface of the led pins 155, 156, 157, 158, as well as physical contaminants such as dust, grease, oil, dirt, and chemical byproducts of oxidation including oxides, sulfides, and process materials such as as CF4, and the like. The pins must be cleaned prior to using the devices for any purpose whatsoever since the presence of plastic material or residue, or contaminants will make soldering impossible and could be severely detrimental to circuit performance.

Fig. 6 illustrates a single lead frame assembly 161 still having a portion of the lead frame 165 surrounding it and including an encapsulated portion 163 having a plurality of lead pins 164 extending therefrom. The lead pins 164 must be cleaned prior to subsequent use.

Lastly, Fig. 7 indicates a finished dual inline semiconductor package 166 wherein the encapsulated body portion 167 which houses the integrated circuit itself, has a plurality of lead pins 168 extending from opposite sides thereof. In the preferred embodiment, the lead pins 168 are normally bent to a predetermined angle so that the upper portion 169 extends only a short distance from the side of the encapsulated body portion 167 and the elongated socket engaging pin portion 171 extends downwardly therefrom and is adapted to be inserted within a socket or aperture or printed circuit board or the like for soldering or otherwise electrically connecting thereto.

The method and apparatus of the present invention enables a far superior, a dry, an inexpensive, a low maintenance, and a low cost system for cleaning the surface contaminants from at least selected portions of semiconductor packages, lead frames, and the like prior to soldering operations or the like. The reverse sputtering technique employed herein provides a highly efficient method of cleaning which can be used in a high speed, high volume continuous or in-line process and which provides a high degree of surface purity for improved solder connections, a lower rejection or failure rate, and a higher yield of high grade circuits utilising the semiconductor packages. The harsh chemical and mechanical treatments used in the prior art are avoided thereby preventing damage to the packages, reducing the cost of the operation, and increasing the speed of the process.

With the specific method and apparatus shown for the purpose of illustrating the preferred embodiment of the present invention, it will be readily apparent to those of ordinary skill in the art that various modifications and changes may be made in both the method and apparatus disclosed herein without departing from the spirit and scope of the present invention which is limited only by the appended claims.

Claims (54)

1. A method for cleaning the lead pins on a multiple lead dual in-line semiconductor package prior to soldering said lead pins into the pin-receiving sockets of a printed circuit board or the like comprising the steps of: providing a vacuum chamber; operably disposing the dual in-line semiconductor package into the vacuum chamber; cleaning contaminants and impurities off of the surface of the lead pins of the dual in-line semiconductor package by reverse sputtering; and removing the cleaned semiconductor package from the vacuum chamber for subsequent soldering operatings.

2. The method of claim 1 wherein said cleaning step further includes: providing an anode electrode; operatively coupling at least the conducting lead pins of the dual in-line semiconductor package as a cathode electrode; evacuating the vacuum chamber; introducing an inert ionizable gas into the vacuum chamber; applying an electrical potential between the anode and cathode electrodes to establish an electrical field therebetween for initiating and sustaining a glow discharge between said electrodes for ionizing said inert gas; and accelerating positive ions produced by the ionization of the inert gas through said electrical field to bombard at least selected surface areas of the lead pins of the semi-conductor package for cleaning purposes.

3. The method of claim 2 further including the step of bending the leads of the semiconductor package to a desired angle prior to the cleaning operation.

4. The method of claim 2 further including the step of separating individual dual inline semiconductor packages from a strip of lead frame assemblies comprise thereof after the encapsulation process which formed the body portion of the semi-conductor package.

5. The method of claim 2 further including the step of masking the encapsulated portion of the dual in-line semiconductor package for shielding at least portions thereof from undesired ion bombardment.

6. The method of claim 5 further including the steps of: restoring atmospheric pressure to the vacuum chamber; opening the vacuum chamber; and transporting the removad dual in-line semiconductor packages for insertion into the pin receiving sockets of a printed circuit board prior to soldering.

7. The method of claim 6 wherein the process further includes at least one of mechanically and chemically cleaning at least selected portions of the dual in-line semiconductor package.

8. The method of claim 4 wherein the sequence of steps proceeds in the order of separating individual semiconductor packages from a lead frame strip, bending the lead pin to a desired angle, and reverse sputtering at least selected surface areas of the pins.

9. The method of claim 4 wherein the sequence of steps proceeds from reverse sputter cleaning a strip of dual in-line semiconductor packages, separating the individual semiconductor packages, and bending the lead pins to a desired angle.

10. The method of claim 4 wherein the sequence of steps proceeds from separating the individual dual in-line semiconductor packages from others in the strip, reverse sputter cleaning the individual semiconductor packages, and bending the lead pins to a desired angle.

11. The method of claim 2 wherein said step of soldering includes dip soldering.

1 2. The method of claim 2 wherein said soldering includes wave soldering.

13. The method of claim 2 wherein said soldering includes hand soldering.

14. The method of claim 1 wherein said step of cleaning further includes the steps of: providing an anode electrode; providing a cathode electrode; operatively positioning a conductive mass intermediate the anode and cathode electrodes; evacuating the chamber; injecting an inert ionizible gas into the evacuated chambers; establishing an electrical field between the anode and cathode for initiating and maintaining a glow discharge therebetween for ionizing the injected gas; and accelerating the ions produced by the ionization of the gas toward the selected surfaces to be cleaned by ion bombardment.

15. The method of claim 14 further including the step of separating individual dual in-line semiconductor packages from a strip thereof prior to said cleaning step.

16. The method of claim 15 further in cluding the step of bending the lead pins of the separated semiconductor package to a desired angle.

17. The method of claim 14 further including masking the encapsulated portion of the dual in-line semiconductor package for shielding same from undesirable ion bombardment.

18. The method of claim 17 further including the steps of restoring atmospheric pressure to the vacuum chamber, opening the vacuum chamber, and transporting the removed dual in-line semi-conductor package for insertion into the pin receiving socket of a printed circuit board prior to soldering.

19. The method of claim 18 wherein the process further includes the step of at least one of mechanically and chemically cleaning at least selected surface areas of the dual inline semiconductor packakge prior to reverse sputtering cleaning.

20. The method of clain 16 wherein the sequence of operations proceeds separating, bending, and reversed sputtering.

21. The method of claim 16 wherein the sequence of the process proceeds reverse sputter cleaning, separating, and bending.

22. The method of claim 16 wherein the sequence of steps proceeds separating, reverse sputter cleaning, and bending.

23. The method of claim 14 wherein the soldering operation includes dip soldering.

24. The method of claim 14 wherein the step of soldering includes wave soldering.

25. The method of claim 14 wherein the step of soldering includes hand soldering.

26. A method of cleaning at least selected portions of a lead frame assembly having an encapsulated semiconductor package and a plurality of electrically conductive lead pins extending from said encapsulated semiconductor package comprising the steps of: providing a vacuum chamber; providing an anode electrode; inserting said lead frame assembly to be cleaned into said vacuum chamber; establishing a cathode electrode incorporating at least a portion of said lead frame assembly; evacuating said vacuum chamber; introducing an ionizable inert gas into the vacuum chamber; applying a DC electric potential between the anode and cathode electrodes to establish an electrical field therebetween; initiating and sustaining a glow discharge within said electrical field for ionizing said inert gas;; accelerating positive ions produced as a result of ionizing said inert gas towards at least selected portions of said lead frame assembly to be cleaned for bombarding same with said positive ions to remove surface contaminants and impurities therefrom; restoring normal pressure to the vacuum chamber; and removing the cleaned lead frame assembly for subsequent processing.

27. The method of claim 26 wherein said step of establishing a cathode electrode further includes the step of electrically coupling and physically positioning at least a portion of the lead frame assembly onto a cathode support for connection to a negative source of potential for creating a cathode electrode therefrom.

28. The method of claim 26 wherein the step of establishing a cathode includes coupling said lead frame assembly as the cathode element; and operatively positioning a conductive mask intermediate the anode and cathode electrode for shielding at least parts of the lead frame assembly from undesired ion bombardment.

29. The method of claim 28 further including the step of positioning an electrically insulated mask over said lead frame assembly to insure that only selected areas thereof such as the lead pins are bombarded with positive ions, the aperatures of said insulating mask generally corresponding to and being aligned with the aperatures of the conductive mask.

30. Any method of encapsulating a lead frame assembly to form a duel in-line semiconductor package including the steps of operatively coupling an integrated circuit to the input/output leads of a lead frame assembly, encapsulating the integrated circuit and lead portion of the lead frame assembly with plastic material such that only the electrically conductive metal lead pins protrode therefrom, separating the individual lead frame assemblies from a strip of lead frame assemblies, bending the lead pins of each individual semiconductor package to a predetermined desired angle and cleaning the lead pins prior to subsequent processing, the improvement residing in said cleaning step and comprising the steps of: : providing a vacuum chamber; providing an anode electrode; inserting the lead frame assembly to be cleaned into the vacuum chamber; utilizing the lead frame assembly and establishing a cathode electrode; evacuating the vacuum chamber; introducing an ionizable inert gas into the evacuated vacuum chamber; applying a DC electrical potential between the anode and cathode electrodes to establish an electrical field therebetween; initiating and sustaining a plasma within said electrical field for ionizing said inert gas; accelerating positive ions toward at least selected surface portions of the lead frame assembly to be cleaned for bombarding same with positive ions to remove surface impurities and contaminates; restoring normal pressure to the vacuumchamber; and removing the cleaned lead frame assemblies for subsequent processing.

31. The improved method of Claim 30 wherein said step of establishing a cathode electrode includes electrically coupling and physically positioning at least a portion of the lead frame assembly a space distance from the anode electrode and electrically coupling same for establishing a cathode element.

32. The improved method of Claim 30 wherein said step of establishing the cathode includes: coupling said lead frame assembly as a cathode element; and operatively positioning a conductive mask having apertures therein intermediate said anode and cathode electrode for shielding at least parts of the lead frame assembly from undesired ion bombardment.

33. The improved method of Claim 32 further including the step of positioning an electrically insulating mask over said lead frame assembly and proximate thereto to ensure that only selected areas thereof such as the lead pins are bombarded by said positive ions, the apertures of said insulating mask generally corresponding to and being aligned with the apertures of said conductive mask.

34. The improved method of Claim 30 further including the steps of: inserting the individual bent and cleaned lead pins of a semiconductor package into pin-receiving sockets of a printed circuit board; and soldering the lead pins within the sockets for mechanical and electrical soldering.

35. The improved method of Claim 34 wherein said soldering includes dip soldering.

36. The improved method of Claim 34 wherein said soldering includes wave soldering.

37. The improved method of Claim 34 wherein said soldering step includes hand soldering.

38. In a method for soldering the electrically conductive metal lead pins of a dual inline semiconductor package into the lead pinreceiving sockets of a printed circuit board including the steps of inserting the lead pins of a dual in-line semiconductor package into the lead pin receiving sockets of a printed circuit board and soldering the lead pins to mechanically and electrically bond said pins within said sockets while completing an electrical connection therebetween, the improvement comprising: providing a vacuum chamber; placing the dual in-line semiconductor package within the vacuum chamber; and removing surface contaminates off of at least selected portions of the electrically conductive lead pins thereof by reverse sputtering.

39. The improved method of Claim 38 wherein said step of removing surface contaminates includes the steps of: providing a vacuum chamber; providing an anode electrode; inserting said dual in-line semiconductor package to be cleaned within said vacuum chamber; utilizing said dual in-line semiconductor package to establish a cathode electrode; evacuating said vacuum chamber; introducing an ionizible inert gas into the evacuated vacuum chamber; applying a DC electrical potential between the anode and cathode electrodes to establish an electrical field therebetween; initiating and sustaining a glow discharge within said electrical field for ionizing said inert gas; accelerating positive ions produced as a result of said ionization process towards at least selected surfaces of the lead pins to remove surface contaminates and impurities therefrom;; restoring normal pressure to said vacuum chamber; and removing the cleaned dual in-line semiconductor package for subsequent processing.

39. The improved method of Claim 38 further including the step of selectively masking portions of said dual in-line semi-conductor package for shielding same from potentially harmful ion bombardment.

40. The improved method of Claim 38 wherein said step of soldering includes dip soldering.

41. The improved method of Claim 38 wherein said step of soldering includes wave soldering.

42. The improved method of Claim 38 wherein said step of soldering includes hand soldering.

43. A system for cleaning the metal surfaces of an encapsulated lead frame assembly including at least a portion of the electrically conductive metal lead pins thereof prior to a subsequent soldering operation comprising: a vacuum enclosure means; anode means operably disposed within said vacuum closure means; means for operatively coupling said encapsulated lead frame assembly to be cleaned within said vacuum enclosure means as a cathode means; means for evacuating said vacuum enclosure means; means for suppyling an ionizible inert gas into the evacuated vacuum enclosure means; means for applying an electrical potential between the anode and cathode for establishing an electrical field therebetween; means for initiating and sustaining a plasma discharge between the anode and cathode for ionizing said gas to produce positive ions; said electrical field accelerating said positive ions toward said cathode and said surface to be cleaned using the electrical field established between said anode and cathode; said accelerated positive ions impacting with at least selected surface areas of said lead frame assembly including the lead pins thereof for cleaning same by ion bombardment; means for restoring normal atmospheric pressure to said vacuum enclosure means; and means for removing the cleaned lead frame assembly for subsequent processing such as a soldering operation.

44. The system of Claim 43 further including means for masking at least portions of the lead frame apparatus for shielding same from undesired ion bombardment.

45. In a system for manufacturing printed circuits comprising means for preparing a printed circuit board with electrical connections and pin-receiving sockets thereon, means for manufacturing a duel in-line semiconductor package including a plurality of lead pins extending therefrom, means for bending the lead pins to a desired angle for engaging the pin receiving sockets of said printed circuit boards, means for aligning said lead pins with said pin-receiving sockets, means for inserting said lead pins into the pinreceiving sockets of said printed circuit board, and means for soldering said lead pins within said sockets for forming both a mechanical and electrical connection therebetween, the improvement comprising means for dry cleaning at least the lead pins of said semiconductor package prior to inserting said lead pins into said pin-receiving sockets of said printed circuit board by reverse-sputtering.

46. The improved system of Claim 45 wherein said means for dry cleaning further includes: vacuum enclosure means; anode means operably disposed within said vacuum enclosure means; means for operatively coupling said semiconductor package to be cleaned as a cathode means; means for evacuating said enclosure means; means for supplying an ionizible inert gas into the evacuated enclosure means; means for applying an electrical field between said anode and cathode for initiating and sustaining a glow discharge therebetween for ionizing said gas to produce positive ions; means for accelerating the positive ions toward the cathode means surface to be cleaned through the use of the polarity of the electric field between the anode and cathode means;; means for removing surface contaminates from at least selected areas on the semiconductor package including the lead pins thereof by ion bombardment; and means for removing the cleaned semiconductor package for a subsequent soldering operation.

47. The improved system of Claim 46 further including means for selectively masking portions of said dual in-line semiconductor package for preventing undesired ion bombardment thereof.

48. The improved system of Claim 46 wherein said soldering operation include dip soldering.

49. The improved system of claim 46 wherein said soldering operation includes wave soldering.

50. The improved system of claim 46 wherein said soldering operation includes hand soldering.

51. A method of cleaning parts of a lead frame assembly during a component encapsulation process, substantially as hereinbefore described with reference to the accompanying drawings.

52. Apparatus for cleaning parts of a lead frame assembly during a component encapsulation process, substantially as hereinbefore described with reference to the accompanying drawings.

53. A method of encapsulating a component and lead frame assembly, including a method of cleaning lead frame parts, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

54. A method of soldering parts of a lead frame of a component/lead frame package, substantially as hereinbefore described with reference to the accompanying drawings.