JPH0243578B2 - - Google Patents

Description

[Detailed Description of the Invention] Background of the Invention and Description of Prior Art Radar soldering is still rudimentary;
In other words, the possibility was known. This method has proven useful over mechanical soldering techniques, as it allows soldering to difficult-to-access soldering assemblies, such as those located deep within the housing. There is. Laser soldering was first used to mount microchips onto common plastic (epoxy-containing fiberglass) plates to prevent heat transfer to other parts of the plastic plate. In conventional configurations, very small diameter wires are welded to micro solder pads on easily accessible conductors on the circuit. These early applications used a very low power laser beam (7-50 watts) and focused the beam to a tiny diameter (.005-.010 inch) onto the soldering pad to effect the melting. It was hot. Early applications of these are described in the following publications: (1) F. Burns and C. Zyetz, “Laser Micro-Soldering”, Apollo Laser Company, Report.

(2) ER Goodrich, “Lasers in Electronics,”
This method allows assembly workers to easily perform soldering without strict restrictions.

Circuit Production, 6th 1980, Volume 21, No. 7.

(3) R. Saunders et al., “Laser Operation, Equipment, and Applications,” Coherent Company Technical Staff Paper,
Published by McGraw-Hill in 1981.

(4) T. Kujawa, “Productivity of Laser Soldering Boosters”, Lasers and Their Applications, September 1981.

Unfortunately, the processing techniques used in these initial applications are not applicable to soldering newly designed assemblies in automotive applications. For example, when assembling highly sensitive electronic elements, thick electrical conductors (thickness.
030 inches or more and often flat,
(i.e. of rectangular cross-section) are used, and the conductors must be soldered with great strength in order to withstand the fluctuating thermal expansion in the engine compartment of a road vehicle. The low power, finely focused laser beams conventionally used present obstacles to effective soldering of newly designed assemblies such as those described above.
A laser beam controlled to a micron diameter cannot sufficiently melt the solder pad across the bottom of a thick flat electrical conductor to reflow the fillet, so the solder joint formed is weak. Become. Reflow of solder around thin circular conductors is relatively easy due to surface tension around the circular surface. If the interfacial diameter of the laser beam were adjusted to expand, the beam energy per unit area would be insufficient to melt the solder and form a reflow seam.

In newer automotive electronic units to be soldered (e.g. electronic ignition systems mounted in the engine compartment), the soldering seams are located on the ceramic substrate and absorb the heat generated by the module during normal operation of the vehicle. can be derived from this ceramic substrate. This prevents damage to printed circuits and solder joints due to temporary high currents. However, since such ceramic substrates act as heat sinks and facilitate rapid heat dissipation when soldering is performed, they prevent thermal control of suitable diameter lasers and, when attempting to perform suitable soldering, The use of low power laser beams is not possible. The increase in laser beam power itself is important because a high-power beam focused to the required diameter will overheat the solder, leaving less room for handling errors and thus damaging adjacent electronic components. , does not provide an answer to the above-mentioned problems.

The preferred laser soldering method is supported on a ceramic substrate and allows concave soldering assemblies using thick conductors to be effectively soldered with strong seams, and can be soldered within a wide range of processing parameters. The method is such that the process can be easily used by assemblers without strict limitations.

In a preferred manner, the present invention is used in soldering integral electrical conductors to thick-walled film igniters useful in controlling the starting of internal combustion engines. The equipment to be soldered is particularly shown in FIGS. 1 and 3.

Essentially a microelectronic device 10
has an annular lead frame 11 embedded within a plastic annular housing 12 with integral conductor wires 13 projecting inwardly through the sides of the housing and descending downwardly therefrom. The device has nine inwardly projecting electrical lead strands supported on a ceramic substrate 15 and soldered to a printed electrical circuit 14 located recessed below the top opening of the housing. The base material is alumina, which accounts for 75% of the total substrate, and beryllia is also used. The ceramic is glued to a support aluminum plate 16 which acts as a heat sink.

Each electrical conductor has a brass conductive strip nominally consisting of 70% copper and 30% zinc. The protruding part of this conductor has an inverted U-shape 17, the end of which has a foot 18 to be soldered to part 14a of printed circuit 14. This foot of electrical conductor has a flat cross-sectional shape, with dimensions in this case being 0.04 inch wide and 0.009 inch thick. A number of adjacent sensitive electronic components include a resistor 19;
There is a capacitor 20, an electronic chip 21, etc.

In this device, a ceramic substrate is required because it is necessary to maintain a heat sink during operation of the ignition device when starting the engine. Ceramic has a higher electrical conductivity than the plastic (epoxy-impregnated fiberglass) substrates commonly used in the past. Alumina _is nominally 94% _Al2O3
More and more. The beryllia substrate is nominally at least 99.5% BeO and is approximately 0.02 inches (.56 mm) thick; 02
Inches. The aluminum support plate 16
Made of AA3003H14 aluminum alloy. Used in 0.6 inch thickness.

The electrical conductor has larger dimensions than those commonly used in the prior art, with a width 22 of . Have a flat rectangular cross-section (i.e., .04" x .009") that is greater than or equal to 0.03". Such electrical conductors require different processing techniques in order to produce a complete solid solder seam over the entire interface (underside) around its legs and edges.

The method essentially consists of two steps in this case: 1. A printed electrical circuit path 14 is arranged on the ceramic substrate 15 and at least one soldering pad 24 is attached to a portion 14a of this printed electrical circuit path. The solder pads can be deposited by conventional techniques, including silk screen printing. The pad in this case has a solder composition of 10% tin, 88% lead and 2% silver. The pad has a generally flat interface 25 whose width is greater than the width of the electrical conductor.
(except for the curved portion due to surface tension). Each pad in this case has a width. 08 inches, length. 1 inch and height.
It has a rectangular shape of 006 inches.

2 The surface portion 23 of the electrical lead is fully engaged with the soldering pad (against surface 25!) with an engagement force of 50-150 grams.
4 has a width 26 greater than the width 72 of the electrical conductor and an engaging surface portion of the electrical conductor. It is important to have a width 22 greater than 0.3 inches. In order to apply a force to the pad by means of an electrical conductor, the electrical conductor is curved with a constant radius or into a U-shape as shown in FIGS. 2 and 3, thereby causing the conductor to can apply a spring bias against the top surface 25 of the pad. The ends of the pads have curved foot portions 18 to allow mating of the engagement planes 23-25.

3. An unfocused laser beam is directed onto the soldering assembly, with the laser beam at least
having a beam input of 100 watts, reflowing only the spot diameter 27 and a predetermined portion of the soldering pad, which is larger than the width 22 of the electrical conductor but smaller than the pad width 26, and between the pad and the electrical conductor portion; Have a beam duration to form a soldering seam with a strength of at least 400 grams.

The laser beam 28 used was generated by a 370 Watt _CO2 laser and focused at a point 29 at a distance of 30 (.4 inches) above the soldering assembly. This beam was directed onto the foot 18 and solder pad 24 assembly. The central axis of the laser beam was aligned with the center of the electrical conductor and the mass 31 at the engaging portion thereof, and aligned with the center 32 of the soldering pad. A small amount of soldering flux was added to the area between the soldering surfaces before soldering. This flux is typically an inert rosin microflux.

The soldering assembly was preheated to 150°C to reduce power requirements and eliminate the risk of thermal shock. A coaxial flow of nitrogen gas is used to (a) prevent the solder joints from oxidizing and protect the solder joints from being coated with black deposits, and (b) prevent the focusing lens from becoming cloudy with smoke. I tried to protect things. The beam itself is almost the same as when it is emitted from the generator. It had a diameter of 6 inches and was defocused just above the soldering assembly so that the spot diameter on the soldering assembly was 0.030-0.080 inch. Best results are achieved with a spot diameter of 0.050.
Obtained when 0.060 inch. Various soldering operations were performed with varying beam intensities and durations in order to find out how to obtain satisfactory soldered joints with sufficient strength. Solder joints with a strength of 2000 grams or more have a power change of at least 100 watts to 500 watts,
The beam duration is 50-500 milliseconds (.
005-. 25 seconds). The above combinations of beam intensity and duration for large electrical conductors and soldering pads are plotted in FIG. 4 (spot diameter in this case 0.060 inch). The shaded area in Figure 4 shows the manufacturing range, within which parameter combinations can be used to create sufficient fillet 33 (second fillet) all around the seam without melting the entire soldering pad or substrate.
Figure) can be formed.

Room temperature samples without preheating required longer beam durations or higher power levels than preheated samples. Room temperature samples required 50-75% more power than samples preheated to 150°C for the same beam duration. Therefore at least
It is preferable to use a room temperature soldering assembly preheated to 100°C. At a temperature of 150℃, the power required is lower than that of a sample at room temperature (20℃).
It can be reduced by 20% (Figure 8).

The required energy or power level of the laser beam can be reduced by increasing the surface absorption power of the soldering leads. This power level can be reduced from 400 watts to 200 watts, for example, by placing a black coating on the conductor legs or by not providing a nitrogen gas shield.

The relationship between the beam irradiation time and the beam spot is such that the minimum time required to form a sufficient fillet gradually decreases as the beam spot diameter increases. The widest time range for obtaining a satisfactory seam is approximately the diameter of the beam spot. This is the case for 05 inches (1.27 mm). Long beam exposure times (eg, .3 seconds) and large spot diameters (eg, .085 inches) can cause beam reflection problems. Such beam reflections can cause thermal damage to adjacent components, such as resistors or capacitors. This beam reflection problem can be alleviated by using a suitable unfocused beam (appropriate spot diameter) and ensuring that the focus is above the soldering assembly. The flow of shielding gas increases the power requirements, but without the shielding gas combustion of the flux produces a black residue. This black residue is difficult to remove, so it is desirable to use a shielding gas. Gently blowing nitrogen gas is suitable for preventing combustion or carbonization of the flux.

Critical Thermal Radius In laser heat processing, the beam spot diameter is a guideline for determining the size of the beam relative to the workpiece. However, the magnitude of the thermal work done cannot be predicted from the beam diameter itself. When drilling holes in various materials, such as glass, by means of a laser beam, the hole formed usually differs from the beam spot diameter on the glass material.

The critical thermal spot radius of the present invention refers to the radius of the area on the workpiece that can provide the required thermal effect. That is, this radius is the radius of the melted area when laser soldering is performed.

An analysis of the input heat distribution is required to obtain a mathematical formula that gives an accurate critical heat spot radius. For this analysis, we assumed a simple thermal model of one-dimensional heating and a certain Gaussian beam shape. When a laser beam impinges on a metal absorption surface, the laser beam is absorbed by interaction with electrons. A quantum of optical energy is absorbed by an electron, which further dissipates its energy by colliding with lattice phonons and other electrons. Since the average residence time of colliding electrons is about 10 ^-12 to 10 ^-13 seconds, it can be considered that optical energy is converted into heat almost instantaneously. The penetration depth of the optical region into a good conductor, as used in a preferred version of the invention, is in the range of 10 ^-6 centimeters or 10-100 atomic layers. Therefore, in the case of laser soldering, laser absorption is considered to occur on the processed surface.

Further assumptions were introduced to simplify the analysis of the heat flow system. That is: (a) the thermal properties and thermal resistance of the material do not change with temperature;
(b) convection and radiation losses are negligible;
(c) Heat flow can be considered one-dimensional;
It was assumed that (d) the thickness of the soldering pad was uniform throughout, and (e) that the aluminum substrate could be considered a heat sink. With these assumptions, as shown in FIG. 5, this heat flow system is proportional to two component heat flows: integrated heat and conductive heat. The heat input q is partially integrated into the soldering pad q ₅ and the alumina substrate q _A , and the remaining heat is conducted to the heat sink q ₃ (aluminum substrate). As shown in Figure 5, T ₂ is the average temperature in the thickness direction of the alumina substrate, and if the temperature profile within the alumina substrate is assumed to be parabolic, then the average temperature through the substrate is
T ₂ is given by the formula that T ₂ is equal to T _1/3 . The energy balance of the heat flow system shown in Figure 6 is given by the first-order differential equation: q=q ₅ +q _A +q ₃ q=(C ₁ +C ₂ /3)dT ₁ /d _t +T ₁ /R where: C ₁ = heat capacity of the soldering pad C ₂ = heat capacity of the ceramic substrate.

From this formula, the value of T ₁ can be calculated, and if the soldering temperature is changed from T 1 to T ₁ within T _c seconds,
If we define another quantity, q _c, with the heat flow per unit area as the velocity to make T _n , we get the following equation: q _c = T _n /R[1- ^exp (-t _c /RC) ] Additional energy is required to melt the solder after it reaches T _n and this additional energy is provided by an extended beam with a duration approximately 3% longer than the time t _c given previously. Supplied.

The residual Gaussian beam is typically defined by the radius at which the intensity falls to 1/e ² of its peak value (86.5% of the total beam energy is contained within the area of this radius). . The intensity profile of the Gaussian beam is shown in FIG. When a Gaussian beam melts a spot of radius c on a soldering pad as shown in Figure 6, the beam intensity at the edge of the melted spot is given by: P _c = 2P/πa ² exp ( -2γ ² /a) This radius c is chosen as the critical heat spot radius. The reason is that this thermal radius defines the area over which the required thermal effect is obtained. Using the formula obtained earlier, the radius c (critical thermal radius) can be transformed into the following formula: In the formula: c is the critical heat spot radius a is 1/e Gaussian radius at ^two points l _o is the logarithm T _n is the temperature obtained by subtracting the sample temperature from the melting temperature of the solder P is the laser beam power A is the solder temperature at 10.6 microns Surface absorbency R is the thermal resistance per unit area of the system, t _c is the critical time to bring the solder to T _n , and c is the heat capacity of the system.

The minimum power required to melt the solder at a predetermined temperature _T1 can be obtained by setting c=0 in the above equation.

FIG. 7 shows the theoretical c radius as a function of beam duration for a beam spot diameter of 0.060 inches. The plot was made by substituting appropriate values of parameters and material properties into the equation for the critical heat spot radius.

[Brief explanation of the drawing]

FIG. 1 is a photograph showing a microelectronic device with several seams soldered by the method of the invention. 2 is an enlarged photograph illustrating some of the electrical conductors of FIG. 1 successfully soldered according to the present invention; FIG. Third
Figure 3a is a cross-sectional elevational view of a portion of the apparatus of Figure 1, showing one soldering seam and an insert (Figure 3a represents a plan view of the soldering seam); FIG. 4 shows a graphical representation of the variation in beam duration and beam power for various combinations (horizontal shading indicates useful allowable manufacturing combinations); 5 is a diagram illustrating the flow of heat between the various materials forming the soldering assembly of FIG. 2; FIG. FIG. 6 is a diagram showing a schematic diagram representing a Gaussian energy beam distribution passing through a soldering pad. FIG. 7 shows a graphical representation of critical thermal radius varying as a function of beam duration at various power levels. FIG. 8 shows a graphical representation of the beam power as a function of the soldering assembly at various preheating temperatures T ₀ . 13... Conductive wire, 14a... Conductive path, 15... Ceramic substrate, 24... Pad, 28... Laser beam.

Claims (1)

Claims: 1. A method of soldering one or more electrical conductors to a printed conductive track, comprising: (a) disposing said conductive trace on a ceramic substrate and placing at least one soldering pad on said conductive trace; (b) pressing a surface portion of the electrical conductor against the pad, applying sufficient engagement force to the conductor and the pad to enhance heat transfer between the conductor and the pad during soldering; has a width greater than the width of the conductor, and the interengaging surface portions of the conductor have a width greater than the width of the conductor. (c) directing an unfocused laser beam onto said soldering means, wherein said conductive wire portion and pad are adapted to form a soldering means; Beam output at least
100 watts, the spot diameter of the beam on the soldering means is no smaller than the width of the conductor and no larger than the width of the pad, and the spot diameter of the beam on the soldering means is controlled by the beam. irradiate the beam for a predetermined beam irradiation time to reflow only a specific portion of the pad and form a solder joint between the pad and the conductor portion, thereby forming a solder joint. the seam having a strength of at least 400 grams. 2. In the method described in claim 1,
The conductive wire has a flat engagement surface and the pad has an interlocking, generally flat engagement surface. 3. In the method described in claim 1,
The laser beam in step (c) is 0.03-0.08
A method in which the beam is focused a distance of 0.2-0.8 inches away from the engagement surface using a lens blurred to an inch diameter, i.e., with a focal length of 5 inches. 4. In the method described in claim 3,
A method in which the laser beam is blurred at a point above the engagement surface of the soldering means. 5. In the method described in claim 1,
A method in which the radius of the heat spot is increased by coating at least a portion of the soldering means with an energy absorbing material. 6. The method according to claim 1, wherein the radius of the heat spot is
(c) by preheating to at least 100 degrees C. 7. The method of claim 1, wherein the ceramic substrate of step (a) is made of one or two materials selected from the group consisting of alumina and beryllia, and wherein the ceramic substrate comprises at least... How to have a thickness of 0.5 inches. 8. In the method described in claim 1,
The beam duration used in step (c) is
Methods within the range of 50-200 milliseconds (.05-2.0 seconds). 9. In the method according to claim 1, the beam spot diameter used in step (c) is .
03-. How to be within the range of 08 inches. 10. The method of claim 1, wherein the center of mass of the surface portion of the conductor is aligned with the center of the pad upon engagement. 11. In the method of claim 1, the parameters of beam power, beam spot diameter and beam duration are optimally correlated by: where c is the critical heat spot radius a is the Gaussian radius at 1/e ^{2 l} _o is the logarithm T _n is the temperature obtained by subtracting the sample temperature from the melting temperature of the solder P is the laser beam output in watts Laser beam output A is at 10.6 microns The surface absorbency R of the solder is the thermal resistance per unit area of the system, t _c is the critical time to reach the solder temperature T _n , and C is the heat capacity of the system. 12. The method of claim 1, wherein the conductive wire is made of tin or brass coated with solder, the pad is made of tin/lead/silver or a tin/lead alloy, and the printed conductive path is method in which palladium/silver components are used. 13. The method of claim 1, wherein the heat spot radius melts a portion of the pad to form a surrounding fillet between the surface portion of the conductor and the pad. Method.