JP4582444B2 - Solder balls with excellent anti-adhesion properties and wettability and methods for preventing fine adhesion of solder balls - Google Patents

Description

  The present invention relates to a solder ball excellent in fine adhesion prevention characteristics and wettability used for connecting electronic parts and the like, and a method for preventing fine adhesion of solder balls.

  At present, electronic components for mounting a semiconductor or the like on a substrate are manufactured by a package called BGA (Ball Grid Array) or CSP (Chip Size Package). In these, solder balls are used for terminal joining. Generally, terminal bonding by solder balls is often applied to bonding at intervals of several hundred microns, and high dimensional accuracy is required. In addition, when a soldering failure occurs in these terminal joints, a package that is very expensive compared to the solder cost becomes defective, and thus high reliability is required.

  In order to achieve high dimensional accuracy and reliability in terminal bonding, it is important to manage the composition of the solder balls, the solder bonding conditions, and the surface state of the solder balls. Among these, the surface state of the solder ball needs to have a low degree of oxidation on the surface. In manufacturing a package using solder balls, the solder balls are mounted at predetermined terminal positions on the BGA substrate, and then the solder balls are melted by a heating device to form bumps. At this time, if the surface of the solder ball is oxidized, the wettability of the solder ball at the time of melting decreases, the solder does not completely adhere to the terminal on the substrate, and the bonding strength is insufficient. In addition to the oxide film, the wettability of the solder ball is reduced when a film of dirt, organic matter or the like is formed on the surface. Insufficient solder joint strength causes poor soldering, which in turn causes defective packages.

  This oxidation of the solder ball surface is called blackening because the surface of the solder ball after oxidation appears black. However, the surface of the solder ball is different from that of the solder ball because of the problem of reduced solder joint strength due to blackening. Various methods for suppressing the surface oxidation of the solder balls by coating with the use of are described. As materials for coating the solder balls, lubricants such as aliphatic hydrocarbon lubricants, higher aliphatic alcohol / higher fatty acid lubricants, fatty acid amide lubricants, metal soap lubricants, fatty acid ester lubricants (for example, Patent Document 1) The use of rust preventives such as metals such as Au, Sn, and the like (for example, Patent Document 2) has been studied.

  The above-mentioned lubricant and Au coating both suppress the oxidation of the solder ball surface. As a specific action, the lubricant is scraped on the surface of the solder ball by friction with the outside, and the surface is chemically active. It has the effect of suppressing the state. In addition, the coating with Au suppresses the oxidation of the solder ball surface by covering the solder surface with Au which is difficult to oxidize.

On the other hand, the coating with Sn improves the wettability of the solder ball at the time of melting by covering the solder ball with the metal having high wettability at the time of melting the material used for the terminal, rather than suppressing the oxidation. To do.
Japanese Patent Laid-Open No. 2000-287771 JP 08-16496 A

  In recent years, in addition to blackening, the problem of fine adhesion of solder balls has been pointed out as a cause of poor package bonding. The fine adhesion of solder balls is a phenomenon that is particularly noticeable in small solder balls having a diameter of 400 μm or less, but the solder balls that were originally separated are bonded with a weak adhesive force. It is. As described above, when mounting the solder balls on the substrate, it is necessary to mount the solder balls one by one at a predetermined terminal position. Solder balls are mounted by an automated device. However, if there are solder balls with slight adhesion, solder balls are not mounted at the terminal positions to be mounted, or conversely, two or more solder balls are mounted. As a result of being mounted, the bonding failure of the package occurs.

  The coating with the material disclosed in Patent Documents 1 and 2 is an excellent material in improving the deterioration of the wettability at the time of melting accompanying blackening and the resulting poor bonding of the package. However, as a result of studies by the present inventors, it has been found that coating with these lubricants and the like can not only solve the problem of fine adhesion of solder balls, but also increases the frequency of fine adhesion in some cases.

  The present inventors have studied in detail the problem of fine adhesion of solder balls solidified and spheroidized in the gas phase, and the fine adhesion described above is particularly affected by the humidity to which the solder balls are exposed, It was found that fine adhesion hardly occurs in a dry atmosphere with low humidity. And as mentioned above, it was confirmed that various coatings conventionally performed have an effect of reducing friction, but conversely promote adhesion. In addition, it has been found that the wettability of these coatings decreases as the film thickness increases.

  An object of the present invention is to provide a solder ball capable of solving the problem of fine adhesion in a solder ball and simultaneously imparting good wettability, and a method for preventing fine adhesion of the solder ball.

  The present inventors examined a method for solving the problem of fine adhesion due to the surface treatment of solder balls, and found that the surface adsorption of metal soap molecules can significantly reduce the occurrence of fine adhesion. And it discovered that favorable wettability could be achieved simultaneously by controlling the film thickness appropriately, and reached the present invention.

That is, the present invention is a solder ball connection terminals spheronised solidified in the gas phase, the solder ball surface, magnesium stearate, barium stearate, thickness 3nm following metals consisting of either Calcium stearate made to adsorb soap molecular film, an excellent solder balls to a fine anti-adhesion properties and wettability. For example, calcium stearate, magnesium stearate, or barium stearate can be used as the metal soap. The present invention is preferably applied to solder balls having a diameter of 400 μm or less.

Further, the method for preventing fine adhesion of solder balls according to the present invention comprises a solder ball for connecting terminals which is solidified in a gas phase and spheroidized, and a metal soap concentration of less than 5 ppm made of magnesium stearate, barium stearate or calcium stearate. in immersed in a solution obtained by dispersing, after removal of the solder balls from the solution, evaporation of the solvent in the solder ball surface, a shall adsorbing the following metal soap molecules film thickness 3nm dried, this Excellent wettability can be maintained. Preferably, after the solvent on the solder ball surface is volatilized, the solder ball is then dried in an atmosphere with a relative humidity RH ≦ 40%.

  According to the present invention, the problem of fine adhesion in solder balls can be solved, and sufficient wettability can be maintained. Therefore, poor bonding when producing packages such as BGA and CSP using solder balls. This is an industrially useful technique in which the wettability is reduced or the wettability is maintained or improved, and the package yield can be improved.

  As described above, the feature of the solder ball of the present invention is that metal soap molecules having a predetermined thickness are adsorbed on the surface of the solder ball. The special effects of metal soap have not been fully elucidated, but are considered as follows.

  First, the problem of micro-adhesion between solder balls occurred remarkably in solder balls that were solidified in the gas phase and spheroidized, and were manufactured by the oil-in-oil cooling method that solidifies spherically in oil that has been commercially available. Since solder balls do not generate so much after sufficient degreasing, it was predicted that this would be due to the difference in surface condition due to the environment during the formation of the balls.

Although the solder balls actually solidified in the gas phase generate an oxide layer, they have a depth of about 2.5 nm in terms of the sputtering rate of SiO ₂ . For this reason, it is presumed that the surface is active and it is very easy to adsorb atmospheric moisture. Combining this with the fact that fine adhesion due to an increase in humidity in the atmosphere increases, it is strongly speculated that the adsorbed moisture promotes fine adhesion between the solder balls.

  On the other hand, the coating of the surface with the coating has a problem with the viscosity and adhesion of the coating itself. According to the study by the present inventors, it has been confirmed that sufficient results cannot be obtained even when a highly water-repellent fluororesin is used. Therefore, the present inventors have reduced the adsorption point by adsorbing molecules that do not have a polar group in order not to promote moisture adsorption, and as having a structure that makes the adsorbed molecules difficult to entangle with each other. We studied to prevent the occurrence of adhesion caused by adsorbed molecules and reached metal soap.

  In the metal soap, the polar group portion such as Na in the so-called soap is replaced with Ba or Mg, and the polar group portion disappears, and the surface active characteristics are lost. Since the non-polar organic acid part exists on the surface of the solder ball where the metal atomic part of the metal soap with a large atomic force is adsorbed, it has the effect of reducing the adsorption point and keeping strong polar water away. is there. In particular, when the organic acid portion of the metal soap is a simple straight-chain carboxylic acid such as stearic acid, it is optimal because adhesion, reaction, and entanglement between the metal soap molecules hardly occur.

  The frequency of occurrence of fine adhesion of solder balls increases as the diameter of the solder balls decreases. This is because when the solder ball has a large diameter, even when fine adhesion occurs, the load applied to the bonded portion increases due to the weight of the solder ball itself. This is because such a load becomes small, and once fine adhesion occurs, it is difficult to separate. Further, when the diameter is smaller, the curvature is increased, the surface energy is increased, and the affinity with water vapor in the air is increased, so that it is considered that fine adhesion is more likely to occur. In particular, in the case of a solder ball having a diameter of 400 μm or less, the fine adhesion increases, but the fine adhesion can be greatly reduced by coating the surface with a metal soap and drying the surface of the solder ball.

  Next, the inventors evaluated the wettability required for the solder balls. As described above, the solder balls of the present invention solve the problem of mutual fine adhesion by adsorbing a metal soap molecular film on the surface thereof. However, on the other hand, it was found that the wetness of the metal soap molecular film was lowered when it was too thick. Therefore, it has been found that the thickness of the metal soap molecular film has an optimum coating thickness for maintaining good wettability as well as sufficiently solving the problem of fine adhesion. That is, the solder ball of the present invention has a metal soap molecular film having a thickness of 3 nm or less adsorbed on its surface.

  Here, the coating thickness of the above-described metal soap of the present invention is as defined below. The metal soap molecule film of the present invention coated on the surface of the solder ball exhibits the above-mentioned effects while being maintained at the predetermined film thickness, but the metal soap molecule film itself at that time is coated It is not easy to measure the thickness. Therefore, in the present invention, in order to reproduce the thickness of the metal soap molecular film at that time, an accurate identification / notation method was examined. And by analyzing the surface depth direction of the C content of the solder ball surface by combining surface sputtering with Auger electron spectroscopy (AES), it correlates with the coating thickness of the metal soap molecular film, and conversion The technique that can be used was adopted.

The coating thickness of the metal soap molecular film of the present invention has a first order correlation with the C detection amount that represents the metal soap concentration of the solution during the coating process, which will be described later. On the other hand, the perforation speed by sputtering on the solder ball surface can be estimated in terms of SiO ₂ . More precisely, the actual drilling depth needs to be estimated by the drilling speed of, for example, Sn constituting the actual solder ball, but does not have a standard that can be calibrated at present (because it is difficult to prepare a reference sample). In the present invention, SiO ₂ is used as a reference sample, and the thickness of the metal soap is defined by the depth converted from the perforation speed. And the upper limit of the coating thickness of the metal soap that can provide both the fine adhesion prevention property and the good wettability according to this notation is 3 nm. The preferred coating thickness is 0.1 nm to 1.5 nm, more preferably 0.1 to 0.8 nm.

  The solder ball of the present invention is obtained, for example, by immersing a solder ball solidified and spheroidized in a gas phase in a solution in which metal soap is dispersed at a concentration of less than 5 ppm and taking out the solder ball from the solution. It can be obtained by volatilizing the solvent on the surface of the solder ball and drying, but after volatilizing the solvent, it is preferable to further dry in an atmosphere of relative humidity RH ≦ 40%. This is because by setting the atmosphere with a low relative humidity, water molecules once adsorbed by the solvent and the atmosphere during handling are removed as much as possible. Preferably, an atmosphere of RH ≦ 20% is applied. It is desirable to perform drying for 1 hour or more.

  Here, what is important in the above method is the concentration control of the metal soap dispersed in the solution, which is less than 5 ppm. When the metal soap concentration is 5 ppm or more, the effect of preventing fine adhesion of the solder balls is increased. However, as a result, when the coating amount of the metal soap after dipping / drying becomes too thick, a problem of reduction in wettability occurs. . In recent small package forms such as CSP, the pitch of the terminals is further narrowed, and good wettability under a smaller amount of flux used is required. Then, under the use of this very small amount of flux, if the solder ball has poor wettability, the solder ball will fall off significantly.

Therefore, the solder ball of the present invention has excellent wettability sufficient to cope with the use of this very small amount of flux, and in order to achieve the coating thickness of 3 nm or less, The concentration of the metal soap dispersed therein is less than 5 ppm. The concentration of the metal soap dispersed in the solution is preferably 0.5 ppm or more in terms of the effect of preventing fine adhesion due to immersion, but 3 ppm or less is desirable in terms of maintaining wettability. As the solvent used in the dispersion of the metal soap, for example, can be applied or an alcohol-based solvent, a solvent consisting of _{C n F 2 (n + 1} ) fluoride and the like.

  Metal soap molecules were adsorbed on the surface of a solder ball having a composition of 3% Ag-0.5Cu-remainder Sn by mass% obtained by solidification in the gas phase under the following conditions, and the surface was further dried. Thereafter, the frequency of occurrence of fine adhesion was evaluated.

Treatment agent: Barium stearate (Invention Example 1)
Magnesium stearate (Invention Example 2)
Calcium stearate (Example 3 of the present invention)
Solvent: 6 conditions of C ₅ F ₁₂ fluoride system concentration: 0, 1.0, 3.0, 4.5, 10, 50 ppm Treatment conditions: after immersion for 180 seconds, relative humidity RH = 10%, room temperature RT = 22 Volatilized solvent at ℃

  First, the thickness of the metal soap molecular film adsorbed on the surface of the solder balls according to the six conditions after the volatile drying was measured. The measurement procedure was examined by the surface depth direction analysis method of C content by the combination of AES and sputtering described above. The result is shown in FIG.

  From FIG. 1, in any metal soap type, there is a first-order correlation between the solute concentration of the metal soap and the adsorbed film thickness after immersion drying, and by adjusting the metal soap concentration in the solute. It can be seen that the adsorbed film thickness can be controlled.

  Next, the frequency of occurrence of fine adhesion was evaluated. As shown in the schematic diagram of FIG. 2, the evaluation procedure is as follows. The height adjustment block 4 tilts the bat 5 by 0.2 ° with respect to the horizontal plane, applies vibration to the table 6, and The solder ball 1 was rolled. When the solder ball 1 rolls on the bat 5, as shown in FIG. 2, the solder ball (mon ball 3 in FIG. 2) that does not cause fine adhesion advances straight downward, whereas the fine adhesion does not occur. Most of the generated solder balls (slightly bonded balls 2 in FIG. 2) do not go straight. This was performed by measuring the number (number of pairs) of finely bonded balls contained in about 100,000 solder balls by a method of visually confirming the slightly bonded balls 2 that do not go straight downward. The experimental results are shown in Table 1.

Furthermore, for evaluation of wettability, a solder ball was mounted on a pad on which a very small amount of flux was applied on a BGA substrate, a reflow process was performed in an N ₂ atmosphere, and the wet state of the ball was observed. The evaluation conditions are as follows.

Pad: Diameter 240 μm, base electroless Ni plating, surface Au flash plating,
Number of pads 100
Flux: Water-soluble flux WS-9160-M3 manufactured by Alpha Metals,
Application amount 0.0004 mm ³ applied with a dispenser Reflow conditions: N ₂ atmosphere, preheat 150 to 170 ° C./180 seconds
+ 220 ° C or higher / 60 seconds (peak temperature 240 ° C)

  Judgment of wettability is based on whether the pad surface is completely wet after reflow and shows a good bump shape, partially wet, bump shape is distorted, or not wet at all. The percentage of bumps was determined at 100 minutes. The experimental results are shown in Table 2.

  From Table 1, the solder balls with a large coating amount or a high solute concentration when immersed and with a diameter of 300 μm are significantly less adhered to each other than the condition of a solute concentration of 0 ppm for which any metal soap type is not coated. Can be reduced. Regarding the reduction of the number of fine adhesion, the higher the solute concentration, the better, but the effect is almost saturated at 10 ppm or more.

  On the other hand, from Table 2, the wettability is better and the bump shape is better as the coating amount is thinner or the solute concentration during immersion is lower. If the solute concentration is 3 ppm or less, good wettability equivalent to the condition in which the coating treatment is not performed is obtained, and a suitable upper limit of the solute concentration is 3 ppm. In terms of the coating thickness, 0.8 nm or less is suitable.

  And if it is a solder ball which satisfy | filled the coating thickness of the metal soap molecule | numerator of this invention, or the solute density | concentration at the time of immersion, it turns out that the favorable fine adhesion prevention characteristic and wettability are compatible. The range suitable for this coexistence is 0.5 to 3 ppm in terms of solute concentration, and 0.1 to 0.8 nm in the case of film thickness.

It is a graph which shows the relationship between the density | concentration of the metal soap disperse | distributed in the solution which immerses a solder ball | bowl, and the film thickness of the metal soap molecule | numerator adsorbed on the surface of the solder ball | bowl dried after this immersion. It is a schematic diagram which shows the method for measuring the frequency of occurrence of fine adhesion used in the examples.

Explanation of symbols

  1. 1. solder balls, 2. Adhesive balls (solder balls); Single ball (solder ball), 4. 4. height adjustment block; Bat, 6. Stand

Claims (4)

  A solder ball for a connection terminal which is solidified in a gas phase and spheroidized, and adsorbs a metal soap molecular film having a thickness of 3 nm or less made of magnesium stearate, barium stearate or calcium stearate on the surface of the solder ball Solder balls with excellent anti-adhesion properties and wettability characterized by The solder ball having excellent anti-adhesion properties and wettability according to claim 1, wherein the diameter is 400 μm or less.   The solder balls for connection terminals solidified in the gas phase and spheroidized are immersed in a solution of magnesium stearate, barium stearate, or calcium stearate dispersed at a metal soap concentration of less than 5 ppm. A method for preventing fine adhesion of a solder ball, comprising: volatilizing a solvent on the surface of the solder ball after drying the ball and drying to adsorb a metal soap molecular film having a thickness of 3 nm or less. 4. The method for preventing fine adhesion of solder balls according to claim 3 , wherein the solvent on the surface of the solder balls is volatilized and then dried in an atmosphere having a relative humidity RH ≦ 40%.

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