JP2009034692A - Solder ball, and solder bump using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solder ball which can solve the problem of defects in the deletion judgement, the height measurement, and the visual inspection of a solder bump defect due to the discoloration of the solder bump in soldering, and further to provide the solder bump using the same solder ball. <P>SOLUTION: The solder ball comprises 0.5 to 4.0 mass% of Ag, 0 to 1.0 mass% of Cu, 0.2 to 3 ppm of Cr, and the remainder composed of Sn and inevitable impurities. It is preferable that the density of Cr is higher in the surface than in the inside. Further, it is preferable that in the solder bump formed from the solder ball, the density of Cr in the surface is higher than that in the inside. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solder ball used for soldering an electronic component or the like and a solder bump using the same.

  In recent years, with the reduction in mounting area and the increase in the number of I / O terminals in mobile devices such as mobile phones, semiconductor packages are also becoming smaller and more highly integrated. As a result, the mounting form in which the semiconductor package is connected to the mother board is also changing from the conventional peripheral terminal type using leads to a type in which electrode terminals are formed in a lattice shape on the bottom surface of the semiconductor package. A typical example is BGA (Ball Grid Array). The electrodes of the BGA are connected to the electrodes of the mother board using solder formed in a spherical shape, that is, solder balls.

Connection by solder balls is performed in the following procedure. First, after mounting solder balls on a plurality of electrodes arranged on a semiconductor package, the solder is melted by heating to a temperature equal to or higher than the melting point of the solder, and solder protrusions called solder bumps are formed on the electrodes.
Next, after mounting the semiconductor package on the motherboard so that the position of the electrodes on the motherboard and the position of the solder bumps previously formed on the semiconductor package overlap, the semiconductor package is heated again to a temperature above the melting point of the solder. And the mother board electrode are joined.

When the electrodes of the semiconductor package and the electrodes of the motherboard are joined by solder bumps, if the solder bumps are not formed on the electrodes of the semiconductor package due to poor mounting of solder balls or poor solder wettability, they are missing to be treated as defective products. Rate management is important.
Even if the solder bumps are not missing, if the bump heights differ greatly from one electrode to another, the bumps with a low height cannot be joined because they cannot contact the electrodes on the motherboard. For this reason, when solder bumps are formed on the electrodes of the semiconductor package, the defect of the solder bumps can be confirmed by an image recognition device such as a CCD camera, or the height variation of the solder bumps can be measured by a laser length meter. The solder bumps are formed on all the electrodes, and only those whose height variations are within an allowable range are mounted on the motherboard.

As a typical solder alloy used for joining, an alloy mainly composed of Sn and added with Ag or Cu as disclosed in Patent Documents 1 and 2 can be cited. These alloys have a melting point of about 220 to 225 ° C., and a general soldering temperature is 240 ° C., which is 15 to 20 ° C. higher than that. Since these temperatures are about 5 to 10 ° C. lower than the case of Sn alone, there is an advantage that deterioration due to thermal damage of a resin material used for a semiconductor package or a mother board can be reduced.
Patent No. 3027441 JP2002-239780

As described above, the solder alloy mainly composed of Sn and added with Ag or Cu can reduce the thermal damage of the semiconductor package or the mother board at the time of soldering.
However, as a result of studying a solder ball made of a solder alloy mainly composed of Sn and added with Ag or Cu, the present inventor has turned yellow when solder bumps are formed on the electrodes of the semiconductor package (hereinafter referred to as yellowing). )), And there was a problem that good metallic luster was lost.
In this yellowed solder bump, the reflection of the irradiation light in the image processing apparatus and the laser light in the laser length meter is disturbed. For this reason, there is a problem that the defect determination and the bump height measurement cannot be performed, and this problem becomes remarkable particularly when the oxygen concentration in the atmosphere at the time of soldering is high.
In addition, even if the solder bumps that have been yellowed can pass the defect determination and bump height measurement somehow, they do not have a metallic luster, so they do not look good. There has occurred. As a result, the productivity and yield of semiconductor packages has been significantly reduced.

  The object of the present invention is to solve the problem of defect in solder bump defect determination, height measurement, and visual inspection due to discoloration during soldering in a solder alloy mainly composed of Sn and added with Ag or Cu. An object of the present invention is to provide a solder ball and a solder bump using the same.

  The present inventors have observed that the discoloration at the time of soldering in a solder ball made of a solder alloy mainly composed of Sn and to which Ag or Cu is added is due to oxidation of Sn, and a small amount of Cr that is strictly controlled. As a result, it was found that the occurrence of discoloration can be greatly suppressed without impairing the solderability, and the present invention has been achieved.

That is, the present invention is a solder ball composed of 0.5 to 4.0% Ag by mass, 0 to 1.0% Cu, 0.2 to 3 ppm Cr, the remainder Sn and inevitable impurities. .
Moreover, it is preferable that Cr is concentrated on the surface of the solder ball of the present invention.
In the solder bump formed from the solder ball of the present invention, it is preferable that Cr is concentrated on the surface.

  According to the present invention, in a solder ball made of a solder alloy mainly composed of Sn and added with Ag or Cu, solder bump defect determination and height measurement caused by discoloration during soldering without impairing solderability. In addition, the problem of defects in visual inspection is drastically improved.

  As described above, an important feature of the present invention is that a small amount of strictly controlled Cr is added to a solder ball made of a solder alloy mainly composed of Sn and added with Ag or Cu.

  In the case of a solder ball made of a solder alloy mainly composed of Sn and added with Ag or Cu, the most easily oxidized element is Sn, which reacts with oxygen in the atmosphere during soldering to form an oxide film on the surface of the solder bump. To do. The color of the solder bump changes depending on the thickness of the surface oxide film, and changes from a silvery white with a metallic luster to a yellow with no luster as the thickness increases. That is, yellowing of the solder bumps due to soldering occurs as a result of the reaction between Sn and oxygen in the solder resulting in a thickened oxide film.

  On the other hand, when Cr is added to the solder alloy, since Cr is an element that is more easily oxidized than Sn, Cr in the solder preferentially reacts with oxygen to form an Sn oxide film in which Cr is concentrated. . This Sn oxide film having a high Cr concentration is thermally stable, and its thickness hardly changes even when heated. For this reason, a solder bump formed of a solder ball made of a solder alloy to which Cr is added can obtain a silvery white color with a metallic luster substantially the same as that of the solder ball even after soldering. The silver-white solder bumps are greatly reduced in the frequency of occurrence of defects in solder bump defect determination and height measurement, and are not problematic in visual inspection. In this way, the problem of semiconductor package productivity and yield due to discoloration during soldering is dramatically improved.

The reason why the amount of Cr added is 0.2 to 3 ppm is as follows. When the addition amount is less than 0.2 ppm, Cr is insufficient to thermally stabilize the Sn oxide film, and the oxide film grows and yellows due to heating during soldering.
When the addition amount is 0.2 ppm or more, the amount can be sufficiently concentrated to thermally stabilize the Sn oxide film, and the progress of yellowing is suppressed.
On the other hand, Cr is an element that remarkably increases the melting point of Sn even in a small amount. Therefore, if it is added more than the necessary amount, the melting point of the solder alloy increases and the soldering temperature must be increased. When the soldering temperature is increased, there is a risk that the resin material used for the semiconductor package or the mother board is deteriorated due to thermal damage. The present inventor has confirmed that soldering at a general 240 ° C. as a soldering temperature is possible up to 3 ppm of Cr.

The reason for setting the Ag content to 0.5 to 4.0% is that the soldering temperature can be lowered because the melting point of Sn is decreased by adding Ag to Sn. Thereby, deterioration by the heat damage of the resin material currently used for a semiconductor package or a motherboard can be reduced.
However, addition of Ag exceeding 4.0% causes an increase in melting point, and it is necessary to increase the soldering temperature, which causes thermal damage.

The reason for setting the Cu content to 0 to 1.0% is that, similarly to the case of adding Ag, the melting point of Sn is lowered by adding Cu to Sn, so that the soldering temperature can be lowered. It is. However, if it is not necessary to lower the soldering temperature, the Cu content may be zero.
However, addition of Cu exceeding 1.0% causes an increase in melting point, and it is necessary to increase the soldering temperature, which increases thermal damage.

An alloy in which Ag, Cu and Cr were added to Sn having a purity of 99.9% or more was produced, and then formed into a solder ball having a diameter of about 0.4 mm. Table 1 shows the alloy composition of the evaluated solder balls. A uniform droplet spray method was used for forming the solder balls. The uniform droplet spraying method is a method of producing microspheres by melting the solder alloy in the crucible and discharging the molten solder from the crucible, and discharging it by applying vibration to the molten solder when discharging. In this method, the molten metal is made into a microsphere having a uniform volume. The atmosphere in the molding method was performed in nitrogen.
Further, since the amount of Cr added was very small, the Cr content of the solder balls was measured by GD-MS analysis using VG9000 manufactured by VG Elemental. The isotope used for the analysis of Cr is Cr (52), and the lower limit of detection is 0.001 ppm.

The yellowness of the produced solder balls was measured to evaluate the degree of metallic luster.
The measurement is performed with a colorimeter, and the solder balls are uniformly arranged on a 15 mm square tray so that they do not overlap each other, and then exposed to white light, and yellow in the L ^* a ^* b ^* color system defined by JIS Z 8729 The degree b ^* was determined.
Next, in order to evaluate the degree of yellowing during soldering, a discoloration test was performed in which the solder balls were heated at 230 ° C. for 30 s or 60 s, and the yellowness b ^* was measured. When soldering a solder alloy mainly composed of Sn and containing Ag or Cu, it is usually performed in a nitrogen atmosphere, but this time, heating was performed in the atmosphere as a severe condition in which yellowing due to surface oxidation is more likely to proceed.

  The measurement result of yellowness is shown in Table 2, and the yellowness after 60 s as a function of Cr addition is shown in FIG. As shown in Table 2 and FIG. 1, the solder ball made of a solder alloy to which Cr is added in an amount of 0.2 ppm or more in the present invention is extremely heated in the atmosphere as compared with the solder ball having an addition amount of less than 0.2 ppm. Even in an environment that is easily oxidized, the yellowness after heating is less than 10, indicating that the progress of yellowing is greatly suppressed. In this way, by adding a small amount of Cr that is strictly controlled, the metallic luster is maintained even after heating, so that the productivity and yield of the semiconductor package can be drastically improved.

In order to clarify the effect of Cr addition in suppressing yellowing, a solder ball composed of the alloy composition shown in Table 1 is assumed to be a solder bump, heated at 240 ° C. for 5 minutes, and then near the surface of the solder ball using EPMA. The Cr concentration of was measured. The heating atmosphere here was the same nitrogen as in normal soldering (purity 99.99%, the balance being mainly oxygen). In addition, the detection limit of Cr in EPMA surface analysis is 50 mass ppm.
Table 3 shows the surface Cr concentration of the solder balls before and after heating. From Table 3, the solder balls of the present invention that are not yellowed have a significantly increased surface Cr concentration by heating, which is 10 times or more and two orders of magnitude higher than the amount of Cr added to the solder shown in Table 1. That is, it can be seen that Cr in the solder positively reacts with a very small amount of oxygen in the nitrogen atmosphere, concentrates on the surface of the solder ball, and suppresses yellowing.
In addition, the smaller the amount of Ag in the solder, in other words, the higher the proportion of Sn as the base material, the more likely it is to oxidize. However, when Cr is added, the oxidation of Cr is promoted and the concentration on the surface is rather enhanced. Has been confirmed to be remarkable.

The relationship between the yellowness b * of a solder ball and the amount of Cr added is shown.

Claims (3)

A solder ball comprising, by mass, 0.5 to 4.0% Ag, 0 to 1.0% Cu, 0.2 to 3 ppm Cr, the balance Sn and inevitable impurities. The solder ball according to claim 1, wherein Cr is concentrated on the surface. 3. A solder bump formed by the solder ball according to claim 1, wherein Cr is concentrated on the surface.