JP5071802B2 - Solder balls, solder layers and solder bumps - Google Patents

Description

The present invention relates to a solder layer and the solder van flop formed by using a lead-free solder balls and it is used for soldering electronic components.

  In recent years, as the mounting area of mobile devices such as mobile phones is reduced and the number of I / O terminals is increased, 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 a 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 into 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 semiconductor package electrodes and motherboard electrodes are joined by solder bumps, if solder bumps are not formed on the electrodes of the semiconductor package due to defective mounting of solder balls or poor solder wetting, they are treated as defective products It is important to manage the defect rate. 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.

  In order to prevent these defects in advance, when solder bumps are formed on the electrodes of the semiconductor package, chipping of the solder bumps can be confirmed with an image recognition device such as a CCD camera, and the height of the solder bumps can be measured with a laser length meter. An inspection for measuring the thickness variation is performed, and only the solder bumps are formed on all the electrodes and the height variation is within an allowable range is mounted on the motherboard.

Typical lead-free solder balls or solder alloys used for joining semiconductor packages include those mainly composed of Sn and added with Ag or Cu as disclosed in Patent Document 1 and Patent Document 2. It is done. 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.
Japanese Patent No. 3925554 JP 2004-1100 A

  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 found that the solder ball is mounted on the electrode of the semiconductor package or the bump is formed at the time when the bump is formed. There was a problem of discoloration (hereinafter referred to as yellowing) and loss of good metallic luster. In the yellowed solder balls and solder bumps, 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, the soldering property of the yellowed solder balls with the electrodes on the semiconductor package is significantly reduced. In the case of a solder bump, even if the defect determination and the bump height measurement can be passed, the appearance is poor because it does not have a metallic luster, and a defect that is determined to be defective by subsequent visual inspection has occurred. As a result, the productivity and yield of the semiconductor package are greatly reduced.

  The object of the present invention is to improve the mounting ratio of solder balls due to discoloration at the time of soldering, to determine the defect of the solder bump, to measure the height, and to visually inspect the solder alloy mainly composed of Sn and to which Ag or Cu is added. An object of the present invention is to provide a solder ball that solves the problem of defects in the soldering.

  The inventors of the present invention have found that yellowing in a solder ball made of a solder alloy mainly composed of Sn and to which Ag or Cu is added occurs when the Sn oxide film on the surface is thickened, and by controlling the Sn oxide film thickness. The inventors have found that the occurrence of discoloration can be significantly suppressed without impairing the solderability, and have reached the present invention. Furthermore, when a small amount of Cr that is strictly controlled is added, the solderability is improved and the occurrence of discoloration can be further reduced.

  That is, the first invention of the present application is composed of 0 to 4.0% Ag, 0 to 1.0% Cu, the balance Sn and unavoidable impurities, and the surface yellowing degree is 10 or less. Solder balls characterized by In the present invention, the thickness of the oxide film on the surface is preferably 20 nm or less.

The second invention of the present application is composed of 0 to 4.0% Ag by mass, 0 to 1.0% Cu, the balance Sn and inevitable impurities, and includes an amorphous SnO layer and crystalline SnO _2. A solder layer characterized by having a layer.

The third invention of the present application is composed of 0 to 4.0% Ag by mass, 0 to 1.0% Cu, the balance Sn and inevitable impurities, and includes an amorphous SnO layer and crystalline SnO _2. A solder bump characterized by having a layer.

  The solder ball, solder layer and solder bump of the present invention preferably contain 0.2 to 3 ppm of Cr, and the surface is preferably enriched with Cr.

  According to the present invention, in a solder ball made of a solder alloy mainly composed of Sn and added with Ag or Cu, chipping determination and height measurement of solder bumps 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 solder ball mainly composed of Sn and made of a solder alloy to which Ag or Cu is added strictly manages the storage state after manufacture, and the surface is yellowed. This is because the surface Sn oxide film is controlled.

  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, and immediately after the solder ball is manufactured, it reacts with oxygen in the atmosphere to form an oxide film. . Further, it reacts with oxygen in the atmosphere during soldering to form an oxide film on the surface of the solder bump. Both are Sn oxide films. The former is naturally formed by moisture and oxygen atmosphere at the time of storage, and as the thickness increases, the solderability decreases, and changes from silvery white with metallic luster to yellow with no luster. The latter is formed by heating, and the color of the solder bump changes from silvery white with metallic luster to matte yellow as the thickness of the surface oxide film increases. That is, yellowing of the solder balls and solder bumps occurs when the oxide film becomes thick as a result of the reaction between Sn and oxygen in the solder.

Immediately after manufacture, a bottle in which solder balls are enclosed in an inert gas and an oxygen scavenger are enclosed, and further sealed with a gas barrier film to block outside air. Thereby, the growth of the Sn oxide film formed naturally can be suppressed, and the yellowing of the solder ball can be reduced. The inert gas is preferably argon rather than nitrogen. As the gas barrier film, it is desirable to use oxygen permeability <0.5 (cc / m ² / day / atm) and water vapor permeability 0.3 to 0.7 (g / m ² / day). If at least any two of the inert gas, oxygen scavenger, and gas barrier film are used in combination, an effect can be found in reducing yellowing.

  On the other hand, the Sn oxide film formed by heating, when Cr is added to the solder alloy, is an element that is more easily oxidized than Sn. Therefore, Cr in the solder preferentially reacts with oxygen, and Cr is concentrated. An Sn oxide film is formed. 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 by 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 greatly reduce the frequency of occurrence of defects in solder bump defect determination and height measurement, and do not cause a problem in visual inspection. In this way, the problem of semiconductor package productivity and yield due to discoloration during soldering is drastically improved.

The reason why the surface oxide film thickness is 20 nm or less is that the Sn oxide film formed naturally is amorphous and elastically deformed. When the inside is in a semi-molten state during the soldering process, the oxide film is destroyed due to internal deformation if it is 20 nm or less. If it is thicker than 20 nm, the oxide film is destroyed, and the molten solder is always covered with the oxide film and cannot be joined.

  The reason why the content of Ag is set to 0 to 4.0% is that the soldering temperature can be lowered because the melting point of Sn is lowered 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% leads to an increase in melting point, and it is necessary to increase the soldering temperature, thereby increasing thermal damage.

  Further, when 0.2 to 3 ppm of Cr is added to the solder balls described above, a remarkable effect appears in suppressing discoloration due to heating. The reason why the addition amount 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.

  An alloy in which Ag and Cu were added to Sn having a purity of 99.9% or more was prepared, and then formed into solder balls having diameters of about 0.4 mm and 0.085 mm. The alloy composition of the evaluated solder balls was 3% Ag, 0.5% Cu, and the balance Sn. 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.

The solder ball sealing conditions were changed for the combination of inert gas, oxygen scavenger, and gas barrier film, and the temperature and humidity cycle test of JIS C 0028 was performed, and then the yellowness and surface oxygen intensity were measured. The yellowness is measured with a Konica Minolta CM-2600d colorimeter, and the solder balls are uniformly arranged on a 15 mm square tray so that they do not overlap each other, and then the white light emitted from the xenon lamp is applied to reflect the reflected light. was determined yellowness ^{b *} in ^{the L} * ^a * ^{b *} color system by spectroscopic sensor. In order to confirm the oxidation state of the surface, the surface oxygen intensity was measured. The measurement is the characteristic X-ray intensity of the oxygen kα ray obtained from the surface of the solder ball by using an EPM-1610 type electron beam microanalyzer manufactured by Shimadzu Corporation and adjusting the electron beam to an acceleration voltage of 5 kV and a sample current of 100 nm.

  Solder balls subjected to a temperature and humidity cycle test using any two of the inert gas, oxygen scavenger, and gas barrier film in the present invention are 1 in 3 of inert gas, oxygen scavenger, and gas barrier film. Compared with seed balls or solder balls that are not used, the yellowness after the test is less than 10 even in a highly oxidative environment such as the temperature and humidity cycle test of JIS C 0028. You can see that In this way, even in a harsh environment, by controlling the moisture and oxygen atmosphere during storage, the metallic luster is maintained after the test, so that the productivity and yield of the semiconductor package can be dramatically improved.

In order to evaluate the degree of yellowing during soldering of the produced solder balls, a discoloration test was performed in which the solder balls were heated at 240 ° C. for 2 minutes or 5 minutes, 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 condition that yellowing due to surface oxidation is more likely to proceed. The degree of metallic luster was evaluated by measuring yellowness and surface oxygen intensity.

  FIG. 1 shows a graph showing the yellowness measured as a function of the surface oxygen intensity in accordance with the yellowness measurement results of the solder balls having a diameter of 0.4 mm shown in Table 1. As shown in FIG. 1, the temperature / humidity cycle test and yellowing by heating are in a linear relationship, but the slope of the straight line is different. When compared with the same yellowness, the surface oxide film formed in the temperature and humidity cycle test takes in a large amount of oxygen. Even with a small amount of oxygen, a strong natural oxide film is formed depending on the storage conditions, and the solderability and metallic luster deteriorate as the thickness increases.

  Next, FIG. 2 and FIG. 3 show the results of observation of the cross section near the surface after the temperature / humidity cycle test with a yellowness of about 13 and heating at 240 ° C. using a transmission electron microscope. The oxide film after the temperature and humidity cycle test is amorphous and has a thickness of about 40 nm. On the other hand, the heated oxide film was almost crystalline and was observed at a thickness of about 25 nm. The striped pattern observed in the oxide film after heating is a lattice pattern, indicating that it is crystallized.

The mechanism by which the solderability with the electrode is improved by suppressing the yellowing of the solder balls below a predetermined value will be described with reference to the schematic diagram shown in FIG. An amorphous SnO oxide film 1a is formed on the surface of the solder ball 1 by being exposed to residual oxygen in the UDS recovery chamber or the atmosphere at the initial stage of manufacture. In the discoloration due to the temperature and humidity cycle, the SnO oxide film 1a grows thicker due to oxygen intervening through the packing material. When heated to 240 ° C., which is a reflow condition, the outermost surface is covered with an oxide and the melting point is raised, so that melting starts from the inside. Solder balls that have become liquid inside are deformed. Since the oxide film formed during the temperature / humidity cycle is amorphous, some of the oxide film undergoes plastic deformation, but is easily elastically deformed and deforms with the inside. Since the coating is strong, the coating is unlikely to be damaged even when the solder ball whose interior is melted contacts the electrode, and the melted solder and the electrode are not bonded to each other. On the other hand, an SnO ₂ oxide film 2a is further formed on the surface of the SnO oxide film 1a during heating. Since this SnO ₂ oxide film is crystalline, it cannot withstand internal deformation and is damaged due to plastic deformation, and the inner molten solder 1b and the electrode terminals 3a and 4a are joined at the portion in contact with the electrode terminals 3a and 4a. The semiconductor package 3 is connected to the circuit board 4 through the bumps 2. Therefore, in order to prevent the solder ball solderability from deteriorating, natural oxidation of the solder ball before reflowing is performed by means of using at least any two of inert gas, oxygen scavenger, and gas barrier film. It is important to suppress film growth.

The relationship between the yellowness b * of a solder ball and surface oxygen intensity is shown. The cross-sectional TEM observation image of the solder ball after a temperature / humidity cycle test is shown. The cross-sectional TEM observation image of the solder ball after 240 degreeC heating is shown. The form figure of the soldering process of a solder ball is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Solder ball 1a Amorphous oxide film 1b Solder 2 Bump 2a Crystalline oxide film 3 Semiconductor package 3a Electrode terminal 4 Circuit board 4a Electrode terminal

Claims (3)

A solder ball comprising: 0 to 4.0% Ag in mass, 0 to 1.0% Cu, the balance Sn and inevitable impurities, and the surface yellowing degree is 10 or less. It consists of 0 to 4.0% Ag, 0 to 1.0% Cu, the balance Sn and inevitable impurities, and has an amorphous SnO layer and a crystalline SnO ₂ layer. Solder layer. It consists of 0 to 4.0% Ag, 0 to 1.0% Cu, the balance Sn and inevitable impurities, and has an amorphous SnO layer and a crystalline SnO ₂ layer. And solder bump.