George A. Riley
Substrate bond pads or board bond pads that are plated with thick gold intended for wire bonding may create reliability problems if used with tin-based solder bumps for flip chip connections.
Wire-bond pads are often plated with several microns (micrometers) of soft gold, for easier bonding with gold wires. While this does make wire bonding easier, it also may make the pads unsuitable for tin-lead solder bumping. The thick gold on the pads is dissolved by the molten tin of the solder during solder reflow. The gold-tin mixture forms gold-tin intermetallic compounds in the solder bumps, which may lessen the reliability of the solder connection.
These intermetallic compounds are solid solutions of gold and tin, with a different crystalline structure from the solder. As the proportion of gold-tin intermetallics in the solder bump increases, they significantly alter the properties of the solder joint. Changed characteristics include the fluid characteristics of the solder, the solidus temperature, the joint microstructure, and the mechanical properties.1
The changes in mechanical properties resulting from gold-tin intermetallics are a particular concern with solder bump assembly. The intermetallics are more brittle than the original solder, increasing the likelihood of joint cracking under mechanical stress. Stress from differential thermal expansion between chip and substrate can more easily cause mechanical fatigue fracture of those solder bumps. Figure 1 shows a fractured solder bump after temperature cycling.
FIGURE 1. A fractured solder bump after temperature cycling. (Courtesy Ken Gilleo)
Tin-lead solder bump brittleness increases rapidly with increasing gold content. Figure 2 plots the change in the measured elongation of eutectic tin-lead solder as the gold content increases. Elongation decreases from 54 at zero percent gold to 1.5 at 10 percent gold, by weight.2
Table 1. Change in eutectic tin-lead solder elongation with increasing percent gold. (Courtesy of McGraw-Hill)
Since increased solder joint brittleness correlates with decreased solder elongation, to avoid excessive brittleness the reflowed tin-lead bump should contain less than 3 percent gold by weight.3 The gold content of the reflowed bump can be controlled by limiting the thickness of the gold layer on the pad to the minimum required for oxidation protection. The weight of gold on the pad can be calculated from the pad dimensions and the gold thickness. Boards already coated with thick gold can be pre-treated to reduce the thickness or even to remove the gold coating before soldering.
(1) Hwang, Jennie S., “Modern Solder Technology for Competitive Electronics Manufacturing,” McGraw-Hill, New York, 1996, page 412.
(2) Hwang, op. cit., Figure 13-14.
(3) Lee, Ning-Cheng, “Reflow Soldering Processes and Troubleshooting,” McGraw-Hill, New York, 2002, page 6/115.
FOR FURTHER INFORMATION
Reference 1, Section 13.1 and 13.2, gives a detailed treatment of intermetallics in solder joints, including several charts on the effects of gold-tin intermetallics.
Reference 3, Chapter 8, describes a wide range of solder bumping techniques.