A customer called the EMPF Helpline with a question regarding the successful soldering of components finished with tin silver copper with tin lead solder...

There have been many inquiries over the last several years to the EMPF Helpline by electronics manufacturers, asking whether it is possible to successfully solder a component finished with tin silver copper (SnAgCu - SAC305) with tin lead (SnPb) solder and whether there are any reliability concerns.  Eutectic tin lead solder has a melting temperature of 183°C. A typical reflow soldering thermal profile for tin lead solder has a peak temperature of approximately 220°C, which is only slightly higher than the tin silver copper alloy melting point of 217°C. A typical profile for tin silver copper solder would have a peak of about 240°C.

During reflow, the peak temperature is usually held between 30 and 90 seconds.  For a chip component with relatively low thermal mass, the time at peak temperature would be sufficient to melt both solder alloys. The tin lead and tin silver copper should form an acceptable solder joint. No drastic reliability concerns are expected of the chip component joints.

However, other Helpline callers said they would be using tin silver copper finished Ball Grid Array (BGA) packages with eutectic tin lead solders and again questioned the reliability risks. While reviewing these calls, EMPF staff consulted one of its Industrial Advisory Board members, who encountered precisely this scenario during participation in the most quoted and thorough military-environment qualification study of lead-free electronics solder joint reliability yet conducted.  The study is called the JGPP study done by NASA, Joint Council on Aging Aircraft, and defense industry leaders. By leveraging these industry partnerships, the EMPF resources available to its Helpline callers are greatly enhanced.

Again, a typical reflow soldering thermal profile for tin lead solder has a peak temperature of approximately 220°C – just above the 217°C melting point of SAC 305 and less than the recommended peak temperature of 240°C for this alloy. Unlike the chip components previously mentioned, the BGA package has a high thermal mass which inhibits heat transfer. There is also a high tin silver copper solder volume in the solder balls, which inhibits solder joint homogenization. This prevents the BGA’s tin silver copper solder balls from melting and collapsing. At best, parts of the ball may enter the “pasty” range, where they are beginning to melt, but there is not sufficient heat to melt and collapse the entire solder ball.

A microsection of the solder joint was performed to determine what was happening inside the solder ball (Figure 2-1). This analysis revealed that the tin lead solder paste had only diffused from the ball/board pad interface into the bottom third of the BGA’s tin silver copper ball, rather than all the way through the ball. Also observed, were distinct areas within the tin silver copper ball where the solder microstructure and intermetallic phases changed. This indicated that the top of the tin silver copper solder ball was cooler than the bottom. The high thermal mass of the BGA package acted like a heatsink, causing these temperature differences within the solder ball. It should be noted that this solder joint would pass external visual inspection to commercial standards.

To confirm the hypothesis that this situation creates a non-reliable solder joint, thermal cycling was performed from -55°C to 125°C. After approximately 250 cycles, the solder joint failed. This might have been good enough for some benign commercial applications, but for the harsh environment of military applications, it is inadequate. In examining the failed joint, a large crack was found at the ball/board interface.

Summary and conclusions
The EMPF Helpline staff investigated whether tin silver copper (SAC 305) finished chip components and BGA components could be soldered with tin lead solder.  If you are manufacturing hardware with chip components finished with tin silver copper metallization and using tin lead solder paste, you should ensure that the thermal profile reaches at least 220°C to properly form the solder joint. However, if you are building assemblies with area array packages containing tin silver copper (SAC 305) solder balls and standard eutectic tin lead solder paste, a typical tin lead solder temperature profile will result in an unacceptable solder joint for military reliability. This combination should be avoided.