Question: Is it possible to successfully solder a component finished with tin silver copper (SnAgCu) with tin lead (SnPb) solder?
Answer: Yes, SnPb and SnAgCu should form an acceptable solder joint. This study explains why.
The EMPF Helpline has investigated the lead free finishes which component manufacturers are using to be compliant with European and Asian lead free legislation. The predominant finish of choice is tin (Sn), since the tin plating process is identical to that of tin lead. However, tin finished components are susceptible to tin whiskers, which present a major hardware reliability concern. As an alternative, component manufacturers are considering using other alloys such as tin silver copper. Tin silver copper finishes are an attractive choice for SMT components because this alloy was selected by several industry consortiums as the preferred solder alloy for reflow soldering applications.
One of the Helpline callers said that they would be using tin silver copper finished chip components with eutectic tin lead solders and questioned whether this presented a reliability risk. 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. However, during reflow, the peak temperature is usually held between 30 and 90 seconds due to the chip components’ relatively low thermal mass; therefore, the chip components are above the melting temperature for both solder alloys, and the tin lead and tin silver copper should form an acceptable solder joint.
The other 3 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 this scenario during a recent component qualification test sequence. By leveraging this industry partnership, the EMPF resources available to its Helpline callers were 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 tin silver copper 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 3-1). 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, which 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.
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. 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 finished components could be soldered with tin lead. The components in question were chip components and BGA components.
The results of the investigation concluded that 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 balls with tin lead solder paste, using a typical tin lead solder profile will result in an unacceptable solder joint. This combination should be avoided until further failure analysis is completed.
