| The SAC-SnPb Dilemma |
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| Written by American Competitiveness Institute | |||
| Saturday, 31 May 2008 19:00 | |||
When can SAC-finished parts be soldered with SnPb solder?
Over the past several years, electronics manufacturers have made many inquiries to the EMPF Helpline about whether it is possible to solder a component finished with SnAgCu (SAC 305) with SnPb solder, and whether there are any reliability concerns. Eutectic SnPb solder has a melting temperature of 183°C. A typical reflow soldering thermal profile for SnPb has a peak temperature of approximately 220°C, which is only slightly higher than the SAC alloy melting point of 217°C. A typical profile for SAC solder would have a peak of about 240°C. During reflow, the peak temperature usually is held between 30 and 90 sec. For a passive chip component with relatively low thermal mass, the time at peak temperature would be sufficient to melt both solder alloys. The SnPb and SAC 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 SAC-finished BGAs with eutectic SnPb solders and again questioned the reliability risks. While reviewing these calls, EMPF staff consulted its Industrial Advisory Board, which encountered precisely this scenario during participation in the most quoted and thorough military-environment qualification study of Pb-free electronics solder joint reliability yet conducted: the JGPP study by NASA, the Joint Council on Aging Aircraft, and other defense industry leaders. Again, a typical reflow soldering thermal profile for SnPb 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 aforementioned passives, the BGA package’s high thermal mass inhibits heat transfer. There is also a high SAC solder volume in the solder balls that inhibits solder joint homogenization. This prevents the BGA’s SAC solder balls from melting and collapsing. At best, parts of the ball may enter the “pasty” range, where they begin to melt but lack sufficient heat to collapse the entire solder ball. A microsection of the solder joint was performed to determine what was happening inside the solder ball (Figure 1). This analysis revealed the SnPb solder paste had diffused only from the ball/board pad interface into the bottom third of the BGA’s SAC ball, rather than all the way through the ball. Also observed were distinct areas within the SAC ball where the solder microstructure and intermetallic phases changed. This indicated the top of the SAC 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 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° to 125°C. After approximately 250 cycles, the solder joint failed. This might have been sufficient for certain 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. The EMPF Helpline staff investigated whether SAC 305-finished chips and BGAs could be soldered with SnPb solder. If manufacturing hardware with passives finished with SAC metallization and soldered with SnPb paste, ensure the thermal profile reaches at least 220°C to form the solder joint properly. However, if building assemblies with area array packages containing SAC 305 solder balls and standard eutectic SnPb paste, a typical SnPb solder temperature profile will result in an unacceptable solder joint for military reliability. Avoid this combination. The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.
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| Last Updated on Friday, 30 May 2008 11:48 |
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