Vibration Testing of Repaired Lead-Tin/Lead-Free Solder Joints

Abstract

Due to growing environmental concerns and recent legislation, tin-lead (Pb-Sn) solders are being phased out by lead-free (LF) solders. The most common Sn-Pb replacement is the tin-silver-copper (Sn-Ag-Cu, SAC) alloys. During the transition phase, it is expected that there will be a period where both Sn-Pb and LF solders will be used side by side, and in conjunction with one another, during assembly processes. Repaired solder joints may also be expected to contain a mixture of Sn-Pb and LF solders, especially in military systems. Very little has been reported on the vibration testing of solder joints in printed circuit boards utilizing LF solders and even less on the vibration testing of solder joints with mixed alloy systems. Aerospace systems typically experience vibration frequencies ranging from the tens of hertz to the thousands of hertz. Given the long life cycle of aerospace vehicles, there exists a need for circuit board repairs and component replacement, which adds to the complexity of the LF transition. Solder joints on these printed circuit boards have a high likelihood of containing a combination of Sn-Pb and LF solders. The vibration fatigue properties of hand-repaired solder joints containing LF solder has not been directly compared with that of hand-repaired Sn-Pb solder joints. Also a correlation between vibration endurance and the solder metallurgy in a repaired joint has not been reported. In this study vibration testing was used to determine how as assembled and repaired Sn-Pb/SAC solder joints withstand dynamic vibration. A frequency sweep from 20Hz to 2000Hz and back down to 20Hz at a constant acceleration of 15g's determined the resonant frequencies of an assembled printed circuit board and its individual components. A 30 minute resonance dwell test at 25 g's determined the vibration resistance of solder joints. The interconnect resistance of the solder joints was measured before and after vibration testing. Tested printed circuit boards were visually inspected for solder cracking and delamination. Metallographic analysis was done on areas where visible cracking or physical damage had occurred. All vibration testing was done at room temperature.