Testing and analysis of solder joint reliability for BGA assembly under flexural loading

The solder joint reliability for BGA (Ball Grid Array) assembly is becoming a more concerned issue as these packages are featuring higher density interconnections, multiple functionality and higher speed combined with smaller size. The traditional test methods for second level PCBA (Printed Circuit Board Assembly) mechanical reliability monitor the electric resistance changes of Daisy chains in the test samples under 4-point bending. The method has been documented by Interconnecting and Packaging Electronic Circuits and Joint Electronic Devices Engineering Council in PC/JEDEC 9702 standard. The effectiveness of the test has been questioned when applied to the new lead-free soldered packages. Due to the failure mode shift from solder joinicopper pad interface cracking in Tin-lead PCBAs to pad-cratering cracking in leadfree packages, the electrical continuity monitoring becomes ineffective in detecting the interconneCt failure. On the other hand, the strain gauges recorded PCB strains during bend tests show little increase that would be indicative of an onset failure. This project applies Fiber Bragg Grating (FBG) strain sensors to detect the pad-cratering failure. FBO have been employed widely in different areas of engineering due to its advantages of small size, light weight and high sensitivity. In this project the FBG sensors are laid to the vicinity of the BGA substrate comers. By detecting and recording the solder joint fracture induced strain release, the onset of pad-cratering is explicitly revealed. The study has demonstrated that the FBG sensors are much more sensitive than electric resistance strain gauges in detecting the substrate strain release in BGA assembly 4-point bend testing due primarily to the sensor's much smaller geometric size. By placing the sensors very close to the comer solder joints, the new test obtains accurate strain information related to the first solder joint cracking. Furthermore, the recorded strain release enables the detecting, understanding and analysis of the critical load of the solder joint fracture, the brittle and ductile fractures and related strain relaxation phenomenon during the PBGA flexural loading, etc.