A Study On The Fatigue Life Estimation Of Electronic Chip Solder Joints In The Harsh Environments

As a major packaging method, electronic chips are often packaged on a printedcircuit board (PCB) through QFP and BGA solder joints. During the service of aPCB, the solder joints are subject to cyclic stresses due to temperature andalternating stresses due to random vibration, which may result in fatigue crackpropagation in the solder joints themselves or the inter metallic compound (IMC)layer, further may induce fatigue fracture failure. Failures of solder joints have beenone of the major problems that influence the reliability of electronic devices,especially for the electronic devices working in extreme temperature and vibrationenvironments, e.g., the aerospace environment. To evaluate the residual life anddetermine the maintenance cycle of a PCB, it is important to study the methods forpredicting the service life of a solder joint and the measures to increase its servicelife.This thesis introduces the structures of electronic chips and PCBs packaged usingQFP and BGA solder joints and reviews the state-of-the-art of them. A simplifiedcomputation model for predicting the service life of a solder joint is proposed,which accommodates both cyclic stresses due to temperature and alternatingstresses due to random vibration. It is verified that the model can easily be used forservice life prediction of a solder joint after the parameters being calibrated basedon experimental results.For the purpose of calculating the failures of QFP and BGA solder joints on aPCB, the equations for calculating stresses in plats and beams are reviewed; thesizes, materials of typical PCBs and chips are summarized; the characteristicparameters of harsh working conditions are collected. Based on the classic theoriesof beams, plates and mechanics of fracture, the PCBs and the chips, the QFP weldlegs, and the BGA solder joints are simplified as plates, beams and Winkler elasticfoundations, respectively, a computation model for calculating the failures of solderjoints subject to cyclic stresses due to temperature and alternating stresses due torandom vibration is established based on the available equations. This modelincorporates the influences of warping of the chips and the assembly constraints ofthe PCB. This model can be easily programmed and used for calculating the fatiguelives of solder joints.By taking the reliability problem of a avionic system working in extremetemperature and vibration environment as an example, the reason of the failures ofsolder joints were identified by calculating the stresses and the service life and experimental calibration. An improved design of the weld legs and an improvedungerfill process for BGA packaging are proposed and verified that can satisfy theworking conditions by experimentation.