Board level reliability of PBGA using flex substrate

Assurance of board level reliability is necessary and required for adopting any new packages into products. This paper presents board level reliabilit y test results of a flex substrate EGA under thermal and bend cyclic tests. It is well known that solder joint reliability is affected by many factors , such as the size of chip, joint stand-off height, pad design, test board surface finish, substrate gold plating thickness and the utilization of und erfill material, etc. However, most of the works have been conducted are EG A on rigid substrates. In this work, thermal cyclic test is performed to re examine these factors using package housed on a flex substrate. Bending tes t with two deflections is also performed to investigate solder joint fatigu e life and failure modes under mechanically repetitive loading. Two-parameter Weibull model is used to analyze joint fatigue life. Failure analysis is conducted and discussed for each case. Under temperature cyclin g test, chip size, polyimide thickness and underfill material utilization w ere found to have significant impacts on joint fatigue life, especially the effect of applying underfill material to the joint. Epoxy thickness was fo und to have little effect on the joint fatigue life for this case. The effects of test board surface finish and substrate gold plating thickne ss on the joint fatigue life were found coupled. The term "substrate" here refers to the chip carrier, while the "board" here refers to motherboard, w hich is the board to assemble test vehicles on. The gold thickness here all refers to the electrolytic gold plating on the substrate. Using organic so lderability preservative boards, substrate gold plating thickness affects j oint fatigue life slightly, but with Au-Ni test boards, the effect is treme ndous. The difference is due to different intermetallic compounds (IMC) for med. In other words, different IMC systems are formed due to different comb ination of test board surface finish and substrate gold plating thickness. As a result, different IMC induces different failure modes. The joint fatig ue life under cyclic bend test with different deflections is also probed an d shown. The corresponding failure modes are also discussed. (C) 2001 Elsev ier Science Ltd. All rights reserved.