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 reliability 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 underfill material, etc. However, most of the works have been conducted are EGA on rigid substrates. In this work, thermal cyclic test is performed to reexamine these factors using package housed on a flex substrate. Bending test with two deflections is also performed to investigate solder joint fatigue 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 cycling test, chip size, polyimide thickness and underfill material utilization were found to have significant impacts on joint fatigue life, especially the effect of applying underfill material to the joint. Epoxy thickness was found to have little effect on the joint fatigue life for this case. The effects of test board surface finish and substrate gold plating thickness on the joint fatigue life were found coupled. The term "substrate" here refers to the chip carrier, while the "board" here refers to motherboard, which is the board to assemble test vehicles on. The gold thickness here all refers to the electrolytic gold plating on the substrate. Using organic solderability preservative boards, substrate gold plating thickness affects joint fatigue life slightly, but with Au-Ni test boards, the effect is tremendous. The difference is due to different intermetallic compounds (IMC) formed. In other words, different IMC systems are formed due to different combination of test board surface finish and substrate gold plating thickness. As a result, different IMC induces different failure modes. The joint fatigue life under cyclic bend test with different deflections is also probed and shown. The corresponding failure modes are also discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.