Thermal cyclic fatigue of the interconnect of a flex-type BGA

Abstract

Assurance of board level reliability is necessary and required for adopting any new packages into products. Therefore, there is a great interest to study solder joint reliability of a ball grid array package using flex-type substrate, which is developed and named "Film BGA" in ASE. This paper presents board level reliability test results of this flex-type package under thermal cyclic test. The test vehicle is a package that uses a polyimide substrate with a die-up, wire-bonded and over-molded configuration. It has a dimension of 1 2~1 2~1 . 1 " and has 132 solder balls, which are in a 3-row peripheral format with a size of 0.45" and a 0.8" ball pitch. It is well known that the solder joint reliability is affected by many factors, such as the size of chip, stand-off of the joint, pad design, test board surface finish, substrate gold plating thickness and underfilling, etc. However, most of the work has been conducted on BGA packages housed on rigid substrates. In this work, reexamination of these factors will be made using package housed on a flex substrate. The factors investigated include the size of chip, test board surface finish, the Au plating thickness on the substrate pad, epoxy thickness, polyimide thickness and underfilling.Weibull plots, probability and cumulative failure functions, are shown for each case. Failure analysis is also conducted and discussed. Chip size, polyimide thickness and underfilling were found to have significant impacts on joint fatigue life, especially the effect of applying underfill material is tremendous. Epoxy thickness was found to have little effect on the joint fatigue life. The effects of test board surface finish and substrate gold plating thickness on the joint fatigue life were found to be coupled. For OSP-processed test boards, substrate gold plating thickness affects joint fatigue life slightly, but for test boards with Au-Ni plating finish, the effect of substrate gold plating thickness on the joint fatigue life is tremendous. The reason for that is due to different IMC system. The IMC system formed was found to depend on the test board surface finish and substrate gold plating thickness. As a result, different IMC systems are formed due to different combination of test board surface finish and substrate gold plating thickness. Furthermore, different IMC also induces different failure modes.

Thermo-mechanical reliability tradeoffs for deployment of area array packages in harsh environments

“…Packages with compliant elastomeric substrates (e.g., BGA) may also exhibit this effect. Hung, et al [2000] investigated the effect of surface finish on the solder joint reliability of 12 mm BGA with polyimide substrate, die-up, wire bonded and over-molded configuration. Thermal cycle of 40 C to 125 C with 15 min ramps and 15 min dwells was used for the test.…”