Investigation of various board-level underfills and adhesives for CTBGA bend performance enhancements in lead-free portable electronic products

This paper discusses the influences of various underfills and adhesives on board-level bend performance of ChipArray
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Thin Core Ball Grid Array (CTBGA) assemblies through an the monotonic three-point bending test. It is found that the bonding materials do enhance the bend performance for all the test categories as compared with the control assemblies which are without bonding materials, but the degrees of improvements between different strategies vary quiet a lot. The specific bend strength is affected by a number of factors, in which dispending patterns and properties of the bonding materials used influence it obviously. In general, the partial capillary flow underfill and edge bond adhesive with relatively lower storage modulus and higher apparent strengths of adhesion (ASA) are preferred. In order to further understand the failure mechanism of the CTBGA during the bending test, failure analysis on tested devices are also conducted using dye-and-pry and cross-section, the results indicate that the dominant failure mode is PCB pad cratering, regardless with or without bonding materials.