Effect of circuit board flexure on flip chips before underfill

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Abstract
Manufacturing operations in high volume production induce flexure in circuit boards. Flip chip assemblies prior to underfilling are particularly vulnerable to stresses induced in the solder bumps due to the absence of a protective underfill layer. While excessive board flexure during manufacturing can be controlled, it cannot be completely eliminated. The need therefore arises to evaluate the vulnerability of any given flip chip assembly to board flexure. In addition, it may be necessary to optimize flip chip assembly design early in the design cycle to ensure robustness during manufacturing. To achieve these objectives, flexure experiments have been conducted on various types of flip chips mounted on test boards in order to quantify the board flexure limit for each assembly. Utilizing these test results, a computer model has been developed for the purpose of simulating the effect of circuit board flexure on flip chip assemblies before the underfill process. Comparison of simulation results with experimental test data from flip chip flexure testing show excellent correlation. Simulations have been conducted for numerous flip chips intended for use in high volume production at Delphi Automotive Systems. Design guidelines based on these results have been summarized.

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