Mechanical Considerations of Shin-Etsu Elastomer As a Z-Axis Interconnect

This research effort addresses the integration of a dense z-axis interconnect technology, Shin-Etsu, into a 3D thermo-mechanical multi-chip module with two to eight stack layers. Shin-Etsu is a matrix of vertical beryllium-copper wires embedded in silicone. Since it is a composite material, accurately determining Shin-Etsu’s mechanical properties is extremely important for estimating long-term reliability of the electronic package.

Five percent compression of the Shin-Etsu elastomer was specified to maintain proper electrical contact between substrates. To determine the corresponding force, Tinius-Olsen compression tests up to 15% were conducted on both single and double layer samples of Shin-Etsu. In addition, these data were used to infer Shin-Etsu’s Young’s modulus. Both series of test were also conducted with and without alumina (A1₂O₃) to simulate the contact surface boundary condition between Shin-Etsu and the LTCC substrate (2 pieces were used for the single Shin-Etsu case, and 3 pieces were used for the 2 layer Shin-Etsu case). All compression tests follow the same trends, though the sample to sample scatter is relatively large (+/−75%). Because Shin-Etsu’s Poisson’s ratio is near 0.5 (volume is conserved), the frictional forces between the contact surfaces is important. By carefully accounting for these frictional forces, we infer a Young’s modulus for Shin-Etsu of 3.25 MPa. Using this value with the appropriate contact model (we found that a simpler model with infinite friction is suitable in our case), we are able to successfully design a two-board test vehicle which incorporates Shin-Etsu.