New package/board materials technology for next-generation convergent microsystems

Current printed wiring boards (PWBs) are all organic, the most common being epoxy-glass laminate FR-4 due to its cost effectiveness and overall performance. However, for high-density wiring (HDW) and assembly of flip-chips directly to the substrate without the use of underfill, substrate materials with low CTE and high elastic modulus are needed. Novel low CTE-high stiffness organic (carbon-epoxy) and inorganic boards (carbon-SiC) have been evaluated for flip-chip on board technology without the use of underfill. Standard liquid-liquid thermal shock tests were carried out on test vehicles with different board materials. In-situ warpage measurements and optical microscopy were used to analyze the observed failure modes. The low CTE low stiffness ceramic and organic boards did not fail from solder joint failure but from cracking in the dielectrics and copper wiring. The high stiffness (>350 GPa) ceramic boards did not fail even after 1000 cycles. The failure modes indicate that a combination of high dielectric stresses and warpage results in crack propagation in conventional epoxies. The novel high stiffness low-CTE ceramic (C-SiC) is also processable in large-area at low cost and is hence a promising board material for future microsystems.