Core Cracking in Advanced Organic Substrates Due to Free Edge Effect at Cold Temperatures

Advanced field programmable gate arrays (FPGAs) utilizing substrates with a thick organic core showed substrate core cracking after thermal cycling. These substrates are laminate structures with a core made from stacked layers of glass fiber and resin with routing layers made of copper and a dielectric build-up polymer. This type of cracking typically occurs on the glass fiber–resin interface, can penetrate several millimeters and across thru-vias to cause electrical failures, and was found on several test vehicles, four of which are detailed here. This work observed numerous examples to identify the characteristics of the cracking and notes the best observation practices. We hypothesize that this cracking is due to free edge effect at cold temperature. Since the free edge cannot have normal stress, a large out-of-plane stress exists at the edge and concentrates at the corner. Models were built to study the effects of various package parameters on the failure mode. The primary contributions to the free edge effect are the low-coefficient of thermal expansion (CTE) glass fiber, the high-CTE resin, and the high-CTE build-up layers. The trend toward lower CTE substrates has demanded lower CTE glass fiber material, which exacerbates this problem. Increasing complexity of routing demands additional layers, which also increases the out-of-plane stress at the corner. Other less significant factors that increase the stress include lower resin CTE, higher build-up CTE, and decreasing core thickness. Thru-via placement and dicing optimization had small impact and the substrate had negligible impact.