Experimental and Theoretical Assessment of Thin Glass Substrate for Low Warpage

Abstract

This paper compares the warpage after lead-free solder assembly of ultrathin glass and organic substrates for microelectronic packages. Smart mobile devices demand packages to be thinner, which exacerbates warpage, which in turn affects interconnect pitch scaling and reliability. Although low-coefficient-of-thermal-expansion (CTE) organic laminates have been developed to reduce the warpage during assembly, glass substrates offer added benefits of much higher modulus and thermo-mechanical stability, with the potential to reduce warpage beyond low-CTE organic materials. The focus of this paper is on modeling and measurements to quantify the warpage of glass and organic laminates at 100- $\mu \text{m}$ core thickness after thermo-compression bonding at 260 °C peak temperature. In the experiments, four-metal-layer glass substrates were fabricated at the panel level, diced into $18.4 \times 18.4$ -mm2 coupons, 10-mm $\times $ 10- $\text {mm}\,\,\times 630-\mu \text{m}$ silicon dies were thermo-compression bonded, and the warpage of the assembled packages was measured using shadow moire interferometry. In parallel to the experiments, numerical models that account for the viscoplastic behavior of the solder as well as the sequential build-up materials and processes have been developed. The predicted warpage from the models was compared to the experimental measurements. It was seen that the low-CTE glass had lower warpage than the organic core of equivalent CTE, while increasing the CTE of the glass increased the warpage. Also, the underfill fillet size was found to influence the warpage for thin substrate packages, and an optimal fillet size has been identified for minimizing warpage. B-staged and capillary underfill materials were compared and found to have similar warpage values.