Achieving warpage-free packaging: A capped-die flip chip package design

Coefficient of thermal expansion (CTE) mismatch between chip and substrate is the root cause for reliability issues in flip chip packages, such as excessive warpage, low-k dielectric layer cracking, solder mask cracking, and bump cracking. The first and foremost thing in designing a flip chip package is to control excessive warpage to meet the warpage specification. In this paper, a capped-die flip chip package is proposed to control the warpage as well as to reduce the stress. In the capped-die flip chip package, a metal cap tightly covers and bonds with the die through an adhesive material. As a result, the capped-die has a higher effective CTE. By adjusting the thickness of metal cap, the effective CTE of the capped-die can matches with the CTE of substrate, theoretically achieving zero-warpage or warpage-free. To verify the capped-die concept for zero-warpage control, a 45mm×45mm size of capped-die flip chip package is designed and manufactured based on the guidance from finite element modeling, where a copper die-cap with 0.4mm thickness is selected. Then, Shadow Moiré test is performed to measure the warpage as function of temperature from 25°C to 260°C. Experimental data show that in the temperature range, the warpage curve of the capped-die package is almost flat and close to zero, verifying the capped-die concept.