Flip chip assembly method employing differential heating/cooling for large dies with coreless substrates

In this work, differential heating/cooling chip join process was developed for coreless flip chip packaging to minimize warpage change of coreless substrates during the bonding process. A chip was vacuumed to a bonder head and a coreless substrate was vacuumed on a base plate and they were held at different elevated temperatures during the bonding process. The temperature difference provides a substantially matched thermal expansion between the silicon chip and the coreless substrate. This minimizes stress induced by low coefficient of thermal expansion (CTE) mismatch during flip chip assembly. Both thermal and mechanical modeling were performed to provide more detailed information about the temperature distributions and warpage levels for all package components during the chip join process. Mechanical modeling of the chip join process confirmed that by implementing differential heating/cooling chip join process the stresses within the solder bumps can be reduced by more than 20% and the stresses in the low-k layers within the chip can be reduced by more than 25%. Our evaluations used semiconductor chips with a known low-k dielectric and SnAg solder bumps. The size of the test chip was approximately 19 mm × 19 mm with less than 150 μm pitch. The coreless substrate was 55 mm × 55 mm with 8+1 layers. The samples were bonded with an optimized differential heating/cooling chip join process. The experimental results showed that there were no C4 (Controlled Collapsible Chip Connection) bumps bridging, non-wets, nor low-k delamination in the large die with coreless package. Reliability data showed no failures in any of the tested modules.