Reliability assessment of the 20 um pitch micro-joints within a 3DIC assembly under various environments

Abstract

For the demands of multifunction, high density interconnection, high performance and integration of homogeneous or heterogeneous ICs, three dimensional IC (3DIC) packaging technologies by through silicon via (TSV) and microbump were widely studied recently. Intending to learn the reliability performance of Pb-free microjoints, 4 chips were interconnected with one Si interposer by Sn2.5Ag microbumps, sealed by a capillary underfill and then did the reliability assessment under different environments. The 4 chips have the same size of 4.6 mm by 4.6 mm by 100 um, and were assembled on one Si interposer with a dimension of 20 mm by 20 mm by 300 mm by a chip on wafer (CoW) bonder. There were more then 3000 microbumps on each chip and totally over 12,000 microbumps were on the Si interposer. The bump pitch and passivation opening of the test vehicle were 20 um and 6 um, respectively, an under bump metallization (UBM) layer of 5.0 um Cu / 3.0 um Ni was plating on Al trace and then Sn2.5Ag Pb-free solder bump with a thickness around 5.0 um was then deposited on the UBM layer. During bonding, the microjoints were formed at a peak temperature of 280°C, and the microgaps were then filled by a capillary underfill and cured at 150°C for 30 min. Subsequently, the assemblies were respectively inspected by an X-ray and a scanning acoustic microscope (SAM) to determine the quality of microjoints including bonding accuracy, formation of interconnections and the percentage of gas voids within the underfill. Afterwards, the test vehicles were baked at 125°C for 24 h and then stored under the test condition of 30°C / 60% RH for 192 h and finally reflowed at 260°C for 3 times to screen the samples for reliability tests, the SAM was again used to check whether the delamination defect was formed within the microgap. The reliability tests including temperature cycling test (TCT), thermal shock test (TST), high temperature storage test (HTS), pressure cooker test (PCT) and thermal humidity storage test (THST) were done according to the JEDEC standards. The results showed that the thermomechanical stress induced by TCT and TST damaged the assemblies, and the failure mode was also discussed in this investigation.