Achievement of low temperature chip stacking by a wafer-applied underfill material

This paper will highlight recent developments of an efficient assembly technology for chip stacking which utilizes a novel wafer applied underfill (WAUF). The a-stage WAUF was spin coated on an 8” wafer with a bump structure of 5 um Cu / 3 um Ni / 5 um Sn2.5Ag Pb-free solder, and then baked at 125°C for 40 minutes to form a 30 um thick b-stage film. After wafer dicing, four chips with WAUF were assembled via a SÜSS FC-150 bonder on a Si interposer. With the aid of the flux agent in the WAUF, the peak bonding temperature was lowered to 240°C, which was beneficial for the reduction of residual thermal stress as compared to that imposed by direct bonding at 280°C. Previous work showed that no delamination of WAUF was found in the chip stacking architecture after the precondition test. In the most recent study, a temperature cycling test (TCT, JESD22-A104-B, Condition B, Soak mode 2) was used to assess the reliability of the 20 um pitch microjoints sealed by the WAUF. After 250 TCT cycles, 87.5% test vehicles failed with a failure mode of the microjoints shown to be interfacial fracture. This was a similar result to the failure mode of microjoints sealed by capillary underfill, though the WAUF failed much earlier. The root-cause of the failure can be attributed to the inferior bonding accuracy, as the majority of the Sn2.5Ag solder alloy at the contact area completely reacted with Ni to form Ni₃Sn₄ at this stage but did not link up with each other (a proposed failure mechanism will be given in another ITRI´s paper). However, the lifespan of the microjoints could be prolonged if the remaining Sn2.5Ag Pb-free solder alloy is sufficient to sustain the bump structure after bonding, even though the WAUF was not post-cured. The authors believe that the newly developed underfill material and the corresponding assembly technology will reveal a proper technique for stacking homogeneous ICs (such as DRAM) efficiently- - and reliably.