Reliable microjoints for chip stacking formed by solid-liquid interdiffusion (SLID) bonding

In this research, thousands of 20 μm pitch microbumps with a diameter of 10 μm and a structure of pure Sn cap on Cu pillar were electroplated on 8 inch wafers, and those wafers were then respectively singularized to be top chip (5 mm × 5 mm) and bottom Si interposer (10 mm × 10 mm) for stacking. Two methods including conventional reflow and solid-liquid interdiffusion (SLID) bonding were chosen to interconnect the microbumps on the chip and on the interposer. In the former case, the as-plated Sn caps were fluxed with Senju Metal's WF-6400 paste, and the chip was then placed on a Si interposer using a SÜSS FC-150 bonder at room temperature. Afterwards, the Sn caps on the chip and on the Si interposer were melted and interconnected at a peak temperature of 250°C in an ERSA's reflow oven (Hotflow 7). The flux residues were cleaned after reflow, and the microgap between the chip and the Si interposer were fully sealed by a Namics' capillary underfill with an average filler size of 0.3 um. Regarding the SLID bonding, the oxides on the as-plated Sn caps were removed by a plasma etcher first, and then the chip was placed on the interposer by the SÜSS FC-150 bonder as well, subsequently, the Sn caps were heated up to 260°C to react with Cu to form Cu6Sn5 completely. In the final, the intermetallic microjoints were also protected by the same capillary underfill. After assembling, the JEDEC preconditioning test was used to screen the test vehicles for reliability assessment, and then a temperature cycling test was performed to predict the lifespan of the microjoints. The test results showed that the microjoints formed by SLID bonding provided a superior reliability performance to those assembled by reflow. According to the images of focus ion beam (FIB), the intermetallic phases of Cu6Sn5 and Cu3Sn coexisted at the interface between the Sn cap and the Cu pillar after reflow once, and some Kirkendall voids were found at the Cu3Sn / Cu pillar interface concurrently. When the microjoints undergone 3 times more reflow in the preconditioning test, the Kirkendall voids accumulated and was going to speed up the failure of microjoints as experienced just hundreds of temperature cycles. On the other hand, the microjoints produced by SLID bonding have not failed when thousands of temperature cycles passed. Based on those evidences, it is claimed here that SLID is an efficient bonding method to form reliable intermetallic microjoints for chip stacking.