SOLID MATERIAL SOLUTIONS LLC — Department of Energy SBIR Phase II: 26a

Description

This proposal addresses the widespread problem of dead zones and reliability in large format chips caused by the high soldering temperature needed to make highdensity interconnects in thin semiconductor chips using solder bump technology. Highly sensitive pixels and highdensity interconnects between readout and sensor chips are necessary for the continued advancement of high energy physics and upgrades to facilities such as those at the Department of Energy and others. Large chips have challenges forming and or keeping the flip chip interconnects joined at the device edge because of coefficient of thermal expansion mismatch causing dynamic warpage. The high heat need for soldering is being addressed by using solder microcapsules in a supercooled liquid state far below its normal melting point. This allows the application of liquid metal solder at temperatures dramatically lower than normal solder reflow temperatures. The objective of the multiphase project is to replace bumps on both the readout and sensor chips with a supercooled liquid metal microcapsule formulation precisely placed via a costeffective process and to enable solder reflow of a higher melting alloy, composed of SnAgCu, at lower temperatures than traditionally needed ≤130 vs. peak reflow 250 °C with high interconnect density and low edge defects. In Phase I the company accomplished proof of concept for using supercooled liquid metal microcapsules of SnAgCu by demonstrating reflow at 165 °C that has never before been seen using the high melting SnAgCu alloy. The company produced two methods to make deeply supercooled microcapsules each with a flowable liquid window of the metal down to 90 °C. Printing techniques were developed for stencil printing high density arrangements of microcapsules with an encouraging level of consistency to match the 50 µm pitch and 25 µm feature size needed. SAFITech will produce finesized supercooled liquid metal microcapsules of SAC305 that can be precisely and accurately deposited and then activated in a solder reflow process at reduced temperatures compared to normal solder reflow of SnAgCu. The supercooled liquid microcapsules will be created and tested as a replacement for bump bonding and used to create an interconnect that has a similar set of electrical and mechanical reliability specifications. The resulting interconnect will perform comparably to SnAgCu in solder joint quality and better in interconnect yield at the die edge and cryogenic reliability because of the lower reflow temperature. The test devices of readout and sensor chips to meet the topic specifications will be 20mm × 20mm, with a readout chip thickness of 50 µm, and an interconnect pitch of 50 µm.