Highly Reliable Die-Attach Silver Joint with Pressure-Less Sintering Process

Pressure-less silver sintering pastes with high-reliability judged from temperature cycling test (TCT) under -40 degrees C to 175 degrees C were developed. For 3mm x 3mm sized dies, the silver joints formed between the Ag surface finished Si die and the Ag surface finished Si3N4 active metal brazing (AMB) substrate retained the initial shear strength after 1500 cycles, the strength being similar to 25 Mpa even after 3879 cycles. For 5mm x 5mm dies, the average shear strength was very stable and remained at similar to 42 Mpa, even after 3879 cycles. For 10mm x 10mm dies, the dies cannot be sheared off (max shear force 200kg) for all the samples before and after 2389 cycles of TCT. Within a standard bondline thickness (BLT) range of 40 to 60 mu m, the average total voids percentage of the sintered joints for the above different die sizes can be controlled within < 2%. Under ambient atmosphere, the shear strength of sintered joints formed on a bare Cu surface finished AMB substrate reach 62Mpa, slightly stronger than that of Ag- and Au-surface finished ones. TCT shows that these surface finishes have similar decay behaviors, implying that bare Cu surface finished AMB substrate can be used for bonding applications. BLT is critical in determining the bond property and reliability. For a very thin BLT, such as < 10 mu m, the bond strength decays at about five times the rate that of a joint with similar to 50 mu m BLT. For a thick BLT, such as > 100 mu m with 10mm x 10mm dies, a large amount of voids are generated if using a similar sintering profile, where almost no voids are observed for a similar to 50 mu m BLT joint.