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|Conventional Reflow Oven Sinter-able Pressure-Less Silver Paste for “Drop in” Die Attach Assembly|
|Keywords: silver sintering , reflow oven processing, pressure less|
|Silver sintering materials have attracted enormous attention due to their promising use as high temperature power electronics packaging materials in applications such as hybrid and electric cars, high speed train, aircraft/aviation, deep well oil/gas extraction, high power LED and related fields. Development in these areas demands more robust semiconductor power devices that can endure harsh operation conditions such as ≥ 200oC. Recent development in wide band gap semiconductors such as silicon carbide and gallium nitride devices makes it possible to operate theoretically at temperature above 300oC.  Silicon carbide device is enabling technology that not only can operate at high temperature, but also offers fast switching speed, high blocking voltage and high thermal conductivity compared to that of silicon technology. Different materials have been explored for the high temperature die attach applications, including lead-free solder, silver and copper sintering, and transient liquid phase (TLP) materials. Lead-free solders which can operate under ≥ 200oC condition do exist, such as AuGe,  AuSi,  AuIn  and ZnAl.  However, these materials have drawbacks such as high price (Au-based alloys) or poor processability (Zn-Al). In contrast, Ag and Cu sintering materials can withstand a much higher operation temperature due to a much higher melting point. Also, these materials can be processed at a lower temperature comparing to that of solder materials, which will greatly help reduce the thermal stress during processing. They are next generation lead-free die attach materials due to their capability to work under high temperature, their high thermal and electrical conductivities, and good thermomechanical properties. Two types of materials are differentiated based on pressure is applied at joints during sintering or not, i.e., pressure and pressure-less materials. Silver sintered joints obtained under pressure conditions have long been reported to have much better reliability than that of solder materials.  Recently, we have observed the good TCT reliability for the pressure-less silver sintering materials.  For the pressure sintering process, besides that it has the issues of large financial investment for the equipment and the potential die crack due to pressure, the sample sintering has to be batch-by-batch, resulting in a very low throughput. Recently, a lot of pressure-less process has been developed, however, they are mostly processed in box oven or hot plate involving at least one to two hours of total processing time, and they are still batch-by-batch process. A recent approach has attempted to use a reflow oven (Sikama FALCON 5/C four-zone oven, Sikama International Inc., Santa Barbara, CA). However, they includes two processing steps, including one contact-drying process and then reflow oven heating,   where the dwell time in each zone lasts 30 min, totaling 2 hours in the oven, which is very difficult to operate in continuous mode. In addition, specially designed fixture has to be used to help contact heating. For industrial scale mass production, a continuous sintering process for die attach assembly is critical. Till now, to the best of our knowledge, this has not been realized. In this study, we report the development of pressure- less silver sintering materials that can be sintered in the conventional BTU type convective reflow oven that is normally used for solder processing. After paste dispensing or printing, die was placed on the paste; the assembly is directly placed on a belt of reflow oven with a constant convoy speed. No pre-drying or other pre- or post-conditioning process is needed. The total sintering time needed is reduced to less than 15 min, which is one order of magnitude less than that of normally reported. This method is a direct “drop in” method for those customers who already have reflow oven. It is very meaningful in order for customer to save money, increase product yield and throughput. The silver sintering joints prepared by reflow oven are compared to that by other methods. They are characterized according to parameters such as bond line thickness (BLT), die tilt angle, void and shear strength. We focused on evaluating the best paste materials and sintering conditions to achieve high device reliability. We studied different factors; including paste optimization, device structure such as surface metallization of the substrate, joint BLT and die size, and explored their effect on the joint reliability during TCT. Material characterization techniques such as X- ray and optical microscopic analysis were used to understand the failure modes, and the relationship between joint properties and the microstructure evolution of the silver sintering materials.|
|sihai chen, Chemist
Indium Corporation of America