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We have developed a new void free process for making the solder joint between the chip mounted AlN substrate and the metal substrate in large-area, high power insulated gate bipolar transistor (IGBT) modules. This new process consists of two steps. First, Ar+ were used to clean the surfaces of Ni plated film on a metal and AlN substrates which were then coated with 0.5-µm-thick Ag film. Second, 50 wt% Pb–Sn solder was sandwiched between the two substrates and heated to 503 K in a vacuum for 5 min before being cooled in a N2 atmosphere. By using this process, the area percentage of voids in a soldering area up to 130×190 mm2 can be reduced to less than 0.1%. IGBT modules made by this process were also found to exhibit satisfactory current-voltage characteristics.

This paper aims to clarify the process factors for applying the void free soldering process to practical use. Solder cleanliness lessens the influence of the heating rate on the void formation during soldering. The lowest vacuum for the purpose of void free soldering for cleaning and Ag coating, and for soldering were 2 × 10 −3 Pa and 4 × 10 −1 Pa, respectively. Cooling in N 2 lessens the void formation during soldering. The allowable standing time from the Ag coating to soldering is 25 h. The reliability of power modules made with this new process was satisfactory. (Received February 28, 2002; Accepted May 30, 2002) Keywords: void free soldering, silver film, vacuum of the chamber, vacuum of the soldering furnace, cooling in N 2 , reliability

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Texture and surface morphology improvement of Al by two-stage chemical vapor deposition and its integration in an Al plug-interconnect scheme for sub 0.25 μm metallization Selective tungsten filling of sub-0.25 μm trenches for the fabrication of scaled contacts and x-ray masks