Electrification & Power Modules – Innovative High Reliability Solder Materials
With increased electrification in automotive, traction, industrial verticals; power electronics is only set to grow. Mission profiles have become harsher with higher service temperatures, increased cycles, lower Rth. Power module construction is also evolving (traditional IGBT power module with DBC and baseplate, to a transfer molded module attached to a water cooler with pin fins. All of this means that the interconnect solder materials (for substrate attach, clip attach, module attach) will need to be designed for the newer requirements.
For larger area attach (20x30mm – 50x70mm) of DBC/Substrate to Baseplate; Transfer Molded package to Cooler, the traditional Sn/3.5Ag alloy does not have enough fatigue resistance for the harsher mission profiles. Sb-based alloys are better suited as they are harder and melt at a higher temperature, but the Sb content and overall alloy composition are critical to strike the right balance between hardness and ductility. Solder preforms (bricks of solder typically in the shape of a cuboid) are placed between the substrate and the cooler. When the preform melts and the solder is molten, bondline thickness is not uniform. To maintain bondline uniformity, a reinforced composite preform with an embedded matrix was designed. This reinforced preform is a drop-in replacement to a solder preform (wetting, voiding, pick/place, reflow…) and eliminates additional process steps involved in wirebond stitch and trim method, thus lowering the cost of ownership. In addition, the embedded matrix in the reinforced preform enhanced creep resistance and improved thermal cycling reliability by a factor of 2-3X. Reinforced preforms with an embedded matrix can also be used for clip bonding to the top of the die, as well as die attach.
To solder molded packages to the cooler, there is an additional layer of complexity in that the mold can be temperature sensitive. When the process temperature exceeds the Tg (glass transition temperature), the CTE (coefficient of thermal expansion) change could increase by 4X resulting in warpages as high as 500µ. So the process peak temperature has to be capped at a maximum of 2300C. This means that traditional Ag and Sb-based alloys cannot be used due to their higher melting temperature. A unique dual-alloy technology solder preform was designed with a lower melt temperature of 2050C requiring a process peak temperature of 2200C, without diluting the hardness and ductility characteristics needed for the harsher mission profiles. The reliability was further enhanced with a reinforced embedded matrix in the lower temperature alloy system. With very high volumes predicted for EV (electric vehicles), the cycle time taken to solder the molded package to the cooler become critical. Convection belt furnace ovens with vacuum allow for quick throughput but cannot address the oxide formed on solder and the substrates. This can cause poor wetting and unreliable solder joints. A thin uniformly distributed flux coating on the preform surface was designed to address the oxide, achieve good solder wetting in convection reflow. These flux coated preforms are designed to be pick/placed from tape/reel packaging for very high volume throughput.
