Graduate Exam Abstract
October 28, 2011, 1pm - 3pm
Animal Sci 112
Clamping Devices to Enable Concurrent Mechanical and Electrical Connections of a Power Semiconductor
Abstract: In response to the restrictions of lead bearing solders in the EU RoHS Directive of 2002, new strategies for solderless electrical connections are desired. In this work, such concepts are used in the simultaneous electrical connection of power semiconductor leads to a PCBA and mechanical attachment of the device package to a heatsink. These concepts are specifically designed for use in an industrial high power (kW to tens of kW) radio frequency generator. The many constraints of such a system, some of which are diametrically opposed, are considered throughout, including manufacturability, mechanical tolerances, system reliability, and cost.
Theoretical models are used to show that in the expected usage environment, the transistor leads in the clamped connection under consideration will move 5.1 ?m for a thermal excursion of 50 °C. SEM micrographs showing z-axis asperities on the order of 1 ?m and calculations estimating adhesive junctions with surface energies on the order of 100 N/mm2 demonstrate that contact wear is likely. A survey of available materials is conducted, with beryllium copper and polyetherimide (PEI) being the favored options for clamp construction. Four concepts are modeled, noting the benefits and drawbacks of each.
The preferred embodiment is found to be a clamping mechanism fabricated of electrically insulating material, specifically injection molded 30% fiberglass filled PEI, incorporating cantilevered beams which deflect upon installment into the system with fasteners, thus forming the electrical connections. Test regimens are undertaken, including room temperature aging, elevated temperature aging at 50 °C and 80 °C, thermal cycling, highly accelerated life testing, and thermal analysis in both the steady state and transient regimes. The results of the experiments show this clamping system to have a useful life in the intended environment of multiple years.
Adviser: George Collins
Non-ECE Member: Hiroshi Sakurai
Member 3: Steven Reising
Addional Members: Tom Chen,N/A
Program of Study: