Reliability Improvement of Solder Joints Between Ceramic Chip Resistors and Insulated Metal Substrates With Thick Copper Pads

An insulated metal substrate (IMS) is a circuit board comprised of an insulating layer on a metal base plate. The insulating layer is made from epoxy resin incorporating dense inorganic fillers with high thermal conductivity. Because the substrates have high thermal conductivity, they have been used in electrical products that generate intense heat, such as inverters, amplifiers, motor drivers and so on. For using a high power semiconductor, thick copper pads are used on IMSs. In many times, aluminum plates are used for metal base plates in IMSs. The substrates are repeated heating and cooling in ordinary usage. So cracks of solder joints between ceramic chip resistors and IMSs often occur because of coefficient of thermal expansion (CTE) mismatch between ceramic and aluminum. Moreover, SnAgCu solder used to replace eutectic Sn/Pb solder as the joint has become major trend from a viewpoint of earth environmental protection. Finite element analysis was used for development of a high reliable IMS to meet these demand for the IMSs. The accelerated temperature cycling test on IMS was simulated by finite element method. The influence of the CTE of metal base plate and the influence of the modulus of insulating layer were studied. The reliability of solder was predicted to increase with CTE reduction of metal base plate. The modulus of insulating layer must be reduced sharply for a high reliable IMS. However, the insulating layer is difficult to be realized by using the epoxy resin. So the effect by fixing the chip resistor on a IMS using resin was calculated. When the space between the bottom of a chip resistor and the copper pads were filled up with resin such as underfill resin in ball grid array (BGA) mounting, it was predicted that the stress of solder decreased. The effect was confirmed by experiment. Solder joint fracture life of accelerated temperature cycling test on IMS was improved more than three times longer by using the underfill resin.