Thermal stress analysis of insulated metal substrates for reducing crack occurrence of solder in modules

摘要

The finite element method has been used to devise a method for improving the insulating layer of insulated metal substrates (IMS). This is required to reduce cracking of solder used to fix ceramic chip resistors onto an IMS circuit. An IMS is a circuit board comprising an insulating layer on a metal base plate. The insulating layer is made from epoxide resin incorporating dense inorganic fillers with high thermal conductivity. Because the substrates have high thermal conductivity, they are used in applications where electric parts generate intense heat, such as inverters, amplifiers, motor drivers and so on. Aluminum is often used for the metal base plates of IMS. The substrates are subject to repeated heating and cooling in ordinary usage, so the solder used to fix ceramic chip resistors onto the circuit sometimes crack because of mismatch between the coefficients of thermal expansion (CTE) of aluminum and ceramic. Crack occurrence rates in solder for ceramic chip resistors of various sizes are estimated from experimental temperature cycling tests. Stress analysis of IMS has been studied by the finite element method. Thermal-elastic-plastic stress analysis is carried out for thermal loads during the soldering process and temperature cycling test. Using the Weibull distribution plot, the equivalent plastic strain range in the solder during the temperature cycling test matched the crack occurrence rates of solder after the experimental temperature cycling test. Measures to reduce cracks in solder are examined, and it has been found effective to control the parameter El, the product of elastic modulus and CTE, for the stress relaxation layer in the insulating layer.