Effect of lead frame material on plastic-encapsulated IC package cracking under temperature cycling

In temperature cycling tests of plastic-encapsulated ICs, cracks in the encapsulant often originate from the lower edges of the chip pads. For Alloy 42 lead frame packages, it has been shown that cracking is caused by the thermal expansion mismatch between the chip pad and the encapsulant. However, cracking in copper lead frame packages cannot be explained in the same way. In the present work, differences in the package cracking mechanisms with lead frame materials are studied both by analysis and experiment. The results of thermal stress analysis indicate that encapsulant stress in the Alloy 42 lead frame package is increased by delamination of the chip pad's bottom surface. Conversely, stress in the copper lead frame package is mainly determined by delamination of the die bonding layer. The estimated mechanisms for both packages are experimentally verified by ultrasonic inspection of the chip pad's bottom surfaces and thermal deformation observation of package cross sections. The effect on cracking of each packaging material is also studied to find ways to prevent cracking. Recommendations are made for lead frame, encapsulant, and die bonding materials.<
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