Mechanical stress and life for plastic-encapsulated, large-area chip

In order to develop test methods for qualification of commercially available plastic-leaded chip-carrier (PLCC) packages, a special test circuit has been used. This circuit contains piezoresistive sensors for measurements of mechanical stress, resistors for power dissipation and diodes for temperature measurements. The mechanical stress has been measured after die attach on different types of lead frames and after encapsulation in different molding compounds. Alloy 42 lead frames gave rise to less mechanical stress than copper-lead frames, but for alloy 42 there are cracks in the plastic instead. These cracks are due to the large difference in coefficient of thermal expansion between the lead frame and the molding compound. The measured stress level, as well as the aluminium-metal movements, are almost proportional to the coefficient of thermal expansion of the molding compound. The mechanical stress is relatively constant during a temperature cycle. This stress level is reached when the package has cooled down immediately after encapsulation. The highest stress levels during temperature cycling are reached at the lowest temperature.<>