Thermally induced IC package cracking

The presence of plastic package cracks due to the thermal stress of assembly procedures in low-cost plastic packages represents a severe long-term reliability hazard. It has been shown previously that the dominant cracking mechanism is moisture expansion due to thermal processing acting on concentrations of water vapor at the back surface of the die paddle and at the front surface of the silicon die. It is also known that the magnitude of the problem is dependent on the thermal gradient, built-in molding stress, plastic yield strength, and silicon die size. Three methods are described that have demonstrated improved resistance to plastic package cracking in integrated circuits. The advantages and disadvantages of each method are detailed and examined in terms of the overall manufacturability of the components. The problems encountered when trying to sidestep the thermal cracking problem by the use of sockets is discussed. Ceramic packages also suffer thermally induced cracks; an example is given by pin grid array packages which only experience the relatively mild thermal exposure of standard wave soldering. The parameters involved in this phenomenon are detailed