The mechanism of plastic package cracking in SMT and two solutions

Package cracks due to the heat stress of surface mounting technology (SMT) procedures in low cost plastic packages present a severe long-term reliability hazard. It is shown that the dominant cracking mechanism is moisture expansion due to concentration of water vapor at the back surface of the die paddle, and at the front surface of the silicon die. Experimental results are used to discuss and compare each of the commonly assumed cracking mechanisms and to evaluate each solution for its probability of successfully eliminating the plastic-leaded chip-carrier cracking problem. Two methods which have vastly improved resistance to plastic package cracking are demonstrated. One is the elimination of the die paddle through the use of wire-bondable tape. The second is the use of lead-frame surface treatments prior to encapsulation to eliminate the moisture concentration at the backside of the die paddle