NEW MODE CRACK OF LSI PACKAGE IN THE SOLDER REFLOW PROCESS

Generally, the package crack mechanism in the solder reflow process ha s been considered as follows: The plastic absorbs moisture during stor age and the absorbed moisture is vaporized at the interface between th e die pad and the plastic by heat in soldering. The vapor pressure gen erates delamination, deformation, and stress in the plastic. Thus the crack occurs if the stress exceeds the flexural strength of the plasti c. However, a new mode crack that cannot be explained by a conventiona l cracking mechanism has been observed under the current circumstance of the surface mount components being smaller and thinner and requirin g more severe characteristics in the solder reflow process. This new m ode crack is generated from the upper portion of the die pad corner, n ot caused by the delamination between the die pad and the plastic. The new mode crack mechanism can be explained in this way: Absorbed moist ure reaches the die attachment layer during storage and is vaporized b y heat in the soldering; the vapor pressure pushes down the die pad an d causes stress in the plastic of upper portion of the die pad corner; then new mode cracks occur. A new lead frame design that can prevent this crack is described.