Generally, the package crack mechanism in the solder reflow process has been considered as follows: The plastic absorbs moisture during storage and the absorbed moisture is vaporized at the interface between the die pad and the plastic by heat in soldering. The vapor pressure generates delamination, deformation, and stress in the plastic. Thus the crack occurs if the stress exceeds the flexural strength of the plastic. However, a new mode crack that cannot be explained by a conventional cracking mechanism has been observed under the current circumstance of the surface mount components being smaller and thinner and requiring more severe characteristics in the solder reflow process. This new mode crack is generated from the upper portion of the die pad corner, not caused by the delamination between the die pad and the plastic. The new mode crack mechanism can be explained in this way: Absorbed moisture reaches the die attachment layer during storage and is vaporized by heat in the soldering; the vapor pressure pushes down the die pad and 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.
