Package Cracking

Plastic Package Cracking

Plastic package cracking is the occurrence of fracture(s) (see Fig. 1) anywhere in or on a plastic package.  Over the years a vast range of mechanisms that lead to package cracking had already been characterized.  Mechanisms vary from one package type to another, and some may even be unique to certain package groups only.

Most of the known mechanically-induced package cracks come from assembly.  Worn-out DTFS blades and punches can result in large deflections of the leads and tie bar during processing, creating excessive stresses at their plastic interfaces.  Cracks occur if these stresses exceed the molding compound's fracture strength. Tight deflashing coupled with excessive package mismatch can also create excessive stresses at the bottom package during deflashing, leading to cracks.  Single-stage lead forming also creates excessive stresses at the lead-to-plastic interface. 

Leadframes with no anchor holes and with poorly designed bonding fingers are prone to lead pulling, which is usually preceded by cracking at the lead-to-plastic interface.  Debris underneath the package during DTFS can produce large bending stresses on the package,  which lead to cracks if they exceed the plastic's fracture strength.  Inadequate package nesting during DTFS can result in similar package cracks.

Overcuring of the package renders it brittle, and may make it more vulnerable to assembly-induced cracking. Insufficient bond line thickness due to inadequate die attach material dispensing may result in cohesion failure within the die attach material, which can lead to package delamination and cracking.

Figure 1. Photo of an SOIC with a corner crack

Plastic surface mount devices (SMDs) may crack because of the intense pressure build-up generated by the vaporization of the internal moisture inside the package during solder reflow.  Moisture is often absorbed by the package from the environment during storage prior to the solder reflow.  This mechanism is known as 'popcorn cracking,'  since a popping sound may actually be heard during the moment of fracture.

Delaminations between the die and the plastic and between the die paddle and the plastic precede package cracking.  Prior to cracking, the vaporized moisture will expand the gap between the delaminated plastic and the die paddle to form a dome or bulge on the package. The excessive stresses are relieved only after the fracture occurs, which usually originates at the corner of the die paddle.  The crack propagates to the surface when the maximum bending stress exceeds the molding compound's characteristic fracture strength. 

Factors that affect popcorn cracking tendency include the solder reflow temperature, the moisture content of the package, the dimensions of the die paddle, the thickness of the molding compound under the paddle, and the adhesion strength of the molding compound to the die and leadframe.  A package becomes more robust as its die paddle length decreases and its bottom plastic thickness increases.  Recent data however indicate that thin packages with large die paddles do not absorb moisture, and are therefore less vulnerable to popcorn cracking.