IC Lead Failure: Lead Tearing

Lead Tearing

Lead tearing is the ripping apart or severing of the lead in the lateral direction, usually at one of its bended portions. It often manifests as a moderately ductile fracture of the lead propagated by mechanical stresses acting on it. It is often due to a problem in the raw material, i.e., the leadframe used is defective or inherently weak.

Potential lead tearing problems due to raw material issues can not be detected by the incoming QC's optical inspection prior to assembly, mainly because most lead tearing cases manifest only after the leads have been subjected to mechanical stresses, such as those imparted by the trim/form/singulation steps of the assembly process. Lead integrity tests, which apply stresses to the leads to test their reliability, would therefore have a better chance of detecting potential lead tearing issues.

Failure analysis on samples that exhibit lead tearing requires SEM inspection of the fracture surface, the topography of which may indicate metallographic 'weaknesses' or abnormalities in the lead. The failure analyst tasked with determining the distinguishing anomaly of affected samples from normal ones must therefore have a working knowledge of how the microstructure of a metal affects its fracture strength.

Lead tearing damage is cumulative. As mentioned, it generally occurs at the bended portion(s) of the lead, initiating at the edges of the lead. The tearing at both sides of the lead propagates inward as the lead is subjected to more mechanical stresses, until the cracks meet in the middle, at which time the lead breaks into two. Lead tearing is very rarely seen in production, but needs serious attention if encountered.

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