Lead frame Hillock-Induced Silicon Crack

Abstract

End customer in circuit test rejects at a rate of 2-5% was sent back to manufacturer for analysis and inspection. Electrical failure analysis had revealed some leakage on particular pin not included in the production test program. Physical failure analysis on those parts revealed micro-crack on affected pins. The micro cracks were found to be within the bonding vicinity and shown to have been propagating downwards into the silicon chip. The root cause was identified to be hillock on lead frame. Hillocks are formed on the surface of a lead frame during the rolling process of its base metal. Roller's surface irregularities and foreign materials on them could scrape the metal's surface. The scraped material's termination point will then produce an accumulated metal higher than the base surface. To sum up, the lead frame will produce tiny copper spikes on the surface which may cause a potential problem on IC packaging. Presence of these hillocks can cause a serious effect especially when its height already touches the surface of chip backside causing high stresses during wire bonding process. The work presents a new silicon crack mechanism and introduces stress modeling as a good validation tool in search for the root cause