FAILURE MECHANISMS OF PEMs AND HSMs

Moisture Ingress:

The ingress of moisture and contaminants, primarily through the plastic-to-lead frame interfaces of PEMs, can cause electrolytic corrosion of the aluminum interconnect metallization or parametric degradation. The rate of corrosion is a function of the bias voltage, presence of moisture, chip temperature, and conductivity of the penetrating electrolyte [1-3]. Probably the most prevalent and potent ionic contaminant associated with corrosion is chloride [1-7]. Therefore, precautions need to be taken to minimize or eliminate sources of chloride and other halides during the manufacture of PEMs, as well as during their assembly onto printed circuit boards (PCBs). Moisture in the absence of a reactive contaminant will not cause corrosion [8]. This is borne out by the multitude of accelerated humidity test data generated on PEMs. These test methods act to saturate the package with moisture in a relatively short period of time and include Temperature-Humidity-Bias (THB), HAST (Highly Accelerated Stress Test), and Autoclave or Pressure Cooker. The likelihood of this mechanism occurring in field use today is minimal based on advances in cleaner processing, passivation integrity, mold compound purity and adhesion, and leadframe construction. Also considered an important factor is the education of the user relative to the elimination of halides and other highly ionic materials during PCB assembly.