FAILURE MECHANISMS OF PEMs AND HSMs

SMD Package Cracking "Popcorn Effect":

Cracking of certain Surface Mount Device (SMD) packages can occur during board assembly solder reflow operations (i.e., vapor phase, IR, and wave solder), due to stresses created from the sudden vaporization of absorbed moisture in the package [9, 10]. Such cracking creates a path for the possible ingress of moisture and contaminants, which can impact long term reliability. The internal shear stresses encountered can also affect bond wire integrity, particularly at the corners of the die, where the stress is greatest. This phenomenon is a function of the solder process temperature excursion, package moisture content, package dimensions, and mold compound adhesion. The effects have been observed mostly on large, high pin count packages, for which the current prevention is a dehydration bake, followed by shipment in dry-pack containers. Work is ongoing in the industry relative to the development of molding compounds which provide more resistance to this mechanism.