The “popcorn effect” of plastic encapsulated microelectronic devices and the typical cases study

摘要

Plastic encapsulated microelectronic devices are gaining acceptance over traditional hermetic parts in avionics, telecommunications, military, and space applications due to advantages in size, weight, cost, availability, performance, and state of-the-art technology and design. Despite so many advantages moisture induced delamination and cracking (called popcorning) is a real problem influencing reliability. “Popcorn effect” is one of the most common failure mechanisms of plastic encapsulated microelectronic devices. The paper introduced the cause that resulted in “popcorn effect”. During high-temperature moisture present in a plastic package can vaporize and exert stress on the package. This stress causes the package to crack and also causes delamination between the mold compound and the lead frame or die. To prevent damage from “popcorn effect”, plastic encapsulated microelectronic devices should be baked before assembly to drive out the moisture. Then, two typical cases on plastic encapsulated microelectronic devices of different package mode were studied in order to further research “popcorn effect”. One was PBGA package and the other was PQFN package. Both of the failure modes were electrical open or intermittent electrical open. The correlative failure analysis techniques on “popcorn effect” were presented in the cases at the same time. One of the important tools now used to nondestructively inspect the failure feature resulted from “popcorn effect” of plastic encapsulated microelectronic devices was Scanning Acoustic Microscopy (SAM). Applications included nondestructive detection of delaminations between lead frame, die face, paddle, heat sink, cracks, and plastic encapsulant. And another destructive technique was cross section analysis which could validate the result of SAM inspection and more visually present the failure appearance. For case one, by C-mode Scanni--ng Acoustic Microscopy (C-SAM) analysis delamination was observed at the interface of the PCB and molding compound at some outside bonds. And PCB cracks and wire bond cracks could be observed by cross section analysis. For case two, by C-SAM analysis delaminations were observed at the interface of molding compound and die, molding compound and lead frame and molding compound and base board. And die cracks and both wedge bond and ball bond cracks could be observed by cross section analysis.