Effect of Thermal Aging on the Interface Fracture Toughness of the PCB-UF Interface

Abstract

Electronics in military applications increasingly rely on the use of commercial off-the-shelf components for enabling critical electrical functions. Defense electronics may often be subjected to high-g acceleration loads in addition to extremes of temperature, humidity and prolonged storage. Underfills are often used to provide protection and survivability expectations under extreme thermos-mechanical loading, CTE mismatches, moisture. There is a dearth of computational tools to allow for the prediction of the initiation of damage and the progression of damage at the underfill-interfaces under high-g shock loads. Defense electronics and military systems have longer lifetimes in the neighborhood of 20-40 years and higher reliability requirements. New packaging architectures, which often push the edge of the envelope in terms of miniaturization, cannot be compared with the prior generation electronic systems and lack decades of historical data to provide robust proof of their survivability. Tools and techniques are needed to determine the failure envelopes for new component technologies for operation under high acceleration loads in current and next generation military systems. In this paper, the interface fracture toughness of the PCB-UF interface has been studied under exposure to high temperature storage. A three-point composite beam specimen has been used to study the interface fracture toughness. The bi-material strips of PCB/Underfill were thermally aged for 10 days, 30 days and 60 days at temperatures ranging from 100°C to 150°C and then the sample specimens were subjected to quasi-static three-point bending to observe fracture parameters and to determine interfacial delamination of bi-material strips. A 2D Digital Image Correlation (DIC) method was also employed to understand the Crack tip opening displacement (CTOD), crack initiation and the fracture toughness, CTOD were compared with the aging schedule and temperature.