A product based lap shear fatigue testing of electrically conductive adhesives

Abstract

Thermoset-based adhesives are used as thermal and electrical interfaces. These adhesives are filled with different particles in order to meet heat transfer and electrical properties. In automotive applications, they are required to have excellent adhesion since delamination may precipitate other electrical, thermal or mechanical failure mechanisms. A vast amount of literature is available on the investigation of solder die-attach reliability where lap shear experiments are frequently used in microelectronics industry. Both environmental and performance requirements resulted in replacing solder die-attach with lead-free alternatives like electronically conductive adhesives. However, only very few studies so far focus on fundamental understanding of fatigue degradation of these materials. The present paper addresses the above issue. To authors' best knowledge; it is the first time that in lap shear testing of adhesives, the specimens are obtained directly from production line and tested for their fatigue behavior. Authors present a novel, 28 mm long lap shear sample which is made by identical fabrication processes as in the microelectronic component. Thin quad flat packages (TQFP) are inspected with scanning acoustic microscope for possible initial defects right after the production. A cutting process, using a dicing saw, is developed to produce lap shear samples from the packages. The cut samples are investigated by optical and scanning acoustic microscope to improve cutting and test results. Mechanical response of the electrically conductive adhesive is investigated under cyclic loading conditions at 25°C and 100°C for different stress ratios and frequencies. Finally, the stiffness degradation during testing is analyzed and the crosssections of the samples are examined on bulk cracking. The presented method can be used to test different adhesives under production conditions in a fast manner which will decrease product development time and the dependence on time-consuming temperature cycle tests.