Hygrothermal Aging of an ACA Attached PET Flex-on-Board Assembly

In this paper, accelerated temperature and humidity test methods were used to study the long-term reliability of a flex-on-board assembly attached with an anisotropically conductive adhesive (ACA). Test methods included constant 65°C/90% relative humidity (RH) (65/90) and 85°C/85%RH (85/85) tests, 10°C-65°C/90%RH cycling test with moisture condensation (65/90 cycling), and 85°C/85%RH humidity cycling test (85/85 cycling). The resistance of the ACA joints was measured in real time during testing. On basis of the resistance measurement results, failures occurred in the 65/90 cycling test due to ACA delamination and cracking. In addition, failures were also observed in the constant 85/85 test and 85/85 cycling test. However, these failures were mostly related to the properties of the polyethylene terephthalate flex and its embrittlement or latent defects. In addition, a 90° peel strength test was used to study the changes in the mechanical strength of the assembly. The peel strength results showed that with all the test methods, the largest drop in the mechanical strength occurred at the start of the test. After this, the peel strength in 65/90 and 65/90 cycling tests remained almost unchanged, whereas a more significant drop was observed with the 85/85 cycling test. The 85/85 test caused the flex to become brittle, which prevented peel testing. Based on the failure analysis of the peel tested samples, similar failure modes were observed with all the test methods used. With untested samples, failure mostly occurred at the interface between the adhesive and the flex, although cohesive as well as adhesion failures at the ACA-printed circuit board interface were also observed. After hygrothermal aging, the failure location changed toward the ACA-flex interface as testing progressed and, in most cases, finally resulted in the detachment of conductor traces from the flex.