Thermal stress induced failures in adhesive flip chip joints

In order to estimate the reliability of adhesive Flip Chip joints, four different isotropically conductive adhesives (ICAs) on alumina and FR-4 substrates were subjected to thermal cycling tests (-40 °C — + 125 °C). By fitting the failure rate data to Weibull distribution, the number of cycles leading to 50 percent probability of failure (N f50) was estimated. For test structures on alumina substrates, the estimated N f50 varied between 2500 and 7000 cycles, whereas for test structures on FR-4 substrates, N f50 varied between 360 and 870 cycles, depending on the adhesive used. The stresses and strains in an adhesive joint during a temperature cycle were estimated using a previously developed simulation model, which calculates the stress and the creep strain (c cr) in the viscoelastic ICA material. The results of the simulations together with the testing data were used to find a relationship between N f50 and creep strain range (Dc cr) for adhesive joints. The results for the test structures on alumina substrates seem to obey a Coffin-Manson type relationship, N f50 = C • (Dc cr) h , where C and g have values of 97.5 and –1.15, respectively. The test structures on FR-4, however, showed failures at much lower cycle numbers than could be expected using this equation. The reason for this is the different failure mecha-nism in these samples. The initial cause of failure was not in the adhesive joints but in the underfill delamination.