A continuous contact resistance monitoring during the temperature ramp of anisotropic conductive adhesive film joint

In this study, a systematic experimental work was performed to evaluate the reliability of the anisotropic conductive adhesive film (ACF) joint at high temperature for flip-chip-on-flex (FCOF) assemblies. A four-point probe method was developed to measure the contact resistance at high temperature. Measurement was also conducted along the length of the chip. The correlation between the increased resistance and the failure mechanism was investigated using scanning electron microscopy (SEM). Initially, the contact resistance increased linearly with rising temperature, but later, it increased abruptly. This changeover was related to the glass-transition temperature (Tg) of the ACF matrix. The coefficient of thermal expansion (CTE) is very high at temperatures above Tg; thus, the ACF swells too much, reducing the mechanical contact of the particles with the bump and/or pad. Again, as the adhesive strength becomes weaker at temperatures above the glass transition, it is unable to resist the thermal stress of the flex. The cumulative thermal stress at the edges dislodges the particles from the interconnection. Even below Tg, the thermal stress at the edges is higher than the middle point. Thus, the contact resistance varied from the middle joint of the chip to that of the corner at the same high temperature. To reduce the contact resistance at the corner joint of the FCOF packages bonded by ACF, a square-shaped chip instead of a chip with a higher aspect ratio should be used. It was also suggested to use an adhesive with a higher glass-transition temperature and lower CTE.