Effect of temperature on the cure shrinkage measurement of non-conductive adhesives for flip chip interconnects

Non-conductive adhesive (NCA) flip chip interconnects are emerging as attractive alternatives to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine pitch applications. The electrical connectivity of a NCA interconnect relies solely on the pure mechanical contact between the integrated circuit bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extend on cure shrinkage characteristic of the NCA. In addition, to reduce the cost and increase the output, NCA is usually ramped up to 200 degrees C and cured for 1 min during the assembly process. However, fast cure reaction poses a great challenge for the accurate measurement of cure shrinkage. In this paper, to precisely determine the cure shrinkage at high temperature, cure shrinkage was first measured at lower temperatures with slow reaction rate by means of thermomechanical analyzer and then extrapolated to high temperatures. With the increase of cure temperature, the maximum of degree of cure will increase, but the maximum cure shrinkage reduces due to the expansion of materials at higher cure temperature. Furthermore, the slopes of the linear relationship between the cure shrinkage and the degree of cure at different cure temperatures after gelation were found to be similar and independent of the cure temperature. The cure shrinkage from gel-point to complete curing was determined to be 4.275%.