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Evolution of mechanical properties and cure stresses in non-conductive adhesives used for flip chip interconnects

Nonconductive adhesive (NCA) flip chip interconnect is emerging as an attractive alternative to lead or lead-free solder interconnects due to its environmental friendliness, lower processing temperatures and extendability to fine pitch applications. The conductivity of a NCA interconnect is completely dependent on the direct mechanical contact between the IC bump and the substrate pad. The contact force caused by the curing and cooling processes is the sole force that enables electrical connectivity in NCA flip chip. However, as an important contributor to the contact force, cure stress is not well understood and has not been sufficiently investigated. In this paper, the evolution of mechanical properties has been measured with rotational parallel plate rheometer. Therefore, modulus as a function of degree of cure, obtained from differential scanning calorimetry (DSC) measurement, can be determined. In addition, the cure shrinkage was measured by means of thermomechanical analyzer (TMA) and thus the relationship between cure shrinkage and degree of cure was determined. The build-up of cure stress during the curing process was thereby calculated based on the relationship of modulus with shrinkage according to the incremental linear elasticity. It is deduced that when cure temperature is above ultimate glass transition temperature, cure stress is independent of cure temperature