Towards model-based engineering of underfill materials: CTE modeling

Polymeric composite based underfill materials, with well-controlled coeffic ient of thermal expansion (CTE) are critical to flip-chip and other advance d high-density integrated circuit packaging technologies. The use of underf ills beneath the flip-chip integrated circuits leads to an increase in reli ability by reducing the strain on the solder bumps during thermal cycling i mposed by the CTE mismatch between the chip and substrate. A fundamental un derstanding of the composite CTE of underfill materials is critical to the manufacture of high performance underfill materials and is critical to mark et expansion of flip-chip technology for high density packaging application s. This work presents a novel model for predicting the effective CTE of und erfills and other polymeric composite materials by considering the effect o f an interphase zone surrounding the filler particles in a polymer matrix. A microscopic model is also introduced for the volume fraction of the inter phase as a function of filler concentration as well as tiller-filler overla pping. The CTE model resolves several conflicts regarding the effect of fil ler concentration, filler size and filler-polymer interaction on the effect ive CTE of underfill and other polymeric composite materials. The results a re demonstrated to be critical for accurate flip-chip reliability predictio ns based on finite-element and other modeling techniques. (C) 2001 Elsevier Science Ltd. All rights reserved.