Isotropically conductive adhesives and solder bumps for flip chip on board circuits-a comparison of lifetime under thermal cycling

Flip chip on board (FCOB) circuits with solder bumps or isotropically conductive adhesives (ICA) may be subject to joint cracking during thermal cycling. Although epoxy underfill can increase the lifetime significantly, there is still a risk of failure if the thermal expansion of the joint and underfill materials is not well matched. This paper presents experimental measurements of the number of thermal cycles to failure for both solder reflow and ICA joint FCOB circuits. Measurements were carried out for several different material systems with various types of underfill. Measurements of solder bump lifetime are compared to a lifetime model based on analytical solder strain calculations. For an underfill type without filler (CTE=58 ppm/°C), the measurements are in excellent agreement with model predictions, both giving a lifetime of around 1500 thermal cycles between -55 and 125°C. For two filled underfill types with CTE nearly matched to that of solder, measured lifetimes vary from around 2700 to above 3500 cycles. The corresponding model predictions are around 6000 and 7000 cycles, respectively. Lifetime measurements of FCOBs with ICA connections were carried out for two different material systems. Although measured lifetimes vary from sample to sample, the measurements indicate that it is possible to obtain connections that are stable for 2000 cycles (-40-125°C) or more. Analytical calculations of the induced stress in the ICA joints however indicate that the endurance limit of the epoxy material is exceeded in the assemblies, and that the joints therefore may be vulnerable to cracking