Residual Stress in Flip Chip Joining Using Anisotropic Conductive Adhesive

Flip chip packaging with anisotropically conductive adhesive (ACA) joint is increasingly used in the electronic industry because of its environmental benefits, fine pitch capability and simpler process. The mechanical deformation has a great impact on electricity ability of the particles in flip chip assembly. The stress analysis of ACA joining in flip chip assembly is one of key steps towards a better understanding of the performance of ACA joints under various process and service conditions. A new finite element simulation for large deformation and residual stress in ACA joining has been made in this work, and there has been no known prior reported on it. A new method was suggested to simulate the force equilibrium and residual stress in conductive ball and resin matrix in flip chip joining using anisotropic conductive adhesive. The link elements with varied Young's module were introduced in present FE simulation. The results from this simulation in ACA bonding process indicated that after the pressure removed, the deformation ratio R
_D
decreased from 80% to 75.7%, and the deformation recovery ratio DeltaR
_D
= 4.3% only. A force equilibrium between resin and ball was built, and the ball can not revert its original state after the external load on the chip had been completely removed. The residual equivalent stress S
_eqv,ball
in ball center was remained at a high value, 318 MPa, and the average residual stress S
_eqv,resin
in resin matrix was only 25 MPa. Comparing with the residual stress in ball, the residual stress in resin matrix would be not important, due to the simulation result S
_eqv,resin
<< S
_eqv,ball
The assumption S
_eqv,resin
= 0 usually used, i.e. the residual stress in resin matrix can be omitted, may be a useful approximation in reliability analysis of ACA joining. The relaxation of residual stress in conductive ball may play a more important role for reliability of ACA joining. It is necessary to investigate the viscoelastic and plastic behavior of ball material, and to build its constitutional equation more reasonably in the future research. The simulation method suggested in this work can serve as a basis for stress analysis and for lifetime analysis of flip chip packaging with ACA joint in the future work