A New Modified Crack Surface Displacement Extrapolation Method for Analysis of Package Delamination

Based on a new second-order analysis of the bimaterial interface crack, a modified crack surface displacement extrapolation method (MCSDEM) for evaluating the stress intensity factor (SIF) and mode mixity at a crack has been developed. Unlike existing methods which require extrapolation of values of SIFs with the concomitant uncertainties, the new method engenders a constant SIF over a considerable distance from the crack tip. The new method is also much less sensitive to mesh density than existing methods. The new MCSDEM is then used to investigate the effects of temperature, moisture diffusion and interface vapor pressure on the delamination of the interface between the leadframe pad and the encapsulant of a plastic quad flat pack (PQFP) during solder reflow for both eutectic solder and leadfree solder. Using a 160-leaded PQFP as a test vehicle, the effect of initial delamination size varying from 0.1 mm to 3.5 mm was studied. The results show that the relative contributions of temperature, moisture and interface vapor pressure is qualitatively different for solder reflow of leadfree soldered and eutectic soldered components.

The Crack Problem for a Nonhomogeneous Plane

In this paper the plane elasticity problem for a nonhomogeneous medium containing a crack is considered. It is assumed that the Poisson’s ratio of the medium is constant and the Young’s modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy-type kernel. Hence, its solution and the stresses around the crack tips have the conventional square-root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson’s ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible. On the other hand, the results are highly affected by the parameter β describing the material nonhomogeneity in E (x) = E0exp(βx).