Damage Mechanics Approach for the Strength Evaluation of Solder Joint with Voids.

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The effect of voids, which is nucleated in solder joint by re-flow method, on the fatigue strength is studied expermentally and numerically. Follwing results are obtained.(1)Voids are meausured expermantally by changing the solder, flux in three cases. It was found that large voids are nucleared for solder with large flux content. For small flux content solder many small voids are observed. It is impossible to avoid void nucleation during solder joint process.(2)The faitgue life is measured expermentally, and it is found that it is affected the existence of voids. Larger void rate decreaces tha fatigue life of solder joint.(3)On the fatigue fracture, many cracks are observed. They are branched fatigue cracks, which grow to the phase boundary of solder and pat, and terminate at the boundary.(4)The fatigue crack behavior is simulated by FEM applying the damage mechanics approach. The effect of the existence of voids on the fatigue life is estimated based on these numerical simulations. It is concluded that lager voids decrease the fatigue strength. Finally it is recommented to decrease the flux content in solder to decrease the voids nucleation.