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

通过实验和数值模拟研究了回流法在焊点内形成的孔洞对焊点疲劳强度的影响。得到如下结果：在三种情况下，通过改变焊料、助焊剂来实验测量空隙。结果表明，焊剂含量高时，焊料会形成较大的孔洞。对于低助焊剂含量的焊料，可以观察到许多细小的空洞。在焊接过程中，不可避免地会产生空洞形核。(2)对疲劳寿命进行了实验测量，发现其受孔洞存在的影响。空隙率越大，焊点的疲劳寿命越短。(3)在疲劳断裂上，观察到许多裂纹。它们是分支疲劳裂纹，扩展到焊料与焊料的相界处，并在相界处终止。(4)采用损伤力学方法对疲劳裂纹行为进行有限元模拟。通过数值模拟，估计了孔洞的存在对疲劳寿命的影响。结果表明，较大的空隙降低了材料的疲劳强度。最后，建议降低焊料中的助焊剂含量，以减少空洞的成核。

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.