Characterization of after-reflow misalignment on Head-in-Pillow defect in BGA assembly

Head-in-Pillow (HiP) defect is a common issue in the Ball Grid Array (BGA) assembly. The defect is caused by several factors, individually or jointly, which includes warpage, misalignment, oxidation of the BGA ball, solder paste oxidation barrier capability, ect. The influence of before-reflow misalignment on HiP ratio(%HiP) has been studied. However, before-reflow misalignment can't describe HiP defect accurately when other conditions changed. This paper focused on the influence of after-reflow misalignment on HiP ratio. Meanwhile, the influence of several factors like solder paste oxidation barrier capability, reflow time, weight of package(PKG) on after-reflow misalignment had also been investigated. Three kinds of reflow profiles were designed. Different oxidation barrier capability pastes were obtained with different soak times; and different reflow profiles were adjusted by setting different reflow time and soak time. In the experiment, all samples had the same before-reflow misalignment. Results showed that the HiP ratio could be predicted by comparing the after-reflow misalignment with the same solder paste oxidation barrier capability. There would be no or very low risk of HiP defect when the after-reflow misalignment was small. Solder paste oxidation barrier capability is a key factor that impact the after-reflow misalignment; low capability would cause the after-reflow misalignment to be much larger than that before reflow; and %HiP increased rapidly with the increase of after-reflow misalignment. That is, when increasing the solder paste oxidation barrier capability, the %HiP will be more and more insensitive to the change of after-reflow misalignment. With the increase of solder paste oxidation barrier capability and reflow time, the after-reflow misalignment will decrease; while it will increase with the increase of package (PKG) weight.