PREVENTING HEAD IN PILLOW DEFECTS IN AREA ARRAY COMPONENTS

摘要

Head in Pillow (HiP) defects are the most undesirable occurrences in PCB assembly process. HiP occurs when a solder ball of an area array component (BGA or LGA) rests on the mound of solder without forming a good metallurgical bond. These defects are very difficult to detect by standard inspection techniques and may escape most standard electrical tests (e.g. bed-of-nails test). Prevalent hypothesis says that HiP defects form when flux in the solder paste used for joining, is allowed to dry out before the joint is formed, and does not adequately remove all the oxidation from the solder ball, for instance, due to extended dwell in flux activation temperature range[1] One way to reduce HiP occurrence during reflow cycle is to heat the PCBA at a high ramp rate. For very thick multilayered PCBAs however, achieving high ramp rates may be very difficult, especially in localized rework situations. This paper describes a methodical approach adopted to study the underlying causes that can lead to HiP formation. Wetting characteristics of the solder paste were studied using a wetting balance. Dynamic warpage profiles of the BGA component and the circuit board were obtained using Moire Interferometry, for different reflow cycles. A proper selection of paste chemistry and the most optimum profile were used to make assemblies, which were then analyzed with CT Scan (3D X-ray) techniques to detect HiP type of defects. Inspection of several assemblies with CT scan has shown no presence of HiP formation.