Indium Corporation - Addressing the Challenge of Head-in-Pillow Defects in Electronics Assembly

Our thanks to Indium Corporation for allowing us to reprint the following article.
By Mario Scalzo, Senior Technical Support Engineer for Indium Corporation

Abstract


The head-in-pillow defect has become a relatively common failure mode in the industry since the implementation of Pb-free technologies, generating much concern. A head-in-pillow defect is the incomplete wetting of the entire solder joint of a ball-grid array (BGA), chip-scale package (CSP), or even a packageon- package (PoP), and is characterized as a process anomaly, where the solder paste and BGA ball both reflow but do not coalesce. When looking at a crosssection, it actually looks like a head has pressed into a soft pillow. There are two main sources of head-in-pillow defects: poor wetting and printed writing board (PWB) or package warpage. Poor wetting can result from a variety of sources, such as solder ball oxidation, an inappropriate thermal reflow profile or poor fluxing action. This paper addresses the three sources or contributing issues (supply, process and material) of the head-in-pillow defects. It will thoroughly review these three issues and how they relate to result in head-inpillow defect . In addition, a head-in-pillow elimination plan will be presented with real life examples to illustrate these solutions.


Introduction


While the electronics manufacturing industry has been occupied with the challenge of RoHS compliance and with it Pb-free soldering, trends towards increasing functionality and miniaturization have continued. The growing use of ultra-fine pitch and area-array devices presents challenges in both printing and flux technology. The decreasing size and pitch of components create new problems, such as head-in-pillow.


What Does Head-In-Pillow Look Like?


A head-in-pillow defect is the incomplete coalescence of the solder joint between a BGA, CSP, or PoP and the printed solder paste. For some reason, the PWB's printed solder and the package's solder spheres do not come together to form a single mass. At first glance, it looks as if a film has formed on the surface of the molten solder, preventing the merging of the printed and package solders. In fact, this may be true, as in some instances there seems to be an oxide film on the surface of the molten solders. In other instances, it appears that upon cooling, the exterior has already cooled enough to prevent the coalescence of the printed paste and the sphere at re-connect when the warpage subsides. From the cross-sections, it actually looks like a head has pressed into a soft pillow (see Figure 1)...


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