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Addressing Head-In-Pillow Defects
Addressing Head-In-Pillow Defects
The head-in-pillow defect has become a relatively common failure. This paper addresses the three sources or contributing issues.
Analysis Lab

Authored By:
Mario Scalzo
Senior Technical Support Engineer
CSMTPE, Six-Sigma Black Belt
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Transcript
The head-in-pillow defect has become a relatively common failure mode in the industry since the implementation of lead-free technologies.

A head-in-pillow defect is the incomplete wetting of the entire solder joint of a Ball Grid Array, Chip-Scale Package, or even a Package-On-Package device and is characterized as a process anomaly, where the solder paste and BGA ball both reflow, but do not coalesce.

When looking at a cross-section, 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 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 & material) of head-in-pillow defects. It thoroughly reviews these three issues and how they relate to head-in-pillow defects. In addition, a head-in-pillow elimination plan is presented with real life examples to illustrate these head-in-pillow solutions.
Summary
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 Package-On-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 cross-section, 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 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 & material) of the head-in-pillow defects. It will thoroughly review these three issues and how they relate to result in head-inpillow defects. In addition, a head-in-pillow elimination plan will be presented with real life examples will be to illustrate these head-in-pillow solutions.
Conclusions
Through failure analysis and empirical testing, it was determined that there were two major causes of HIP defects, which are poor wetting and warpage. After breaking down the entire assembly process, three areas were determined to contribute to the poor wetting and warpage: supply, process and material.

The most difficult issue for the user to control is the supply. The BGA or CSP manufacturer may provide a component that will always have the tendency to warp or not have controls in place to reduce the oxidation level on the spheres. Therefore, the user must then make sure that our manufacturing processes and controls within the product assembly are optimized.

Viewing and adjusting this process through the use of statistics yields two important objectives. First, an outside perspective on each part of the process arises by focusing on the details of each segment step, sharply increasing the understanding of the process. Secondly, using this data to eliminate problems from the process itself while streamlining each step of the process and discarding surplus, increases process flow and cost savings, while defect minimized and yields increased. Polishing the printing process, where the majority of all solder issue can be traced, sets the foundation for success.

Once consistent printing is assured, then other issues such as graping and head-in-pillow defects may be eliminated through optimization of the reflow parameters or evaluating a solder paste with an enhanced oxidation barrier, longer tack life, or better wetting performance. As shown, head-in-pillow defects can be eliminated through tight process controls and robust materials.
Initially Published in the IPC Proceedings
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