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Solder Pastes to Reduce Head-In-Pillow
Solder Pastes to Reduce Head-In-Pillow
Results of test methods to analyze head in pillow defects to assess BGA/CSP solder ball interactions.
Materials Tech

Authored By:
Jasbir Bath, Roberto Garcia
Christopher Associates Inc., Fremont, CA, USA

Noriyoshi Uchida, Hajime Takahashi
Gordon Clark, Manabu Itoh
Koki Solder, Tokyo, Japan
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Summary
Over the last few years, there has been an increase in the rate of Head-in-Pillow component soldering defects which interrupts the merger of the BGA/CSP component solder spheres with the molten solder paste during reflow. The issue has occurred across a broad segment of industries including consumer, telecom and military. There are many reasons for this issue such as warpage issues of the component or board, ball co-planarity issues for BGA/CSP components and non-wetting of the component based on contamination or excessive oxidation of the component coating. The issue has been found to occur not only on lead-free soldered assemblies where the increased soldering temperatures may give rise to increase component/board warpage but also on tin-lead soldered assemblies.

In order to reduce this affect work was done on a set of tin-lead and lead-free solder pastes which were developed with flux formulations in the solder paste having higher heat resistance which did not lose their flux activation properties as quickly and quicker wetting which helped to reduce the head-in-pillow defect. The results of the evaluation are presented with test methods to analyze the head in pillow defect not only on BGA component test vehicles but also more fundamental studies to assess BGA/CSP solder ball interactions with the developed versus control solder pastes.

As well as reviewing some of the other causes of the Head-in-Pillow defect such as printed solder paste volume, component placement and reflow profile used, an assessment will be done on the industry standards which need to be updated or developed for component and board co-planarity in the as-received and reflowed conditions.
Conclusions
Newly developed tin-lead and lead-free solder pastes have been shown to help to reduce the occurrence of pillow defect by controlling flux fluidity and improved heat resistance of the flux used in the solder paste. However this would only be a partial solution as there also needs to be progress in reducing warpage of components and boards during reflow by updating / developing the industry standards related to this area.
Initially Published in the SMTA Proceedings
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