Planning a BGA Head-in-Pillow Investigation

An intermittent BGA connection should not be labeled head-in-pillow simply because reheating makes the board work. A useful investigation establishes the joint condition and then examines how it formed. Preserve the original evidence before rework changes the interface you need to understand.

Define the suspected defect

Indium describes head-in-pillow as a failure of the package solder ball and printed solder deposit to coalesce into a sound joint. Package movement during heating, wetting behavior, and process conditions can contribute. The defect is different from a small passive standing on one end. Indium: head-in-pillow and tombstoning.

Record the electrical symptom, test conditions, board serial number, assembly lot, and package location. Map suspect ball positions using the correct package view and orientation. If mechanical pressure changes the symptom, document the observation without repeatedly flexing the board; additional handling may alter the evidence or introduce another failure.

Build a comparison set

Select representative failed and passing assemblies from known process conditions. Include material and package lot information, paste and stencil revisions, printing records, placement setup, and measured reflow profiles. A comparison set is more informative than a single damaged board with no manufacturing history.

1. Use appropriate inspection to locate and characterize suspect interfaces.
2. Confirm the inspection method’s ability to resolve the condition of interest.
3. Compare defect positions across packages and boards.
4. Review paste deposits and package coplanarity or warpage evidence where available.
5. Check component handling history and the actual thermal process.

For example, defects repeatedly concentrated in one package region suggest a different investigation from random low-volume deposits across several component types. The spatial pattern guides the next measurement; it does not, by itself, prove a unique cause.

Change one hypothesis at a time

State why a proposed adjustment should address the observed mechanism. A new paste, revised stencil, or altered profile changes different aspects of the process. Evaluate the resulting solder interfaces and the rest of the assembly, not just the original failing test. Raising temperature without checking component and board limits can create a new problem while concealing the old one.

Document the evidence for closure

Retain the original failure analysis, trial conditions, and inspection results. Define the production population covered by the corrective action and how effectiveness will be monitored. A repaired sample demonstrates recoverability, not necessarily a stable production process. Where joint integrity is critical, use the product’s agreed qualification and reliability plan to determine what additional evidence is needed before releasing the revised process.