Separating Package Coplanarity from Board Flatness in Assembly Reviews

An open solder joint beneath a lead or terminal can be associated with an uneven package, an uneven board, insufficient paste, placement error, or movement during heating. Calling every such failure a coplanarity problem obscures the investigation. First establish which physical surfaces failed to approach each other and when that separation occurred.

Define the measurement reference

Package coplanarity describes the relationship of terminals to a defined reference or seating plane under specified measurement conditions. Board flatness describes a different object. A component may satisfy its package drawing while the local board surface beneath it differs from the assumed plane.

Texas Instruments’ power package assembly discussion identifies package coplanarity as a consideration in establishing solder paste contact. Use the current drawing and assembly guidance for the actual package to obtain applicable limits. Do not substitute a number from an unrelated package family.

Preserve evidence before adjusting the process

1. Record the failed terminal locations and whether they repeat across boards or component lots.
2. Inspect incoming terminals for deformation and handling damage using an appropriate measurement method.
3. Review printed paste height and volume at the affected lands.
4. Check local board support and placement conditions.
5. Compare the soldering profile and any available evidence of behavior during heating.

Room-temperature dimensional compliance does not by itself describe all movement during reflow. If the defect appears only after heating, preserve that distinction in the investigation. Avoid assigning a thermal mechanism solely from a room-temperature photograph.

Use the defect pattern to choose a trial

For example, repeated opens along one side of a package invite different checks from randomly distributed low deposits across the whole board. The first pattern may justify a closer look at placement, terminal geometry, and local support. The second suggests examining printing and stencil condition before focusing on one component type.

Increasing stencil thickness may improve contact in a particular trial, but it also changes solder volume elsewhere. Review bridging, component seating, and the behavior of neighboring fine-pitch features before accepting that adjustment. A local process change should have a stated mechanism and measurable outcome.

Close the issue with the package drawing revision, measurement method, affected lots, paste evidence, and comparison trial results. State whether the cause was demonstrated or remains a working hypothesis. This distinction matters when another lot arrives: an action that compensates for one damaged shipment is not necessarily the correct permanent manufacturing setting. Retain representative failed and acceptable samples when practical so future reviewers can compare physical evidence instead of interpreting a short defect code.