Comparing Cold and Heated Solder-Paste Slump Evidence

Slump-test results need the printed pattern and exposure conditions that produced them. A satisfactory observation before heating does not automatically represent the same deposit after a thermal step. Review the test stages separately when investigating paste bridges or selecting a material.

Confirm what was printed

IPC-TM-650 2.4.35 defines a solder-paste slump test with specified printing and specimen conditions. It calls for agreement in the printing method between vendor and user, highlighting the importance of a comparable starting deposit.

Record stencil pattern, thickness, substrate and paste identity. Check that the initial print is valid before interpreting later movement. A bridge already present at printing should not be counted as proof of a change caused by the subsequent exposure.

Keep observations attached to stages

Use photographs with scale and pattern orientation. Preserve individual feature results when comparing two materials. A single overall pass label can hide which spacing or geometry first showed an issue.

Resolve disagreements in the starting conditions

If two laboratories report different outcomes, compare their initial deposits and exposure sequence before attributing the difference to paste age or formulation. Confirm that both used the same relevant test variant and acceptance requirement.

For example, a hand-prepared deposit that is visibly thicker than the intended specimen changes the experiment. Document the deviation and repeat the required preparation rather than adjusting the interpretation to make the result fit.

When evaluating a production problem, preserve boards or images from printing and from later process stages. This helps determine when the unwanted connection first appeared. A bridge discovered after reflow does not, by itself, identify slump as its cause.

When a photograph is used for review, include enough of the test pattern to identify the evaluated gap. A tightly cropped bridge image without its pattern location cannot establish which specimen feature met or missed the agreed requirement.

Use the result within its scope

The test supports a defined material comparison. It does not reproduce every pad layout, stencil release condition or board support arrangement. Follow an acceptable material result with a representative print trial for the actual design.

Agree on the product disposition using its assembly requirements. Do not invent a universal allowable bridge count from an unrelated coupon or use a catalog statement as a substitute for the required test evidence.

Retain the original pattern photographs and stage labels with the selection record. The underside-dryness guide addresses a separate source of smearing after cleaning, while separation trials examine deposit release.