Separate Forward and Reverse Strokes in an SMT Printing Comparison

An alternating pattern of acceptable and marginal solder-paste deposits deserves a direction-based review. Combining both print strokes into one histogram can hide two different processes. Identify the physical squeegee and stroke direction for every inspected board before changing pressure, speed, or stencil geometry.

Define direction in machine coordinates

Use named machine directions and identify each blade independently. Terms such as forward and backward can change meaning after a panel is rotated or a technician stands on the opposite side. A photograph showing board orientation, blade identities, and travel arrows makes the record much easier to interpret.

Indium’s solder-printing guidance identifies squeegee settings, stencil gasketing, and cleaning conditions among relevant process variables. The useful engineering extension is to hold those variables stable while checking whether direction explains the observed deposit pattern.

Build paired observations

Print a sequence with both strokes under normal operating conditions. Record blade identity, actual recipe, stencil cleaning events, paste additions, and elapsed time. Select the same aperture groups on every board. Compare volume, area, height, and reject locations separately; a change in deposit shape may matter even when total volume barely changes.

Use blade-edge inspection to investigate a blade-specific pattern. Check the post-cleaning print condition when the apparent alternation occurs near a wet-clean cycle instead.

Avoid changing the reference frame mid-trial

Suppose narrow apertures near one edge show lower volume on every second board. Swapping blades, rotating the panel, and increasing pressure together might remove the symptom, but would not reveal which change mattered. Choose one controlled comparison and preserve the other conditions. If permitted by the equipment procedure, a blade exchange can help distinguish blade identity from travel direction.

Approve a correction only after both directions perform acceptably over a representative sequence. Do not tune the average by improving one stroke while worsening the other. Save direction metadata with the SPI export so later maintenance reviews can detect recurrence without reconstructing events from operator memory.