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Solder paste misalignment can be caused by poor initial alignment or because of variations on each printed circuit board. It can also occur on old printers when the table supporting the board does not always locate in the same position--hopefully those days are gone. Failure of the operator to pay attention to the process during the first-off or test print can also contribute to misalignment errors.

If the paste deposits meet the 80% coverage rule in inspection stands, the board should not be cleaned off. However the root cause of the problem should be corrected before the problem gets worse. If the board contains fine pitch parts, this amount of misalignment would probably not meet inspection criteria.

Figure 1: Here, the stencil was aligned one pitch out--unacceptable.

In Figure 2, the solder paste is perfectly printed from the stencil with a sharp brick appearance with a flat top and no peaking. Unfortunately there is evidence of sideways shift of the deposit off the pad. This will be caused by poor initial alignment, or variations on each printed circuit board. It could also occur on old printers when the table supporting the board does not always locate in the same position, hopefully those days are gone.

Figure 2: Another sideways shift. Washoff not required.

The solder paste in Figure 3 is not aligned with the pad surface but would be considered acceptable. The washoff of this board would not be required as experience suggests that satisfactory joints would result after reflow. The criteria suggested for paste print inspection would be a minimum of 80% coverage.

The cause of the problem is either initial poor alignment of the stencil to the PCB or inconsistent PCB pattern positioning to mechanical tooling positions like holes or edge of a multi panel.

Figure 3: Another misalignment. Again, washoff would not be required.

The solder paste in Figure 4 is not aligned with the pad and would be considered unacceptable due to excessive error. The wash off of this board would normally be required as experience suggests that solder shorts would result after reflow. Even if shorts did not occur it is likely that the solder would be unevenly distributed between joints.

The criteria suggested for paste print inspection is a minimum of 80% coverage, the example is less that 50% with paste shorting between pads. The cause of the problem is either initial poor alignment of the stencil to the PCB on this 0.025" pitch pads or inconsistent PCB pattern positioning to mechanical tooling positions. This results in error, board after board and makes realignment of the stencil necessary after each board.

Figure 4: This amount of misalignment would be unacceptable.

Figure 5 shows a solder paste deposit which has been misaligned with the pad pattern. The stencil is aligned one pitch out, which would be unacceptable as the rest of the board would also be misaligned and would need cleaning. This should not occur as a first off print on to a plastic cover sheet or set-up board would show up the error.

This type of defect is caused by the operator not paying attention to the process.

Figure 5: Here, the stencil was aligned one pitch out with unacceptable results.

Damaged Stencils	Stencil Blockage	Paste Misalignment	Smudged Paste	Poor Definition
Incomplete Prints	Residues	Bleeds	Ears	Scooping
Shorts	Slump	PIHR Printing