Solder Paste Inspection

The must-do is to inspect for coverage and registration. How much bare pad and registration error you can tolerate is process and product dependant, but you can measure both fairly accurately with a reticle on a microscope.

Developing measurement techniques, calculating coverage and registration error and what-not is part of the fun. Teaching your operators how to do this regularly is the real challenge.

Buying a new inline printer that does it for you is much more gratifying.

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Inspection criteria [B Willis] Solder-paste application * Fine-pitch printing Satisfactory solder-paste should: ** Be aligned with all pads ** Conform to the stencil's pad-aperture size ** Have a smooth, flat void-free surface Acceptable solder paste covers greater than 50 percent of the pad surface. Unacceptable ** Less than 50 percent of the pad surface is covered. ** Solder paste has slumped between the pads. * Chip print deposit Satisfactory solder-paste should: ** Be aligned with all pads ** Conform to the stencil's pad-aperture size ** Have a smooth, flat void-free surface Acceptable solder paste covers <75 percent of the pad surface. Unacceptable ** Less than 75 percent of the pad surface is covered. ** Solder paste has slumped between the pads.

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Thanks for your input. Where do I get information about fiducial dimension? Also is there any IPC standard for SMT process?

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There are several ways to inspect solder paste after deposition. Defects will range - too much paste, not enough, misaligned, bridged, smeared, etc. Some sceen printers have 2D inspection imbedded, otherwise a standalone system for paste inspection can be done after the board leaves the printer. 3D automated solder paste inspection systems will provide the most coverage and typically can keep up with line-rates these days. Depending on what you are inspecting (i.e. the beginning of this echain talks about CCGAs which require volume or 3D inspection) you could consider 2D as well. Just do a web search for "solder paste inspection" and you will find a ton of papers, systems, etc.

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I talked to our quality director and he says every solder joint had to be 50% of the way up on all the parts to be consisdered good.

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