Out of the Box Stencil Quality

Is Dock-to-Stock the right process?

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There are several legendary stories in our industry about miscut apertures making it onto production lines and costing millions of dollars in scrap or field returns. The figures vary depending on who tells the story, but there’s no doubt the impacts were substantial, and there is no excuse for shipping stencils with missing apertures.

How does an assembler know that the stencil they just received from their supplier is correct?? Do they trust the QC paperwork?? Should they?

The stencils that presented these miscut apertures both came with QC paperwork saying they were perfect.? Therefore, the quality certificate was literally not worth the paper it was printed on.

Our QC check is more than a photocopied form with a few initials scribbled on it - we use ScanCAD to automatically inspect every stencil we cut before we ship it.? ScanCAD is a system that scans the cut stencil and compares it to the original CAD file, ensuring aperture presence, size and location.? It is an extra step that some stencil manufacturers opt to use as a final verification, and one that some choose to skip.? Ask your stencil manufacturer if they use ScanCAD or other automated checking functions and ask them to include those reports with your stencil shipment.

There’s more to our ScanCAD program than just QC.? It helps us customize stencils for boards that have stretch or shrink issues.? While stencils are typically accurate positionally to 0.0005,” PCB feature positions can vary widely based on their designs and fabrication processes.? When a stencil’s apertures and a PCB’s pads will not align properly, the source of the variation typically lies within the PCB.? The stencil, especially a laser cut stencil, is far truer to its nominal CAD dimensions than the bare circuit board.?

If the alignment issues are considerable enough to affect production quality, we can scan the PCBs and customize the stencil apertures to match up.? In fact, if our customers have boards that regularly experience lot-to-lot positional variations, we can scan their new lots of PCBs and advise them if their existing stencil will work or if they should scale a new one before the next lot hits the assembly line and creates problems.

What if your stencil supplier doesn’t use ScanCAD??

Some stencil cutting machines have the ability to self-check their work and recut the aperture if there is a problem.? This self-check routine takes up cycle time, and discount suppliers usually can’t afford to spend the machine time on it. ?They may have the feature but don’t use it to save on cost.

It really pays to ask your supplier about their QC and scaling routines.? If they can’t demonstrate something like ScanCAD, going from dock to stock is risky business for a PCB assembler.? The onus to ensure the stencil’s fitness for use then falls upon the user – and buyer beware!?

With an SPI machine, these are some straightforward, but time-consuming recommended best practices:

·???????? When a new stencil comes in for a new design, it should be held in an NPI queue until it can be paired with the PCBs.

·???????? Prior to setting up the production line for the first run, the stencil should be verified by printing several PCBS - at minimum one in each direction - to validate positional accuracy and proper stencil tension.

·???????? If proper alignment isn’t possible, most SPI machines can measure the stencil fiducials and the PCB fiducials and provide the offsets, which the engineer can then communicate to the stencil provider.

·???????? When a new lot of PCBs known to demonstrate excessive positional variation comes into stock, they should be printed with the existing stencil and inspected, to determine if the current stencil is adequate or a new one is required – again, before the full line setup and potential downtime or defects.

While these checks are very easy to perform in-house, they cost the assembler in engineering/technician time and usually line-down time as well.?

If your operation has ever been impacted by a non-conforming stencil, if you never want your operation to be impacted by stencil quality, or if you don’t want to spend your high-dollar engineering time chasing down your supplier’s problems, contact us.? We are a family-run business that values our customers and guarantees our quality before we ship anything we make.