Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

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Manual Stencil Alignment



Manual Stencil Alignment | 27 June, 2001


Can anyone give me advise on the best way to manually align a 20"x20" stencil on a semi-automatic printer? We have a board with a 16 mil pitch QFP and have difficulty aligning the stencil. We have tried various alignment schemes on the pcb and stencil. Adding vision is an option although an expensive one, any tips, advise and helpo would be appreciated.

Thanks Herb

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Manual Stencil Alignment | 27 June, 2001


First, proper alignment can produce an order-of-magnitude increase in the number of boards that can be printed without wiping. The initial alignment can be made either by: * Peering through the stencil apertures at the board underneath and adjusting the board position to align with the stencil apertures. OR * Printing onto a clear, fixed film and aligning the board under the film. OR * Printing either onto the board itself or onto the board covered with clear adhesive tape, until the alignment is correct. Here, the print must be wiped off after each adjustment.

Once acceptable alignment has been achieved on one board, several boards should be printed with no further adjustment. These boards should be inspected for misalignment, and any correction made for the average error.

Second, some semi-automatic printers are dogs that can�t hunt 16 pitch. Adding vision, while techically possible, may be like putting bifocals on a blind man.

Third, if you print 16 pitch too fast, the paste will never "see" the apertures that go east / west and only recognize the north / south apertures.

Forth, HASL is tough to print on for devices with 16 pitch.

Now, let's talk. Tell us about your machine, set-up method[s], board[s], wait time between print and reflow, specific problem in terms issues and breadth, and all the other stuff that you didn't tell us in your original post.

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Steve Vargas


Manual Stencil Alignment | 28 June, 2001


Try oversizing your stencil fiducial openings to maybe twice the size of the board fiducial. When aligning manually, look for uniformity in the annular ring which appears around the fiducial. Hope this helps.....

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Murray Pulman


Manual Stencil Alignment | 5 July, 2001

Dear Herb

Intoduce a decent light source (halogen desk lamp or similar)when aligning.

Then concentrate on the refection as you adjust in the x & y rather than trying to see edges of pads as they pass the appetures.

Start with rapid sweeps to get the contrast in your head! then slow down and settle in the correct position!. Practice makes perfect.

Finaly, it pays to trust the vernier scales on you x-y table a little more for final adjustment!.

regards Murray Pulman

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