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Re: smt stencils

 

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Why cut the stencil??

The common procedure with frameless stencils is to attach the PCB to a flat surface using very thin double sided tape.? If many PCBs are made, you need to create a reference frame so the PCBs can accurately and repeatably be placed in the same position.

Then place same thickness PCB or other material around the PCB so when the stencil is laid on top it is supported.

Put a long piece of tape on one side of the stencil and tape it to the flat surface as a hinge.

Align the stencil.? See below.

Place solder paste as a line a little bit away from the PCB.? Use a squeegee to actually apply the paste.

As you know, the alignment is the tricky part.

Besides eyeballing it, there are basically two options: Either the PCB or the solder mask is taped down to a separate movable piece that can be locked in place by screws or clamps.

After initial alignment, swipe it, lift the stencil and inspect solder placement using a strong magnifier or a microscope. ?Adjust the movable part. Clean off the solder paste and try again.

That is how commercial machines work except the three X - Y – Z adjustments are done with micrometer screws.

Bertho

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From: [email protected] <[email protected]> On Behalf Of Jim Pruitt
Sent: 14 August, 2022 4:05
To: [email protected]
Subject: [homebrewpcbs] smt stencils

?

I have several questions about smt solder mask templates.? A while back I ordered some bare pc boards from/for the Adafruit Si5351 pll modules. The board is small as about an inch by 3/4" or so. When I ordered the boards from JLCPCB I also ordered a stencil (unframed). When they arrived the stencil was on a piece of stainless steel about 11x17 inches.? Did I mention that the board it was for is only an inch by 3/4 inch?? I wondered if I had ordered the framed stencil if it would have been board size.? A few nights ago I was doing some internet searching and ran across an Adafruit series on stencils:





One of those articles recommended this stencil frame which is a glorified screen printing style frame:


Unfortunately that frame is $1000 so that is way out of my hobby budget!? Does anyone know of a much cheaper stencil frame than the Madell tech one?

The next questions deal with how to cut the stencil.? I want to be able to lay the stencil on the board so that the stencil is exactly the size of the pc board and registers correctly.? I do not know how to do that.? If I lay the stencil on the pc board then I can not cut the stainless steel without moving it which looses registration.? So the question is how do I trim the stencil to board size and keep it so it registers correctly?? I asked JLCPCB that question but they did not understand the question and told me to put registration guides in the gerbers (that would not help).

I had also bought a stencil from ProtoAdvantage for the TSSOP10 Si5351 chip but that was a big mistake.? The TSSOP10 footprint is a very thin 2x5 footprint and it was almost impossible to get it registered over the pc lans to apply solder paste because there was nothing else to reference against (in/on the frame) and with the very tiny slits I never was able to get it aligned on the board and apply paste.? I ended up just hand soldering the IC but there has to be a better way to do it.

Do you have any suggestions?

Thank you.

Jim Pruitt
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