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By EmbeddedMan
#93378
When I build prototype boards (New UBW32 versions for example) I design the boards in Eagle and then send them to Gold Phoenix or BatchPCB to be fabbed. They send me the raw boards back. Then I need to solder the components on. Many of you guys/gals do this too.

I sometimes hand solder everything on. But it takes a while. If I need to make a number of boards, I need to get a solder paste stencil made.

The first one I tried was the mylar one made at Polou.com. http://www.pololu.com/catalog/product/446 While it did work, there were burrs on the edges of all of the holes. I couldn't get the stencil to work really well (i.e. nice sharp past edges) until I trimmed some of these burrs with an Xacto knife.

So recently I needed to make a number of boards (including some UBW32s) and I saw a video up at Adafruit that shows Lady Ada using a kapton stencil. Looking around a bit, I find that http://ohararp.com/Stencils.html will make them for you, for a pretty reasonable price too. So I tried them out.

I had 6 different boards that were in the panel Gold Phoenix made for me, and so I sent a single gerber file created by taking each of the six boards, arranging them with some space between them, and drawing boxes around them in the _tsilk layer. Then I output just the tCream layer and _tsilk layer and got a single gerber with holes for the solder paste. The boxes around each board allowed Ohararp to automatically have their laser cut out each board's stencil.

I ordered on Tuesday and got the stencils on Friday for $32.55 including shipping. You can get up to 8.5" x 11" worth of Kapton for this price I guess.

How did it work? Great! There were no burrs, and it went very smoothly. It seemed like there was not enough paste being deposited (compared with the mylar stencils I had used before, which were a little thicker) but all of my parts reflowed perfectly, so the thickness was just fine. Beautiful little fillets on each pin.

One hint is to clean the top and bottom of the stencil after every couple boards with IPA. Because I didn't use the stencil in any kind of a frame, solder paste would slowly get underneath it and so future 'scrapes' would get less and less defined edges of paste. Cleaning solved this problem.

I had some problems because of having too much paste underneath a chip scale package (thermal pad in the middle), which was my own fault for not editing the part's footprint before having the stencil created.

Here are some pics to show you what I did:

This first one shows a board ready to be stenciled. The kaptop stencil is flipped up. I just put the board in the hole (made with old boards), flip the stencil over, and squeegee on the solder paste.
Image

Here you can see three boards with solder paste on them ready for parts.
Image

And here are some of the other boards we made that night, showing the nice joints from the reflow process.
Image

Anyway, I don't mean this to sound like an ad or anything, but I liked this so much I thought I should let you guys all know about it and how well it works.

*Brian
By lukemalpass
#93405
Nice work I like the reflows. I have yet to get around to stencils but thats coming up soon so its good to get some hints and tips on it.

How do you go about scraping and maintaining the solder as I currently just have a syringe so will need to get a tub and I know they are expensive for small amounts so I would want minimal waste.
By EmbeddedMan
#93412
Honestly, anything that's very straight will work I think. I use a 2"x8" piece of stainless steel sold for the purpose, but there's nothing really special about it other than it has a flat edge without a burr (you don't want to scratch the stencil). Some people use credit cards in a pinch. You could use a paint scraper, or better yet one of those metal paddles you use for applying spackle to walls.

*Brian
User avatar
By ohararp
#93415
@lukemalpass - kapton is pretty tough, but I recommend about a 1/16-1/8" piece of plastic. An old credit card or something similar is perfect. The small plastic spackle knives you get at home depot also work well.
By mpanetta
#93435
How do you suggest modifying the QFN center pad to fix the overage of paste with such large pads?
By EmbeddedMan
#93446
It's really simple. By default the tCream layer hole will be the same size as the center pad. What you do in Eagle is edit the footprint of the part to turn off the cream layer hole for that pad, then add your own instead. You make four smaller squares (one for each quarter of the center pad) and you can then control the overall amount of paste.

I've never done this type of chip before (with solder paste) so I'm not sure how much smaller to make them. I'm certain that if you look around a little bit you'll find all sorts of guides on how best to adjust for this.

*Brian
By MichaelN
#93447
I agree with EmbeddedMan about breaking up the center paste area into multiple sections. Many datasheets will have recommendations for the sizes of these areas. It isn't just about reducing the total amount of paste - it also allows much more accurate amounts to be applied (large areas in a stencil make it hard to get a uniform thickness of paste).
User avatar
By ohararp
#93493
Typically I will edit your gerber files for large pads (Think TO-252) to keep solder paste from going under the stencils by breaking it up as suggested. I could also edit this pretty easily if you indicated that you needed this done in your gerber files.
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