Suggestions for 1st time stencil application of solder

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1

I have a small board 50 mm X 50mm. Purchased the board and stencil from JPLPCB.

I've never used a solder paste stencil before but understand alignment an being flat is paramount. It looks like a Type 4 solder might be the best. I'm looking for tips from those who have gone before me.

And does anyone have an opinion on SN42 / Bi53 (Tin Bismuth) vs SN/PB. I know lead is the desired product but for one board I doubt the lead would be an issue.

Thanks.

I have a heat controlled hot plate (actually a 1" thick piece of aluminum with two cartridge heaters. In the past (when reflowing some parts) I let the board sit above the plate by about 1/8 inch. Allowing the board to warm up. Then I drop it on the plate.

2

Hello John,

Over the past few years I have gone from not knowing how to make a PCB to making them with SMD components so small I imagined that they would be impossible for me to solder (spoiler, they are easy to get right).

Before you start make sure you have all your components ready, there's no point placing a few then realising you are short of something, by the time you order them and get them the solder paste will have dried out.

How big is your stencil? I mean, is it the same size as the PCB? My first stencil was way over-size because I didn't understand what I was ordering. Now I order them the same size as the PCB. To apply the paste I put the PCB on my desk, align the stencil over it then, using electrical tape, tape the edges of the stencil to the desk, being careful not to disturb the alignment. Getting the alignment right is a skill to be leant like any other. All I can say is the first time I tried it took me ages to get right, now I seem to be able to do it in minute or 2 without difficulty.

Once aligned I use a small spreader (I can't remember where I got it) to apply solder paste to every opening in the stencil. The hard part is removing the tape and lifting the stencil without smudging the paste. If you mess it up you will have to wipe the PCB clean and try again.

As for solder paste I can't give you a comparison of different types. I just bought some reasonably priced paste 63/37 (tin/lead) T3 paste in a syringe.
(I think it is this:
https://uk.rs-online.com/web/p/solder-pastes/1466197?searchId=517aea4f-e75f-4119-8300-53b4ada2784f )

Despite reading that it only lasts 3 or 6 or 12 (insert random number here) months after several years in the fridge it seems to be perfectly OK. I do take it out of the fridge a few hours before use to let it get to room temperature.

Having applied the paste and made sure it is where it should be and not where it should not be I place the components with a pair of tweezers. Don't be concerned about perfect alignment, they will align themselves when the solder melts. As long as they are about right they will be OK. Just be sure that the solder pad on the component is over the correct pad on the PCB.

As for actually heating the board, I used to use a heavy bottomed frying pan with a lid. Place the board in the pan and heat gently until the solder melts. Once the solder melts turn the heat off, place the lid on the pan and leave to cool. Since then I bought a proper PCB oven, much easier.

3

I've had soldering paste in the fridge for years and it was fine.

Alignment is the hardest thing to get right with metal stencils. With clear Mylar it's easy.

One of the nice things about OSH Stencil is that they give you everything you need. A stencil, a laser cut alignment frame and a paste spreader.

Lacking that, I glue down two little pieces of scrap PCB to a sheet of wood to form an alignment corner and put the board to be stenciled in that corner.

Next, get the stencil aligned with the board. Thus is very time consuming if the stencil isn't transparent. Once aligned, fix the stencil somehow so you can realign it again when needed.

Now you can spread the paste. If you've ever done silkscreening its exactly like that. An old credit card works fine for this.

4

Great tips here. The alignment part seems like the biggest challenge for someone doing stencil printing for the first time. I think taking extra time to set up the stencil properly can save a lot of frustration later. The idea of creating a simple alignment jig from scrap PCB pieces is also a clever and practical solution. It’s interesting how many small improvements come from experience when working with SMD assembly.

5

I glue four pieces to a piece of scrap wood, to box in the target piece.
So the stencil support is on all sides and can't warp during scraping.
I then tape the stencil on one edge to the scrap to form a hinge.
So I can then stencil multiple boards without constantly having yo re-align.
I scrape starting at the hinge.

The biggest problem from keeping it in the fridge is drying out.
I have fixed that by stirring in one or two drops off IPA..
You can only do that with a tub of solder, not with a syringe.

The result depends a lot on room temp and viscosity of the solder.

6

Thanks for the information. I have a small board 50mm x 40 mm and a large stencil 14" x 12". I'm glad to hear your experience as looking youtube videos it seemed like screen printing is a magical art.

I've used my reflow device successfully installing large SMD using solder paste from a syringe. I'm comfortable with the reflow (for large objects). I've never reflowed any small parts.

I'm thinking of setting up the board being flanked by some blank FR4 held down with some double sided tape. Then because my stencil is so large I was going to shim it to be a millimeter or two above the target board, then clamp it tightly to a piece of Masonite. I'm thinking when I squeegee the paste I will be pressing down so the stencil is against the target board then when I release the pressure the stencil will "spring" up above the just applied target board. This works great in my head, I'll have to see if it works in real life.

I will post my success for failure.

John

8

For a one-off board, a stencil makes life much easier. Secure the PCB first. Tape the stencil as a hinge. Use Type 4 paste for consistent deposits. One smooth squeegee pass usually works best. Inspect the paste before placing components. I would choose Sn63/Pb37 for easier wetting and rework. Tin-bismuth works, but remains more brittle. Your hot plate approach is sound. Preheat gradually before reaching reflow temperature. Avoid overheating after solder flows. This solder Paste Printing Procedures article covers stencil alignment, zero-gap printing, and paste handling exceptionally well.

9

Can't you cut it to a more manageable size. Thin stainless can easily be cut with normal scissors. Don't cut it too close to the edges because the cut can wrinkle a bit.