Friday, October 30, 2015

Using a Solder Paste Stencil for a Prototype

If you've seen any of my previous post you'll know I'm in the process of making a PCB that contains a configurable LED Grid.  Why?  Something to do I guess.  Anyway I finally got around to assembling the board I had made by BreadboardKiller.  I've assembled small surface mount PCBs in the past and have always manually applied the solder paste by hand with a syringe.  It's hard to get right, you might not get paste in the right place, you can use to much or too little, but when it's a only a few parts it's not too hard to fix.  This board has 96 surface mount components on it, which means that doing it by hand was going to be a close to impossible.

I decided to get a laser cut Kapton stencil from OSH Stencils.  At 40 dollars it looks expensive for a piece of plastic, but that includes a one off cost for set of board holders the exchange rate wasn't kind either.  These are the black acrylic L shaped pieces in the image below.  The stencil was only 20 dollars US which was well worth it.

There are demonstrations on how to use the stencils online but I'll show my setup.  The board holders are taped down and the stencil is aligned and taped down on on side to act as a hinge.  This allows a board to be put in the holder, have the stencil flipped over it, the paste applied, the board removed, and the process repeated.  Application of the paste is easy.  Squirt some out of the syringe where it needs to go, and use the paste spreader (basically a credit card) to swipe the paste across the stencil.

PCB Assembly
PCB Assembly Set-Up
The stencils are easy to use and make sure you apply the right amount to each pad.  For this project I chose to use lead free solder as I assumed that I'd get it everywhere when using the stencils.  I was right.  I wasn't sure how to clean it but mild soapy water did the job.

Solder Paste Stencil
Solder Paste Stencil

Solder Paste Stencil
Part of Stencil for LEDs

Solder Paste Stencil
Part of Stencil for Resistors

Solder Paste Stencil
Stencil label
Once again I used my toaster oven to solder the boards.  It has no automatic controls, I stand there and watch the board and time the steps by counting aloud to myself.  The temperature is also set manually by turning the dial.  The process is described in a previous blog post.  It helps to put a little bit of solder paste on a fiducial mark so that I can see the moment the solder liquefies through the oven window.

PCB Reflow
PCB in Oven After Reflow
It's important to shield the board from direct IR radiation from the heating elements, that's why there are trays above and below the board.

PCB Reflow
PCB in Oven after Reflow
After the surface mount parts were soldered the through hole parts were added by hand.

PCB
Assembled Board with LEDs
When I designed the board I screwed up and made the holes in the footprint for the terminal blocks too small.  This means they had to be enlarged by hand, and because of the way the tracks were laid out, the positive terminal block has to go on the other side of the board.  No biggie.

PCB
Assembled Board

PCB
Assembled Board

Surface Mount Resistors
Surface Mount Resistors
The parts seem to have been soldered nicely.  There isn't an excess of solder or too little, and there's that nice little fillet you expect to see as well.

Surface Mount Resistors
0.25 Watt 1206 Resistor
The LEDs are harder to judge as the pads are under the board, but they all work.

Surface Mount LED
Osram GW JCLMS1.EC-GUHQ-5L7N-1 LED
Just to prove they all work.

LED Grid
LED Grid
To show the reconfigurable nature of the board I disconnected the jumpers that power the middle section of the LED grid.

LED Grid
LED Grid with Sections Turned Off
Over all I'm very happy with the results.  Obviously boards produced this way aren't going to be of the same standard as professionally made ones, but these are rather sturdy and fine for prototypes.  I'd gladly use OSHstencils again just for the time that it saves me.

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