Fun with PCB stencils

When you’re assembling just one or two pieces of your homebrew electronic stuff you never think about how fast the process is. But then comes a moment when you need to assemble 10 or 15 pieces, and it’s still cheaper to do it at home. The process quickly becomes dull as hell, and the brain starts to think of all the ways to speed up the process.

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The stencil comes in really handy here. Soldering SMD with a stencil looks somewhat like that:

  • Put on the stencil (it has the holes for all the smd pads) on the PCB
  • Take an old plastic card (Or a raquel)
  • Carefully apply the solderpaste to all the pads
  • Remove the stencil
  • Place all your SMD components
  • Heat up with a heat gun or put into your PCB oven ™
  • Solder all the through-hole components
  • Wash with your favorite washing powder in ultrasonic bath (No, really. Washing powder, the one you use for clothes works great for PCBs!)
  • PROFIT

The problem is – professional stencil costs just about the same sum you’ll pay for a few dozens of your PCBs. Even at cheapest Chinese fabs.

Today I’ll tell you how I solve this problem using some relatively cheap hardware. After experimenting with lasercut stencils (they sucked!) I purchased a Silhouette Portrait cutting machine. You can get one for about 150$ on ebay. It’s original purpose -cut paper, cloth, whatever. Designers love these. Folks that use Silhoette for the same purpose as me already created some awesome software called gerber2graphtec. That takes in a gerber file and cuts it out on the Portrait, applying size corrections as needed.

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Consumables for this awesome machine are: Knifes and mats. Original knifes come sealed in a plastic barrel, cost about 25$ each (ouch!). But Chinese folk offer aluminum knife barrel and sets of spare knifes. It was about 30$ for this “barrel” and 2 boxes of replaceable knifes, 5 pcs each. The knifes come with different angles, sharper ones give more precision, so they were my choice for stencils.

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Mats are thick sticky plastic film with grid. Feels like those protective films for phone and tablet screens (the sticky side). They cost about 25$/pcs, so using tablet protective film as a mat is cheaper.

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The cheapest is using the temporary adhesive spray like Krylon Easytack or one of it’s endless clones and a transparent film. This transparent film for laser printers (e.g. lomond brand etc.) are my material for stecils. Their thickness is roughly the same as the IPC recommended for SMT stencils. This brings us really close to serious ™ industry ;). On the photo below you can have a look at the freshly cut stencils.

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At first I used duct tape to secure the stencil, next switched to kapton tape for better results. Everything was great, save for one issue. When applying paste some would go under the stencil and resulted in minor bugs. These could be easily solved by carefully inspecting what you’ve soldered, but that still could use some improvement. Remember that re-positionable adhesive spray I’ve mentioned above? Spraying the bottom side of the stencil with it makes results just perfect! Just have a look:

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All you need is just stick the stencil to your PCB:

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Apply the paste (I use local-made Тиноль, and add the Chinese ripoff of Amtech flux if it gets dry)

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Remove the stencil and have a look at the results:
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Carefully place all of the components (tiresome)

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Heat it up. As you can see on the vid below – TQFP gets soldered in place with not a single pin gone wrong:

This stencil serves for a long time. I’ve assembled over 15 copies of that red board on the pics with the stencil and it’s still sticky. Rinse it with warm water and soap to wash away dust – and it will restick again once dry. The more aggressive stuff you use to wash the dishes washes this adhesive away, so you can always wash the glue and spray it with adhesive once more. How long can these be stored? Mine are already over 6 months old and still sticky!

As for the storage – either stick them on a PCB, or something that will be easy to unstick it from. e.g. that special toner transfer paper.

 

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