Feed on
Posts
Comments

I’ve had some rudimentary experience with surface mount (SMT) soldering in the past. The method I learned basically involves squeezing out solder paste from a syringe onto the PCB, placing the components, and then using a hot skillet to reflow. It’s pretty easy to learn. But if I want to produce assembled PCBs in large quantity, the syringe method is not going to work well. Instead, I need to use a solder paste stencil to quickly apply solder paste onto PCBs.

Solder paste stencils (aka SMT stencils) are expensive if you want to order them. Even a small piece would cost $30-$40, and that’s often the price for just one. I wanted to learn how to make stencils myself, so I can produce them quickly, and as many as I want. I don’t own a laser cutter. Fortunately, I found this really clever method from Instructables. After trying it a couple times, I’ve got a good grip of it. Below I document the process with pictures. The method is essentially based on the Instructables article above, but combined with my own experience and tips.

Preparation

To begin, you need thin copper sheet. The thickness of the copper sheet should be around 3 to 4mil (0.003 to 0.004 inch). I got mine from Amazon: 12 inch by 12 inch for a reasonable price. Use scissors to cut a small piece from the sheet. The size should be twice as large as the actual PCB size, to avoid solder paste from running off the stencil.
Clean the copper surface using some Bar Keepers Friend.
Now use a laser printer to print the color inverse of the top stencil mask onto a piece of magazine paper. Use the highest toner darkness. The color inverse means you want the stencil masks to be white, and the rest to be black. After etching, the solder masks will become holes in the sheet, thus creating a ‘stencil’.
Now use the toner transfer method to transfer the print to the copper sheet. There are many online instructions for the toner transfer method, but according to my experience, the following two tips make it really a breeze. 1) Use magazine papers for the printout. Don’t use glossy photo papers — they are expensive and difficult to peel off. Magazine papers work like a charm. 2) Use a GBC laminator for transfer. It’s much, much easier (did I say much?) than a hot iron. Place the printout face down on the copper sheet, and run it through the laminator at least 10 times. Then throw the copper to hot water. After 5 minute you can peel off the magazine paper. Really easy.

Etching

The next step is very important: you need to cover the bare copper surfaces with tape. This includes both the front and the back sides. You don’t want them to be etched out. Of course the tape should not run too much into the region of the printout — you don’t want to cover any solder mask holes that should be etched out.
Now it’s time to throw the thing into an etchant and let the exposed copper be etched out! Instead of Ferric Chloride, I like using the fast solution that consists of one part muriatic acid + two parts hydrogen peroxide. Very cheap in terms of material cost, and very effective. You can get muriatic acid from Lowes. Just be careful: do not let your skin touch it, and do not breath it directly.
After etching is complete, carefully remove the tapes. Then use Acetone to remove the black toner. Finally, polish the copper surface with a Scotch-Brite scrub sponge. Voila, the solder paste stencil is done! Looks quite pretty.

If you are experienced, the whole process should take no more than 40 minutes. You can make as many stencils as you want. I haven’t tried making a stencil with really small component footprints. I will try that at some point and see how accurate the stencil can be.

Apply Solder Paste

Now it’s time to see some stenciling in action! First, use three spare PCBs to make a tight frame, and insert the target PCB into the frame.
My target PCB today is the AASaver / Breakboard Power Supply. This is a little boost converter that bumps a voltage as low as 0.8V to 5V or 3.3V, which can be used to power bright LEDs or serve as breakboard power supply.
Carefully position the stencil on top of the target PCB, and align the holes with the solder pads. After you are satisfied with the alignment, use a tape to fix the left edge of the stencil. This way, you can easily life the stencil up when you are done, and you don’t have to re-align for the next PCB.
The next step is to apply the solder paste. This is a bit nasty. You need to prepare a putty knife (available in home improvement stores) and some solder paste. I got my solder paste from DealExtreme.com. Cheap and works really well (the only downside is the long shipping time).
I didn’t make a video to show how to apply the solder paste. Fortunately, Sparkfun electronics has a great video tutorial which I included on the left. Check it out!
This is the outcome of applying solder paste to nine PCBs. Click on the image to see a zoom-in view. It’s really fast.

Place SMT Components


Placing SMT components onto the PCB is often the trickiest step. These kids are so tiny that even a small breath from your mouth will blow them away. Not only are they difficult to hold still, but it’s tricky to get them out from their packages.

But don’t worry, I found a good way to hold them still so they won’t get lost easily. The method is very simple. Just use some magnets! Most SMT components attract well to magnets. You can 1) easily dump components from their packages to the magnetic surface, and 2) move or even lift up the magnets, and the components won’t fall off. So next time when you travel, don’t forget to buy some extra magnets!

Use tweezers to carefully move components from the magnets to the PCBs, and place them in the right spots. Here is the outcome of SMT placement.

As some point I will need to buy a pick and place machine to automate this step.

Hot Skillet Reflow


Now it’s time for some hot skillet action. Place several component populated PCBs into an electric skillet. Turn to the maximum power. In about 2-3 minutes the reflow will start to happen. As soon as it’s done, turn off the power and let the PCBs cool down. If you’ve never done the reflow before, check out Sparkfun’s awesome Reflow Skillet tutorial.

You should do this outdoors because the fumes produced during reflow are toxic, and you certainly don’t want to breathe them! The picture on the left shows the outcome of the reflow.

Lastly, perform careful visual checking to see if every component is soldered properly. Also do a basic electric test to see if the circuit functions correctly.

The Final Result

Here you are, a fully functioning AA Saver / Breakboard Power Supply! I will write another blog article shortly to introduce this small yet very useful circuit. Stay tuned.

18 Responses to “Learn to Make Solder Paste Stencils”

  1. Christian Lerche says:

    Hey, nice tutorial, one thing I notice:
    What about the backside? The acid would probably eat this up, if you didn’t cover the whole backside with tape right?

    Lerche

  2. Matt says:

    In the picture under “Apply Solder Paste” where you build the jig, what tape are you using? Standard electricians tape, or something much thinner? Doesn’t the tape cause the stencil to sit above the board slightly, and wouldn’t this cause the solder paste to ooze under the stencil? Or is it that the stencil is too thin to deliver enough paste?

    Matt

    • ray says:

      That’s the standard electric tape. You are right that it does raise the height a little bit. It’s a good point. I should move the tape away from the stencil, or cut out the part that overlaps with the stencil. The stencil itself has sufficient thickness to deliver enough paste.

  3. PCB says:

    Good advice and very detailed pictures and tutorials. Do you definitely have to do it outside or could you get away with doing it in a room that’s very well ventilated?

  4. Jeff says:

    Great process! I wonder why you don’t etch holes in the corners where the screw holes in the boards are– would make for easy registration with pins.
    THanks for the details!

    • Taylor says:

      Registration isn’t that tough, but if you had holes where the screws go, they’d either fill up with solder paste or the alignment pins would get in the way of your pasting. Though you could align, then tape it up, then remove the pins, then tape up the holes.

  5. […] I applied solder paste to the PCB using my home-made solder paste stencil. I then placed the circuit board on the PCB holder of the machine. Make sure you push the PCB all […]

  6. At Porcupine Creek, Fred is pushing through the pain and this the
    lesson of the pain is lost on them. Sowash says these accidents are no surprise, in
    a sport where part of the game means pushing a boat to
    its limits. However, if your usage is strictly recreational, the 14
    day rule may be enforced. Simpson, who won a Gold Grills medal for Canada!
    The start was overdue.

  7. John says:

    This is a great tutorial, but if you don’t mind spending a few dollars there’s a good service called OSH Stencils oshstencils.com that you can have professional laser cut stencils made for next to nothing. I am a big fan of making stencils with aluminum cans much like you did here, but now I just buy from these guys cause it’s so cheap it doesn’t make sense to make them myself any longer.

    • ray says:

      Hi John: yes, oshstencil.com is a great service and I really like them. For production, I am actually using a type of steel stencil (laser cut and with a frame) ordered from China. See this blog post: http://rayshobby.net/?p=6833 for pictures. The home-made stencil is still fun and good for ‘emergency’ (e.g. need to prototype in a hurry and cannot wait for professionally made stencils etc :)

      • John says:

        Cool Ray! Steel stencils are great for bigger runs for sure, and I heard OSH Stencils is working on a plan to offer those real soon. Will be interesting to see if they can maintain ultra low price points like they have with their film stencils.

  8. Andrew says:

    Hello, You mention solder paste from DealExtreme. I assume the BST-506 ?
    This type requires storage in frig. right ? I asked source if shipped in ice. Business said not shipped in ice. Doesn’t it degrade the product if not kept cool ? You mention long shipping time.
    I have reflow oven and in past, bought lead free no clean solder paste. It is rather expensive and for prototype, maybe better to use cheap solder paste.

    • ray says:

      Yes the quality does degrade. So it’s better to use a fast shipping method such as DHL. In general, not refrigerating it just reduces the shelf-life of the solder paste. Unless if you are doing very fine pitches ICs, the degradation probably won’t affect the reflow quality at all.

      Lead-free solder paste requires higher temperature to reflow, and generally requires more care.

  9. ray says:

    Mine is a Dell 2230d laser printer. Check your toner level, and if it’s low, take out the toner, give it a good shake and try again.

Leave a Reply