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Homebrew SMT stencil

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This article describes how to make a SMT stencil at home.

Its capabilities are limited (probably due to my non-optimized processes), but sufficient for 0603 passives, SO ICs, etc.

You need the following stuff:

- a thin brass plate (foil matches better) 0.1mm thick

- "Positiv20" fhoto resist spray (brass plates are also abailable already prepared with photoresist - in this case you don't need the spray)

- FeCl3 solution to dissolve the brass (same as for pcb etching)

- Laserprinter and matching transparent foil

- you pcb's paste data as gerber file (e.g. generated from eagle, etc)

- Additional stuff like some Acetone to clean the brass, etc...

Here is the top layer layout for my prototype 5mm-pitch-led-screen that uses a MY9268 driver IC (more on this another time)

Eagle' s CAM-Processor allows an easy export of the layer's paste data as "Gerber" file. Other pcb CAD programms allow that too.

This leaves me with a .GTP file for the top layers Paste data

Use a free Gerber viewing tool like gerbv to display the layer.

We need to change a few things in this file. the Gerber file format is plain text, so you can open it with any text editor

The first marked region defines, whether the mask is positive or negative. Since we want to use a photopositive mask (remove everything, that was exposed to light) we need to change the "IPPOS" statement to "IPNEG"

The second marked region defines the different apertures (shapes) used in this file. In our example there are just two. The surrounding lines and the pad of the 3528 sized RGB led.

The third and fourth section now lists the use and position of each of these previously defined shapes. Here, we remove the lines, and we also reduce the pad size (in its definition) by ~10%. We don't need much solder paste, and the holes in the brass foil will grow a bit later, so this turned out to be a good value.

After the modifications the file looks like this:

Now open the file again in gerbv and change, and change the color of the layer to black, and the baclgroundcolor to white (found in the "view" menu).

Now use the "export" function of gerbv to generate a .pdf file from the stencil. Then use the "Layer -> modify orientation" tool in gerbv to mirror the image, and generate another pdf file. This is important beacuse we will expose the brass foil from both sides, and the printed side of the transparent foil must have "contact" with the brass.For this layout this is not neccessary, since it is point-symmetric and its mirror is the same...

Glue both foils together and make one side short enough, to stick the brass to the foils with a piece of tape.

Now cut a matching piece from the brass. A normal scissors can do this, take care not to bend the metal too much.

Clean the brass foil with fine sandpaper and acetone (Or anything else you have at hand). Keep the surface dust-free, this is important!

Now apply the Positiv20 to the brass. I have not found the optimal way to do so yet, but the quite irregular result like on the photo is good enough for the relatively large structures on this layout. I also dried the piece in the (reflow/pizza) oven a few minutes afterwards.

For exposure you have to find the optimal time values, that depend on your equipment. In my case (~15cm distance from a face tanner (cheap on ebay) with a 5mm acrylic cover to kep the transparent foils flat) 6 minutes per side are fine.

When flipping the sandwich over, be sure not to move the brass inside the transparent pouch (use tape to stick the brass to the pouch like stated above)

After proper exposure time to both sides, prepare a solution of ~3g NaOh in 300ml water. Using warm water (not hot) will speed the next process up.

Put the exposed brass into the solution (gloves, glasses) and wait/help (for) the exposed parts of the Positiv20 to vanish.

Afterwards, cover the blank brass pieces that you want to keep with some tape (kapton in my case)

Then put the whole thing into the FeCl3 solution (gloves/glasses - spilled drops on clothes are very hard to remove!!). Wait an occasionally stir the cauldron.

After quite some time the etching process is done:

And after some cleaning your STM stencil is done!

Now its time to apply the solderpaste to the pcb

Finally place the components and reflow:

Finally it turned out, there were some few solder joints between adjactent pads, so a maybe pad size reduction of ~20% instead of 10% would be better, but nevertheless this SMT stencil production procedure can easily be used to manufacture low-tech stencils for prototyping, etc. - nevertheless there are professional stencils made of stainless steel avaiable for just around 35EUR - in case you are willing to wait for them ~2 weeks :)


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