Introduction: Solder Stencils
Now that you have your Othermill, there are approximately an oodle of things you can build and create with it. Let’s make something useful for the experienced circuit board designer and novice alike: this tutorial will show you how to make a solder stencil for your custom board.
A solder stencil is a neat accessory that makes it easy to lay down solder for surface-mount components. Tiny components are often too hard or numerous to solder by hand, but a solder stencil makes it easy. It can also facilitate making a bunch of the same board at once.
Solder stencils are used with solder paste, which slicks over the top of the board through the stencil to make reflow-toaster-oven-ready boards for your components.
Traditionally, solder stencils were made on a laser cutter or with etching chemicals, and they sometimes required special stock or sending away to a PCB manufacturer and waiting a week or more until they came back.
With an Othermill, you can make boththe circuit board and the solder stencil in a few minutes with accessible materials.
Keep in mind that due to the way that Eagle exports Gerbers and Otherplan 1.0 currently reads them, Otherplan will render the stencil like it does pads and traces, cutting on the outside of the shape. This means that the diameter of the end you're using will be added to the size of your stencil shape. For instance, with a 0.25" square shape cut with a 1/100" flat end mill, the resulting cutout will be 0.27" square.
There is no option to cut out just the shape itself, as Otherplan can do for holes exported in Excellon files during Gerber export. What this means is that you should try cutting out your stencil with the smallest tool possible, as this will minimize the effect. With thin stencil material you could try using a 30° Engraving bit, which we carry with either a .005" or .003" diameter tip. If thicker, try a 1/100" or 1/64" flat end mill.
Step 1: Tools, Materials, and Files
- Computer with Otherplan installed
- Flat end mill, 1/64''
- Calipers for measuring your material
- Sheet of material of your choice, thin, easily machine-able such as mylar, aluminum flashing, copper foil, etc., that can be used for the stencil itself. We used a piece of laminating pouch material (usually used for laminating paper) that we ran through the laminator (without paper) before we put it in the Othermill. You can even use a soda can as long as it's rigid, impermeable, and big enough to cover your board.
- EAGLE design file for a board for which you'd like to make a stenci
You'll also need to be relatively familiar with EAGLE or willing to learn (in order to generate the file) and have a working knowledge of how to reflow-solder surface-mount (SMD) components with solder paste.
If you've never reflowed solder for SMD parts before, here's a very nice tutorial.
Step 2: Create Your Stencil File
Open up your board in EAGLE. We borrowed this one from Jonathan Foote, an OMC Kickstarter backer and all-around genius, who created it as a high-power LED driver to drive the 10W RGB LEDs in his desk lamp robots.
First create a file for Otherplan that consists of the top and bottom cream layers and the outline of your board. If you're new to this, the cream layers in an EAGLE file are specifically for solder paste. Not all EAGLE boards have them — they have to be created by whomever makes the board.
The easiest way to do this is use EAGLE's computer-aided machining (CAM) processor to export a Gerber file to Otherplan. Gerber files offer a nice, built-in way to isolate the cream and dimension layers from the board and are a format Otherplan can readily import.
- First, Isolate the layers. Click the Layers button (labelled on a screenshot above), and disable everything except for the Dimension layer (20) and either the top Cream (tCream, 31) or the bottom cream (bCream, 32).
- You're welcome to work with both cream layers at the same time if you have 'em, but we find looking at one at a time is a little easier.
- For this particular board, we also enabled the Holes (45) layer so we could be sure to get the alignment right.
- Click on the CAM Processor, either via the toolbar at the top of the window or via the file menu. The CAM window will open.
- While you're in the CAM window, go back up to the File menu and click Open > Job
- Make sure you are in the Open CAM Job window. Navigate to the folder where you installed EagleCAD (usually the Applications folder) and look for the Cam folder. Select Gerb274x.cam. This will open up a handy window with tabs that name parts of your board.
- If you're not in the CAM window when you select Open, it will not offer you the option of opening a CAM job, which can be confusing, so watch out!
Step 3: Set Up Your Gerber
The tabs in the CAM window handle various aspects of the layers and elements that make up your Gerber file.
In the Component tab:
- Under Device, select "GERBER_RS274X"
- Also in the Component tab, click on the File button, and select where you'd like to save the file.
- In the same tab, on the right-hand side of the screen, make sure to select the Dimension layer (20), the tCream layer (31), and, if you have them and want to use them for alignment, the Holes layer (45).
Click on the next tab, Solder Side:
- Leave the device and file settings the same.
- In the Layers tab, select the Dimension (20) layer, and the bCream (32).
Note that the Component side has the Gerber for the top cream layer (tCream) and the Solder side has the Gerber for the bottom cream layer (bCream). If you're only making one stencil for one side, just enter the info for the side you're working with.
Leave the rest of the tabs at their default values.
Once all your entries are made, click on Process Job, and the file will generate and save where you want it.
Step 4: Adjust Your Pad Sizes If Necessary
For most files, just going through the process and clicking on Process Job should be enough.
However, for really small files with tiny, tiny pads, which we had, you might have to adjust the size of the cream layer masks in EAGLE to prevent the Othermill from cutting the holes too big. Holes that are too big will lead to solder where we don't want it, which would not make a very effective stencil now, would it?
You can adjust the sizes of your masks by going into the Design Rule Check (DRC) at the bottom of the left menu panel in EAGLE. Click on the Masks tab, and adjust the Cream mask value to .5mm (millimeters not mils!). If it turns out you need to do this step, you'll need to re-run your CAM job. This will reduce the footprint of your solder stencil, preventing any holes from running into one another and isolating the solder more effectively.
Step 5: Load Your Gerber Into Otherplan
Launch Otherplan and fire up your trusty Othermill.
Click on the Import Files button and import the .cmp file.
The tool you use to cut out your stencil depends on how big the traces are on your cream layer. We cut ours with a 1/64'' end mill, but you can go down to a 1/100'' end mill reliably on this machine.
Take a good look at the pads as rendered in Otherplan. If they look like they won't cut out completely, set the trace clearance to 0.0, and select a smaller tool.
If you have set the smallest tool possible and the traces still aren't cutting, it means you'll have to adjust the pad size and re-run your CAM job as we did in the last step.
The file will show up in your Otherplan window, on the right-hand side. You'll be able to toggle back and forth between the top stencil and the bottom stencil.
Step 6: Cut Your Stencil
Set up your material by clicking Materials Setup and entering the dimensions of your stock.
For material as thin as this, it's a good idea to use some sort of sacrificial layer underneath to protect the machining bed. Selecting single-sided FR-1 as your material and taping your very thin material to the copperless side of the FR-1 will work very nicely. Make sure to measure the thickness of both materials together, including the tape. For tiny, delicate end mills such as the 1/64'' and 1/100'', a few thousandths of an inch make a difference in whether or not you end up with a broken end mill!
Secure the stencil material to the machining bed with double-sided tape or hot glue.
Next, install your tool into the collet and tighten it down. In the Tool Change menu, select the tool you chose in the previous step and follow the tool-locating dialogue boxes.
If you need a refresher on how to set up tools and materials in your Othermill, check our our very fine Getting Started page with links to tutorials on everything you need to know about setting up and running an Othermill.
Once you've reviewed all your settings and dimensions, click Cut in your file's panel and your stencil will take shape!
You can cut both sides of the stencil, if you have them, on the same piece of material if it's big enough. Simply toggle the Top/Bottom button in Otherplan and hit Cut on the applicable file. No need to move the material!
Step 7: A Finished Stencil
Once your file has cut, you should have a lovely finished stencil ready for solder paste. Hurray!
Take the board (no doubt milled on your Othermill) that you made the stencil for, add the stencil to the correct side, and apply solder paste. Re-flow-riffic!
Here we have pictured another board and stencil we made on the Othermill. It's the FabISP board of legend, a nifty little board that allows you to program your microcontrollers on the fly.
If you have questions or comments, let us know! Send us an email at firstname.lastname@example.org. We'd love to hear from you!
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