process_stencils

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Table of Contents

  1. What is a stencil?
  2. How to make a stencil
  3. How to use a stencil

What is a stencil?

A stencil is a material with cutouts that enables solderpaste to be more easily and precisely applied prior to component placement and reflow. (If you don't know what I mean by 'reflow', or want more information about that process, see our “Reflow Oven” page for more.) Stencils can be made with a variety of materials; the images below show two very common ones (polyimide on the left, and stainless steel on the right) (Source for images). Kapton tape (also polyimide) is another low-cost option. The Hive has 5mil polyimide sheets for stencil fabrication that are free for users, as well as Kapton tape.

Stencils are by no means a required or necessary step for completing your PCB. They are very useful if you have many surface-mounted components (SMDs), SMDs with many pins or fine-pitch pins, or have a few SMDs but many identical boards. They do not work for through-hole components! Those must be done by hand.


How to make a stencil

Stencils can be fabricated at The Hive using the LPKF ProtoLaser U4. The process detailed here is for 5 mil/0.125mm polyimide ONLY. The Hive provides this material free of charge to our users. For other accepted materials, you'll have to adapt the process below. Stainless steel is not allowed in the ProtoLaser!! If you require a stainless steel stencil, please email us (hive-pcb@ece.gatech.edu).

(If you have never used the ProtoLaser before, you'll need to be trained prior to use; see this page for details on that. Stencils made of non-metallic materials, like polyimide, can also be made with our laser cutters, but that's beyond the scope of this write-up.)

Note: This process is detailed in the LPKF ProtoLaser's “How-to” document, which can be found both here, and on the ProtoLaser itself (in the “ProtoLaser Docs” folder on the Desktop). See section 4.1, p. 120.

Before you start, you'll want to make sure that you have your solderpaste gerber files ready, along with your piece of polyimide. Note that we (typically) have either full 8×12 sheets, or a 24“-wide roll. If you need a full sheet, use one. Otherwise, use scissors to cut an appropriately-sized piece from the roll, making sure to leave enough room for a sizeable (~1 inch) frame/boarder around the design.

  1. Log into the ProtoLaser. Turn on the tool and/or start the software if they are not already on and running.
  2. Start a new project (“File” > “New”).
  3. Select the “Stencil” template. (Note: I'm not sure what the “short” version is, so just use the regular one.)
  4. Select “Polyimide_0.125mm (non-laminated)” as the Material type, and “0.125mm” as the Material thickness.
  5. “File” > “Import”, and select the solderpaste gerber file(s). Note that you need one stencil per side of the board, so only select a single gerber file, unless your sheet is large enough for both. Make sure they are set to the correct layers (either SolderPasteTop or SolderPasterBottom) before clicking “OK”.
  6. At this stage, we want to position the design in the software so it cuts correctly on the material.
    1. If you have a full sheet, click-and-drag to select the entire design, and then right-click and select “Move object”. Click on a position in the design to use as a reference point, and then click where you want the design to be moved to. Repeat as necessary to position the design within the boarder.
    2. If you have a specially-cut piece (as in, one that isn't the size of the bed), click over to the “Processing” pane and use the pilot laser (the black laser button in the main toolbar) to make sure the material is placed such that the design is in the center of the material.
  7. “Toolpaths” > “Computer All” to computer the toolpath. Click “Close” when completed.
  8. Press the green play button to begin processing. Follow the steps as requested.

How to use a stencil

Once the stencil is fabricated, it must be aligned with the bare board underneath. The stencil should sit against the board as flat as possible! This is very important for applying the correct amount of paste, as well as not pushing paste underneath the stencil (rather than through the cutouts). Because the stencil is often cut out of a sheet of material larger than the PCB itself, often some unused PCB material of the same thickness (scraps or fresh, whatever you have) is used to surround the PCB and provide support to the stencil (see image below left (source)). Make sure to secure everything down so it doesn't move! Typically tape is used for this, but The Hive also has a fancy frame system that you can use (the LPKF ProtoPrint S).

Apply paste over the stencil using whatever tool you have, either a syringe, a wooden spatula/tongue depressor, whatever. You don't need a lot, so start with less than you think you need. (NOTE!! The reflow oven that The Hive has is optimized for lead-free solder paste, so if you're planning on using our oven, please make sure to use lead-free paste, or you'll be sad later.)

Then use a squeegee of some sort (credit card, ID, plastic scraper) to squeegee the applied paste across the stencil and into the cutouts (see image on right (source)). You shouldn't need to press down very hard, and again, maintain the stencil as flat against the board as possible.

You can always lift the stencil to check your progress, but you run the risk of ruining the alignment, so be careful.

Once the squeegee'ing is done, lift off the stencil and confirm paste is where you expect and not where you don't. If you got some excess outside where you wanted, that's okay, just scrape it off (because solderpaste is expensive, so we'll want to re-use as much as possible), then clean the PCB of any remainder, and try again.

NOTE!! The human eye is very good at detecting mismatched edges! However, this process is surprisingly tolerant, especially with regular-pitch components. Try not to be too picky. You can also always reflow again, or cut traces later if needed. It's a bit of trial and error to get a feel for this.

After the paste is applied to your satisfaction, move your board and all the components for placement to near where the reflow will happen (either a bench with a reflow station if you're a masochist, or near the reflow oven in the semi-clean room). You want to do the actual placement of components near to where the reflow will happen because the less you need to move the board with all your non-soldered-on components, the better. Using tweezers (we have plenty upstairs), place each component onto the PCB in the proper position. Double check the component and orientation prior to placement! You'll be sad later if you discover you put a key IC on backwards and fried it. Pin 1 is often indicated with a dot.

Once you've placed all your components, you can reflow the board to melt all the solder in place at once. See our ”How to reflow“ page for how to use the reflow oven.

  • process_stencils.1646153173.txt.gz
  • Last modified: 2022/03/01 11:46
  • by benh