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--- process_stencils [2022/03/01 11:29] – created benh
+++ process_stencils [2022/05/23 10:54] – benh
@@ Line 23: / Line 23: @@
 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 8x12 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.
-  - Log into the ProtoLaser. Turn on the tool and/or start the software if they are not already on and running.
+  - Log into the ProtoLaser. Turn on the tool and/or start the software if they are not already on and running. Brush off the processing surface; it needs to be clear of debris so that the polyimide can sit flat.
   - Start a new project ("File" > "New").
   - Select the "Stencil" template. (Note: I'm not sure what the "short" version is, so just use the regular one.)
@@ Line 36: / Line 36: @@
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 ===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 ([[https://www.sparkfun.com/tutorials/58|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 [[https://www.lpkfusa.com/datasheets/prototyping/zelprint.pdf|LPKF ProtoPrint S]]). Apply paste over the stencil using whatever tool you have, either the syringe, a wooden spatula/tongue depressor, whatever. You don't need a lot, so start with less than you think you need. 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 ([[https://www.pcbway.com/blog/PCB_Design_Tutorial/A_very_easy_way_to_accurately_align_the_stencil_with_the_PCB_1.html|source]])). You shouldn't need to press down very hard, and again, maintain the stencil as flat against the board as possible.
+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 ([[https://www.sparkfun.com/tutorials/58|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 [[https://www.lpkfusa.com/datasheets/prototyping/zelprint.pdf|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 ([[https://www.pcbway.com/blog/PCB_Design_Tutorial/A_very_easy_way_to_accurately_align_the_stencil_with_the_PCB_1.html|source]])). You shouldn't need to press down very hard, and again, maintain the stencil as flat against the board as possible.
 {{ ::stencil-usage.png?direct |}}
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 **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.
+One other thing to note about this process is that it's extremely likely that it will look like the pads are shorted together, i.e. the paste will connect across pads. This is typically okay! When the paste melts, it will naturally wick to the nearby copper, which is hotter than the soldermask and/or the bare FR4. **BE AWARE!!** If you have a bare board, i.e. without soldermask, it's critical to etch copper away from between the pads, or you //**will**// get shorted pads later. This must be done prior to board fabrication. (Soldermasking can be attempted with Kapton tape, but YMMV and it's a very unsupported process. See [[process_kaptonMask|this page]] for how you can try that.)
 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.