I have developed a technique to make my own paste screens using hobby cutting machine (Cricut).
These machines lack the resolution to create a proper paste screen but, after experimentation I have discovered I can get acceptable results for home SMT assembly by using a custom shape. I can regularly solder TQFP-48s, 0603 resistor networks, and so on, without any issues. Rather than modifying the paste layer of the footprint, which may cause issues if I get the boards professionally manufactured, I make my own “special” paste mask (see the picture below. I place this on layer User 1. The problem is, when I plot, Layer 1 is plotted on the top and bottom.
I’d like to know if there is a way I can display a user layer conditional on what side of the PCB it is on.
I tried that after I posted. The problem is that lets say I have a part on top and the same part on the bottom. When I print top or bottom, whatever user layer is show is displayed. I need to be able to print a user layer only when it it on one side.
I guess the good news is it is really hard to hand assemble boards with components on both sides so I tend to only have a small number on the back …
The problem is that if you show User Layer 1 or 2 it shows up on both sides. You’d have to have a “top layer” footprint and a “bottom layer footprint”
I have a Cricut Maker, although I don’t think the actual machine or cutting method makes a difference.
I use (polyester?) transparent overhead projector sheets for a laser printer because they cut well, are strong enough, and are about 100 microns (4 mil) thick. Maybe there is a better material but that is what I use. They are also cheap and available off Amazon.
I make my own paste mask using the following rules
each row of pads with dimension WxL has a separate “strip”. N = number pads in the row,
the area of the strip = N * W * L (sum of the area of the pads under the strip)
the length of the strip SL = ( N+1) * W
the width of the strip = ( N * W * L ) / SL
place the strip over the pads where the pin will contact the pads. In most cases, this is center of the pads. Placement is important for adhesion during placement prior to soldering.
I make a frame out of old PCBs stuck to a piece of masonite with double stick tape. This frame is open at one end to hold my PCB, so it makes an upside down “U”. I place a PCB in the frame and position the paste mask over and tape it on the top with heavy duty packing tape.
I usually do about 4 or 5 boards at a time. I don’t know how long the paste mask lasts because I haven’t worn one out yet.
Using this approach a need to rework is because of solder paste issues (too dry, lumpy, etc.) and I am experimenting fixing that. I have had better luck with this approach for home use than purchased aluminum screens - though that may be because of experience.
I should add that getting the sheet from Kicad to the Cricut software is harder than it needs to be. I plot to DXF, and for utterly unknow reasons Cricut re-scales the drawing and breaks each cut into a separate object. I suspect Cricut software developers are infected by the same idiocy as Arduino IDE developers.
In any event, I include a scale item beyond the max width of the board (i.e. 100 mm) and I scale the drawing so that is 100 mm wide. Then I delete the scale item and “weld” all the parts so they don’t move when they are plotted.
I use 1206 resistors and capacitors as well as SOT-23 transistors. The Cricut can cut these but it is hit and miss and the quality is not that good.
So I make my strips for resistors and capacitor extra long and take care to align the components on the PCB so, as much as possible, the strips of adjacent parts overlap so instead of a bunch of closely spaced pads for each terminal of a 1206, you get 2 strips which cover them all.
The other boss of the house has issues with scales, I’ll ask her tomorrow.
Lumpy solder paste is cured by making a fluid of pine tree rosin dissolved in alcohol and using that to dilute the lumpy paste… works a charm.
I bought some proper SMT flux and alcohol to do some trials.
I also discovered the you can get steel mesh coffee filters which are extremely fine and I suspect that after I mix I can run the paste through that and de-lumpify it. I am waiting for my next batch to arrive from JLC to try.
Interesting to know what she says about scales. I’m not exactly a Cricut expert and I find their software weird to use so maybe there is a better way.
You’d think that the minimum for any hard copy device is it producing what you display …
