In this article I show my process for doing a pre-order fit check on a new PCB design being done in Kicad. This process is easy to do and well worth the effort. You only need a printer and the parts you plan to use on the board.
There have been several recent articles on the web related to PCB layout and ordering. The steps I describe here are good to use for any PCB and are easy to do. You will be surprised at the errors you discover with this process.
The trick is to print the layers at 1:1 scale onto cardstock, then insert all the parts to insure that the lead spacing is correct and verify the placement of the parts. This is especially useful if you have connectors and indicators that must line up with an enclosure.
My board is two sided with the bottom being a ground plane. The design is for a solder-less breadboard power supply module. The PCB will have two sets of headers to mate with both power rails on the typical breadboard.
The first step is finding the best settings to get a single sheet printout with your board design with the silk screen and pad placement in place. I have only through hole components on the component side. I selected to print all the layers in Kicad except the bottom layer as that obscures the silk screen. Also select to print all the layers on a single sheet and the accurate 1:1 printing option.
I like to use 110 weight paper which is like the paper used for business cards. Buy this at any paper supply outlet. You want standard sized paper, not the stuff that is precut for business cards. Most printers can handle this paper without any special care but my laser printer had a separate feed tray for envelopes or thick paper. 110 weight paper is not so thick that it is likely to damage any printer but you should be careful.
If you don’t have 110 weight paper you can use standard printer paper for of the most checks. If you want to fit the board into an enclosure you could glue the paper to some thin cardboard like the covers of most spiral notebooks are made from.
Place the printed paper on top of some foam to support it as you press though the components. I use the lead on a higher wattage resistor to pre-punch the holes for items that have fragile leads, like the IC or smaller resistor parts. Just punch the parts into the center of each pad and verify that everything fits as expected. The foam piece shown in the picture was left by a carpet cleaning company that did some work in my home. I saw them being thrown away and scooped them up. They are very dense foam, perfect for this application. If anyone knows where to get this stuff let us know with a comment.
Using this pre check process you can verify the size, placement, and lead spacing on each part. I caught several errors including component body size, lead spacing, and mating connector placements. As you can see in the pictures the bread board connector spacing was initially wrong.
While investigating the connector placement error I found that the printer has a slight scaling error that showed up over the width of my board. I did have a placement problem which was discovered using this process but when the error was fixed it still did not fit. I got out the calipers and discovered that my printer had a slight error which showed up in the long dimension of my board.
To find the exact printer scale I added two long, 5" horizontal and vertical scale lines to the drawing. The scale lines need to be long so that you can get an accurate measure. Kicad has a X and Y printing scale that allows fro very accurate control. Surprisingly after I added the scale lines, the overall accuracy was fine. Adding the long lines must have corrected some issue in the printer driver. The key thing learned here is that you should always verify scaling in both the horizontal and vertical direction.
I also suggest that you get a good pair of dial calipers if you plan to do PCB layout work. You will have to make PCB footprints for parts and having a way to make accurate measurements is essential. Of course the drawings for parts should be used but they can often be hard to follow. Calipers will make figuring things out much easier. You can pick up a good caliper for about $25. The caliper shown above has a digital readout and easily switches between mm and inches. I even use it just as a conversion tool sometimes. I normally prefer a caliper with a dial but this one has its advantages. I think it costs about $50.
Do you have a useful trick for the PCB design or order process? Leave a comment and let us know.