FISHERS (project pt.5)

 -ASSEMBLING CIRCUIT BOARDS:

Circuit board day–my favorite day! For some reason, waiting for my circuit boards to arrive from JLC PCB always makes me super anxious. I know that they will arrive on time, and be perfectly made to my specifications, but for those 5-10 days between ordering and them arriving, I can't help but feel like something terrible is going to happen, and won't get them because I wake up to a reality where ordering circuit boards for as little as 5 dollars including shipping is not even remotely possible, and I have to go back to perf-board circuits with through-hole parts. Or maybe I'm anxious because I am constantly checking the very detailed progress of the manufacturing processes of my circuit boards on the JLC website. Either way, when they do arrive on my porch in bright blue packaging, it is the closest thing to the feeling of waking up on Christmas morning as a kid, but better! 

All four sets of my circuit boards arrived safely, and I didn't have to pay any trump-tariffs, so that's a plus. I usually just go with green mask and HASL plating for the lower cost and faster turnaround. The circuit boards look great, and seem to fit nicely in the enclosure. 

Solder Paste Dispenser

Populating the circuit boards is a pretty time consuming process since I pick-and-place all of the parts by hand. Each of the four boards will probably take an hour each. To apply the solder paste to the circuit boards, I use a solder paste syringe applicator that I bought on Tindie years ago. It has a motorized plunger thing that can be precisely controlled to apply a small dot of solder paste to each pad. There are probably thousands of pads to dab with solder, and it can take a long time and be kind frustrating if there's an air pocket or the paste is old and crusty. The advantage though, is that I can control how much paste goes on. I like to use extra, because I usually buy cheap lead-free low temp solder, and I don't use it very often, so there is usually a good chance it is old. The solder paste still works fine when it's old, but I don't think it would be consistent enough to apply using a stencil. Then again, I have never tried using old paste with a stencil, so I really don't know.

Moving!

Our new home!

Placing all of the components is actually pretty easy but time consuming. I keep most of my SMD components in little jewel cases that can all be clipped together in groups or categories depending on what I need them for. One small container can easily hold over 1000 0805 parts, so I am able to have all of my parts right next to the work. All I need are my good tweezers and my magnifier headset, and I am ready to go. 

$15 at Harbor Freight!

SMD parts

When populating a board that has more than a few hundred parts, it is really helpful to export a BOM from the circuit board file to a text document, and strip it down to just the SMD parts and quantities, so that it is easier to grab only as many of the individual components as I need from my inventory. That way I know I didn't miss any. It is also useful to have the circuit board file open, so that if I have 35 10K resistors to find the footprints for, I can simply use the search function in KiCad to locate each one in the drawing one-by-one by just hitting 'Enter' successively after typing '10K' in the search box. Before I figured this out, I was spending so much time searching the circuit board for all of the component's footprints, I would start to get motion sickness from looking back and forth between my screen and the circuit board's tiny silkscreened graphics.

Once all of the solder paste and components are placed, I use a programmable hotplate to flow all of the solder to the parts. It is a standard 10x10cm work area model, but I opted for the one that has a nice wide outer area that is useful for putting my smoke absorber right next to the work. It also keeps me from getting burned when I am adjusting components with tweezers when pads get bridged with solder. The hot plate takes up a lot of desk space, but it is pretty light and easy to put away. Before I bought it, I was just using a hot-air wand, which works, but doesn't do a very good job at burning off the flux, so it leaves a gooey mess behind. The hot-air wand can also overheat sensitive parts pretty easily. The hot plate is better because it heats up at a slower rate that is probably closer to what the solder manufacturers recommend. Most of the 'organics' burn off cleanly, and there is hardly ever any solder bridging that can't easily be jostled free with an Xacto knife while the solder is still flowing. 

After the boards have cooled, it is time to install all of the through-hole components and hardware. I like to start with the lowest profile parts first just so that I can flip the board onto a flat surface to solder the other side without having all of the parts just fall out the top side. First the big chips go in. Then the resistors, diodes, capacitors and transistors. Then the push-button switches. The potentiometers and slide switches hold themselves in pretty well, but it is important to make an effort to check that each one is soldered flush against the top of the circuit board, otherwise they have to be de-soldered and reseated, which is no fun.

Top side of 'Main' board

Bottom side of 'Main' board

I usually like to hold off on installing the LEDs just because once they are soldered into place, it can be kind of difficult to line up all of the LEDs with their holes in the faceplate if I should need to remove the circuit board from the faceplate for any reason during debugging–which is always going to happen. This time I went ahead and installed all of the LEDs anyway, because seeing them light up during debugging is actually a lot easier than connecting my oscilloscope all over the place to be sure that everything is working. To make it easier to re-align the LEDs with their holes, I just soldered them in low to the top of the circuit board with extra long leads on the back so that when everything is ready, the LEDs solder can simply be reheated, and the LEDs can be pushed/pulled into position to be flush with the surface of the faceplate. 

The last step for preparing the circuit boards is installing the standoffs. I like to use M2 nylon standoffs bolted to the circuit boards. Then the circuit boards can be attached to the faceplates with countersunk M2 screws. The M2 screws come in various lengths and colors, and are usually pretty cheap in bulk. I usually use black, but I decided to go with chrome this time. I think it will look good, and I need to use some up.

Part 6 coming soon...