With all the planning now done, its time to take the partially completed schematics for each PCB and take them through the whole PCB design process so that the manufacturing process can be started.
With the board specific functionality identified and key components on the schematic, an initial footprint matching the enclosure can be produced and placement of devices into the physical layout can be done. This allows different design decisions to be looked at, including how much space you have, where devices would logically fit best, where are the screw mounting holes, where do the connectors go, etc.
This process also allows you to look at any spare resources that may exist and try to work out what to do with them, could they be taken somewhere useful, can you expand the functionality with them, or add new features, should the come out to a pad that can be connected to something in the future ?
Its worth remembering that a printed circuit board (PCB) design costs the same if its got extra traces and device footprints on it for things you might want, compared to a board that doesn't. It only costs more when you come to populate those footprints with components during assembly, or when you come to respin a board to add on things you forgot the first time around. Empty board space is just wasted space and if the space is not needed, then can the board be made smaller or should other features be added.
Iteration
With the design firming up and the board layout starting to come to life, parts lists were going into shopping carts, so they are ready to order. The circuit diagrams are getting cleaner, missing components symbols and footprints were generated or sourced from SnapEDA
Electrical Rules Check (ERC)
With the circuit diagram done, the first check can be completed - Electrical Rules Check (ERC), this makes sure that obvious electrical mistakes were not made, such as :
- No inputs are floating - i.e. inputs with nothing driving them
- Outputs are only driven by one thing - preventing shorted outputs
- Power rails are connected to everything that needs them
- Ground pins are connected to everything that needs them.
- All crossing or touching wires are correctly shown as joined (with a circle) or not - leaving nothing assumed.
Setting up and laying out traces
With this done and all the inevitable problems fixed, we need to move to laying the traces, but before doing that, I need to set up the layers, so that the software knows what type of signals are on each layer, polygons are added for the different power and ground rails on the inner layers. Then I need to update the settings in the design software to match the PCB manufacturers capabilities, so the design software knows how thin traces can be, how small holes can be, how far apart things must be for their machines to actually manufacture what I ask for. These settings are on their web site and may change over time as their equipment is changed out or problems are encountered.
Once the software is configured, then the next step is the routing of traces. To the uninitiated, its just a matter of hitting the Autorouter button and going to get a coffee and watching whilst it does its magic. Those that have already tried that, know that it never really works like that, particularly on any board with any form of complexity. So, the laborious process of manual routing each of the traces could begin. type in "ratsnest" to the CAD software and it updates all the airwires, these show all the links you need to make as straight lines between A and B, in a light yellow on white (nice and easy to see), suddenly the sheer scale of what needs doing hits you.
I made a start and chipped away at this, it took a long time - as in weeks worth of evenings. Many edits were made, beer and coffee may have helped the design process progress. Some devices were replaced with smaller footprint ones to gain space and shopping carts were updated so that the parts that arrive match what I just changed. Some devices were relocated, additional devices added, some ripped up and the main board became very tight and complex, ending up as a 230mm x 180mm 6 layer board, providing 2 signal layers and 4 power layers (GND, 12V, 5V and 3V3). This was getting expensive as area and layer count drive the manufacturing cost. I nearly had to resort to 8 layers to get two more signal layers, but that just pushed the price up by another £100, However, some creativity, re-laying out what I had already done, and some further time allowed me to prevail.
Design Rules Check (DRC)
Once the traces are all laid out, a Design Rules Check (DRC) is performed on the board, this makes sure that what you have laid out, actually matches the design rules you entered from the PCB manufacturer. This results in a sea of errors on your screen and you work your way through them. Perhaps its a clearance issue here, those two traces there are touching and shouldn't be, this wire doesn't go correctly to that junction. The purpose is to make sure that your traces go precisely where they are supposed to and nowhere else.
Airwires
With that all done, it should be complete, but I need to check how many Airwires (un-routed signals) are left ? There should be 0 as this means that all the features defined in the circuit diagram matches an actual trace going between the right places on the PCB. Any mistakes here means that things will not work properly.
The difficulty, at least in the design software I use (the non-cloud version of Eagle), is that its not good at showing these Airwires, you may need to resort to using some User Language Program (ULP) scripts to hunt the Airwire, but at least the scripts tell you which net its on and you can then go looking. This gets harder though when it says its on the ground or one of the power nets, since they go to lots of places.
Eventually though, I prevail and everything is happy ERC passes, DRC passes, there are zero Airwires. I'm ready to make the files and build the PCB.
Rinse and repeat
Not so fast !.. I need to complete all the board designs so I can save on shipping.
Back to the start of the process and onto the next board. I do the same same end to end process on the front panel layout, then again on the power sensing boards. Both of these go into the same enclosure type, so they both come out as 140mm x 100mm, 2 layer boards, with flood fills for the power and grounds. These boards are thankfully a lot simpler, hence the process took a lot less time.
Ready to order ?
That was a lot of work and I'm keen to see my designs and feel the silky smooth boards in my hands.
This is not the time to rush though, rush now and your hopes will be dashed when you spot the problem soon after your boards turn up, or worse still, when you start trying to bring them up.
Stand back, deep breath. Take your time.
- Re-check the Schematics, make sure nothing was missed and nothing else you want to add.
- Re-check your problem log and notes to make sure nothing was missed for physical or electrical changes
- Re-check the physical layout of the boards - dimensions, mounting hole placements, what about the stand-offs in the case, any components in the area around those ?
- Re-do the Electrical Rules Checking (ERC), there should be no errors reported
- Re-do the Design Rules Checking (DRC), there should be no errors reported
- Re-check that there are still zero Airwires
- Print the board out and check the footprints match the size of the devices (hard without the devices)
- Take a break
- Are you really sure you are ready ? Once you are sure, then proceed.
- Run the Computer Aided Manufacturing (CAM) process to generate the manufacturing files
- Generate the drill files, so it know how big the various holes are - so they could load the correct drill sizes in the equipment, although these days, not all the holes are drilled by spinning bits of metal, more often now its by a laser burning through the board, since the hole sizes are getting smaller.
- Generate the Excellon files, so the machine knows where to drill all the holes.
- Generate the Gerber files for each of the layers, so you get the right traces - by etching off what you don't want to keep and so that the machines knows the border of the board is, to mill out later.
- Get a Gerber viewer on-line / use the one in the instructions above
- Part of checking the design before manufacture is ensuring that what you expect is actually in the files that you plan to send. A Gerber viewer does that, exactly like a PDF reader, reads PDF files.
- You load the files, take a look at the PCB to make make sure that the files look like the board you want to produce. If you are browsing around and looking at the traces, positioning of text, where traces go or don't go isn't right, then go back and check your design, re-run the checks and re-create the files. Do this until it looks good.
- When you are 120% confident that its all OK in the files, then Zip up the files that the PCB manufacturer needs, this includes
- The drill files, which holes and where do they go - the Drill and Excelion files
- The Gerber files
- One per layer, showing what each individual layer looks like
- One for each side of the boards solder resist. This is the Green (or other colour) lacquer coating that covers the copper tracks, it resists the solder sticking to it, that things do not short out, its key for surface mount soldering
- One for the silk screen on the top and bottom - what is drawn on the solder resist.
- One for the board outline (cutting / routing) and one for and one for routing
- Check there are no last minute design changes and that I've not missed something - using your design notes, since this is where it starts to cost real money, not just free time.
Finally, take the decision to upload the design files to PCBWay, make sure you order the non-framework stencils for each board, these are just a thin metal sheet, the framework ones come in a metal frame and are used in manufacturing machinery for full production runs, so not needed for home assembly of a couple of boards.
Although you can select stencils for both the top and bottom surfaces, Make sure you only select it for surfaces that actually have components on. There is nothing to manufacture for a stencil with zero components, it would just be a solid sheet of metal. You will get a manufacturing query from PCBWay if you do this and the manufacturing does not progress until the problem is fixed.
Once you have paid the invoice, its time to wait. You will see it progressing through the manufacturing process and they state it 24 hours for manufacturing, but it always seems to be 2 days until the boards ship, at least for for 2 layer boards. For more complex ones, with additional layers, it takes longer as the manufacturing process is repeated on the addition layers.
Now, you just need to wait for the chosen delivery company to actually move the parcel from their factory to my address. For the more expensive boards, its worth using a recognised courier, but for the cheap boards, shipping costs more than manufacturing, so if you are not in a hurry, save the cash and wait a couple of weeks for planes, trains and automobiles to do their thing.