Questions for making my first PCBs - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: March 11, 2020, 07:48:08 am
content: I'm trying to design my first pcb ever. However i never got a formal EE education so i don't know about any rules i may not know (don't put this so close to that, traces need to be wider/routed differently for xyz reasons, etc)If i design the circuit in EAGLE and let it auto-generate a PCB layout for me, and then put the screw holes where i need them to be, is that "good enough"?I'm trying to make fresh PCB's to replace an old power supply board that's no longer made, and seem like it was "designed to fail" (two 85c capacitors almost touching two regulators running extremely hot) - even i know thats not good for longevity.

author: Rerouter
author_rank: Super Contributor
timestamp: March 11, 2020, 08:02:37 am
content: Lots of little rules, for simpler stuff throwing an autorouter at it can get you by, but be aware it is not also an autoplacer, e.g. if you just drag a pcb edge around the components exactly as they where added, it will struggle,Normally I begin by grouping up things into chunks based on where there doing, e.g. here is an amplifier, here is the power supply, here is a micro, make them look nice, then bring the groups together to make it all hook up,For a power supply, keep the heat sources away from the capacitors within reason, if a trace needs to have 1A flow through it, keep it thicker, if more current, use a trace calculator (they are online if not in eagle)If something is passing a lot of current, usually you aim to keep it shorter, so it drops less voltage and produces less heat.There are many many small rules that add up, but its hard to point at enough rules of thumbs to get people started easily. PCB layout is like art, ask 3 different people to make X and you will get 3 different X's, they may be similar, but depending on what little rules of thumb each person knows, and how they prioritize each of them, you get very different layouts of the same thing.

author: kjr18
author_rank: Regular Contributor
timestamp: March 12, 2020, 02:38:19 pm
content: I think you could try to redesign existing power supply so capacitors wouldn't be so close to heat sources. I guess that you have board size restrictions, so I think dividing schematic into functional blocks could help you. Photos of existing power supply would help.

author: JKKDev
author_rank: Regular Contributor
timestamp: March 12, 2020, 02:50:48 pm
content: In our PCB designing course at UNI professor said multiple times: "If I see you used auto routing it's an automatic fail."When it comes to actual rules I'm afraid I lost what little knowledge I gained there as I didn't practice for the last 5 years (it's a shame but I'll have to relearn).

author: wraper
author_rank: Unknown
timestamp: March 12, 2020, 03:18:56 pm
content: Quote from: Mp3 on March 11, 2020, 07:48:08 amYou'd be far better with just replacing those capacitors rather than replacing whole PCB with autorouted crap. Autorouter won't pace parts for you, and by default will make something with very thin traces completely not suitable for PSU.

author: aeberbach
author_rank: Regular Contributor
timestamp: March 13, 2020, 02:23:19 am
content: I have done boards that I couldn't have designed - give me a schematic and I can do it even if I can't explain the circuitI like to look at the PCB manufacturer information and see if there are price breaks on hole/via/tracks size and proximity.If anything is going to get hot often the manufacturer will offer a thermal design guide and recommend minimum copper weight etc.Always get the designer to review the finished board.Consider how easy your placement will make manufacturing and rework - no 0805 parts right up against big melty plastic connectors, for example.Learn your PCB package but choose carefully - Eagle is fine if you are sure you don't mind paying for it one day, as one day your board might need to be larger than what free allows you to make. I like KiCad.Never, ever autorouter - not that KiCad has one!

author: Rerouter
author_rank: Super Contributor
timestamp: March 13, 2020, 01:54:32 pm
content: Kicad does have a few as plugin options, but yes, I am not a fan of auto routers in there current flavor, too much setup needed to get decent results, however for a small and simple board, beginners are free to lean on them then do some easy cleanup or part shuffling.

author: k8byp
author_rank: Newbie
timestamp: March 13, 2020, 05:22:53 pm
content: "If i design the circuit in EAGLE and let it auto-generate a PCB layout for me,"That doesnt constitute you designing a circuit board, but there's nothing wrong with that, unless you are looking to learn about the process.BTW, there's no such thing as a "printed circuit board" i.e. copper traces on a board, its aPrinted Wiring Board. Printed circuits have literal components placed directly on the board substrate.Just a little Trivia point...Yes, group parts on the board by function ANDas or more critical, you MUST know what signals, currents etc travel on which traces and SEPARATE ones that will have cross-talk and interference.This can easily make the difference between a board that works and one that doesn't. Cheap schematic capture and spit a trace layout out packages cant do that. Thats EMC compatibility and that is $$$$Without any EE knowledge , thats going to be next to impossible to do a proper trace layout. If its just a PS board and you know what the failure was then signal integrity probably isnt relevant, unless there are any sensitive control voltage circuits.Wanna really learn something? Make the board by hand. Ive made many that way, from bare copper boards. Lay down traces, etch it by hand...coat the boards with resist, lay down tape tracks, expose and etch it. Big fun.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2020, 09:19:18 pm
content: Forget auto-routing, it is for the most part a useless gimmick, I have *never* seen what I'd consider to be really good results from an auto router, in the best of cases it's a starting point. For your first boards you should definitely be manually routing though, you will get much better results and learn more in the process.Also forget Eagle, ever since Autodesk acquired them they have gone subscription-only and it's a complete dead end. All the hobbyists have migrated over to KiCad, it's a better system and has no artificially imposed limitations whatsoever. It's quite good and is getting better all the time.

author: twospoons
author_rank: Frequent Contributor
timestamp: March 13, 2020, 11:09:04 pm
content: I think the single most important you have to keep in mind when designing a pcb is "Current flows in loops". Its so obvious it gets overlooked, and what it means is that every signal has a return path to the source of that signal. So when you route a trace you should also be aware of the return path for that signal, and make sure its not going to cause trouble. A lot of the time its really not a big deal, but by always keeping this in mind you'll cover the 1% of cases where the return path is just as critical as the signal path.With that out of the way ...- Choose a pcb manufacturer first and find out their process limits (track width, spacing, drill sizes, annular ring sizes, routing sizes, etc). Ideally you want to not be working right at the limits, but back off a bit. Just because they can do 4/4 mil rules doesn't mean you should use 4/4 rules, unless you absolutely have to.- choose your technology - SMT, through hole, mixed - and assembly method - hand or machine. Design with appropriate footprints and spacing. Be aware that some packages are very difficult to hand assemble.- check the sch for nets with special requirements: e.g high current, low noise, high frequency, controlled impedance, controlled timing, high power parts, connectors, etc- Do a floor plan for your layout - sub-section the sch and keep associated parts together - plan the signal flow from section to section (keep in mind the "current flows in loops" thing)-get your placement done, paying attention to special nets and how they need to be tracked-route any special nets- route everything else- run a "design rule check", fix and errors, and check any warnings.- critique your work - often you will find areas you can improveLastly, spend the next 20 years getting good at this.

author: james_s
author_rank: Super Contributor
timestamp: March 14, 2020, 01:58:43 am
content: There's no need to choose a manufacture first, not for the sort of boards most hobbyists are going to be doing. Virtually all the hobbyist friendly Chinese board houses have roughly the same capabilities and there is almost never a reason to go below 10 mil traces.I don't think it takes 20 years to get good at it either, at least depending on what your definition of "good" is. Within about a year of laying out my first board I had done a few dozen of them and was doing double sided boards with mostly surface mount parts, etched at home. Some people pick it up quicker than others, fortunately most typical hobbyist boards are not cutting edge. Almost nobody is going to be designing 8+ layer PC motherboards or whatever.

author: twospoons
author_rank: Frequent Contributor
timestamp: March 14, 2020, 02:52:31 am
content: Picking a fabricator is just a way to know whats possible and whats not.The 20 years comment was meant to be somewhat facetious. You are right in that 95% of what you need to know you'll learn with the first dozen boards. But learning truly goes on forever, if you are paying attention. There's always something to learn to make your next design incrementally better.