Beginner SMD design  - Page 1
Forum section: electronics / general

author: Hughieandrolf
author_rank: Newbie
timestamp: October 15, 2015, 03:04:44 pm
content: Hi.I'm a first-timer here. please be kind...I'm designing a PCB for a home built clock project around a 44 pin TQF PIC16F887 using Eagle PCB design software.I'm hoping to self etch prototype PCBs, (I've done this a few times using through-hole components) and if all goes well, have 20 or so PCBs professionally made.I want to keep the PCB as small as possible and I have a few questions:Is it a good idea to route tracks via the pads of N/C pins of components to gain access to other pads from the inside of the chip footprint?Can tracks attach to any edge or corner of a pad or should it always be 'axial' to the component?Is it good/bad practice to route tracks between pads and under components as I would do with say a axial lead resistor or between rows of DIP pins?Is it OK to lay components at 45 degrees to others?Cheers.

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 24, 2015, 09:19:21 am
content: Welcome to the forum!Quote from: Hughieandrolf on October 15, 2015, 03:04:44 pmFor this one, it's generally not a good idea because the N/C pins could be grounded, diagnostics, etc.As this stackexchange question states:(http://electronics.stackexchange.com/questions/8496/can-i-run-traces-through-no-connection-pins)QuoteQuote from: Hughieandrolf on October 15, 2015, 03:04:44 pmThat should be fine.This video might be able to help you:Sorry, I can't answer the other questions, hopefully a more experienced member can inform you.

author: Simon
author_rank: Unknown
timestamp: October 25, 2015, 09:08:03 am
content: no solder resist on a TQF ? good luck, what are the pin pitches ?

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 25, 2015, 09:12:54 am
content: Quote from: Simon on October 25, 2015, 09:08:03 amLooks like 0.65mm between centres... eek

author: Simon
author_rank: Unknown
timestamp: October 25, 2015, 09:15:10 am
content: I just get cheap prototype PCB's made these days, no point in prototyping something that will be made in a certain way in production in a way that is near impossible to prove the prototype.

author: tggzzz
author_rank: Super Contributor
timestamp: October 25, 2015, 10:21:41 am
content: Quote from: Hughieandrolf on October 15, 2015, 03:04:44 pmWelcome, and don't be afraid to make mistakes - but try to ensure they arenewmistakes!I've recently returned to fabbing PCBs after a 20 year hiatus - so I've been in your position recently. My experiences are athttps://entertaininghacks.wordpress.com/category/homebrew-pcbs/Toner-transfer is just about good enough for 0.1" PTH double sided boards. I wouldn't try it for fine-pitch SMD components. You will spend a long time debuggin your manufacturing technique.If you are making 20, then you might as well use the same implementation for the prototype - otherwise you will do debugging twice.Cheap'n'cheerful PCBs (double sided, solder mask, silk screen, 0.5mm pitch SMD components) can cost £10 for 10 delivered in 10 days; see the above reference for the source I used.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 25, 2015, 11:40:31 pm
content: Quote from: Hughieandrolf on October 15, 2015, 03:04:44 pmCompletely depends on the component. Most components just say "NC", "no connect", and I leave those alone. Some say "DNC", "do not connect", and youhaveto leave those alone. Some say "NIC", "no internal connection", and you can do whatever you want with them (within reason, of course; no component not rated for it is going to like 1000V appearing on a random pin). And some components flat out don't tell you the whole story in their datasheets. For example, the AD9257 has two "DNC" pins, which should mean they have to be left alone. But ADI engineers say it's OK if they're grounded, as they were on the earlier, otherwise-pin-compatible AD9222. So sometimes you can ground NC pins, sometimes you can't.I just leave the damned things alone, and it's never served me wrong.Quote from: Hughieandrolf on October 15, 2015, 03:04:44 pmTraces coming out straight from the center of a pad are best, but there's no hard and fast rules. SMT is pretty forgiving when it comes to this, at least at larger pad pitches. The smaller things get, the more careful you must be. I have had no trouble with all sorts of crazy trace routing on 0603 component pads.Quote from: Hughieandrolf on October 15, 2015, 03:04:44 pmThis is OK (in general) if and only if you have solder mask. With solder mask, you can run a trace between the pads of an 0603 resistor with no trouble. Without solder mask, I think trying to do that would likely end in frustration. As others have said, this is a very good incentive to get professionally fabricated prototype PCBs. It's so cheap these days that there's little reason to bother doing it yourself.Quote from: Hughieandrolf on October 15, 2015, 03:04:44 pmSure. Modern pick and place machines have no problem with that (as long as the usual mechanical clearances are maintained, of course).

author: KL27x
author_rank: Super Contributor
timestamp: November 22, 2015, 12:51:54 am
content: QuoteNo luck needed. 0.65mm pitch is no problem. I have done this aplenty.None of my production flex boards have soldermask between the pins. I've soldered hundreds of 0.65mm pitch IC's on them by hand. And that was just today.Done hundreds of toner transfer boards with no mask, same pitch, too. I've also proto'd a few 0.5mm pitch pcb's with toner transfer. No problem, at all.Soldermask is important to cover copper that isn't supposed to get tinned. It isn't particularly important to have where there's no copper.Toner transfer can get down to maybe 10 mils with all-day-reliability, if you do it right. Smaller in a pinch, if you care to do more examination/testing and throw away some boards. Down to 6 mils is maybe the practical limit where unevenness in the etching starts to become nearly as wide as your trace - and at this level, you should probably be using 1/2 oz copper, and your etching technique better be good.OP, no problem routing traces thru pins you KNOW are unused and high impedance. If you're not using all your pins, for instance, you might intentionally route a special function pin through the pad for an unused pin and just make sure that pin is left in the digital input state. The only traces I would be wary of doing this are ICSP traces, if you're using them. (Obviously can't do this with the high voltage programming line, but I would avoid doing it with any of them; weird stuff can happen to the unused pin during programming).No problem routing traces under SMD parts. I have routed traces between 0603 resistors (custom footprint to widen the gap) and thru SOT 23 pads (standard footprint), using toner transfer with no problem. The disclaimer is I have been doing toner transfer and etching with a custom CuCl2 tank for quite a while.

author: Christopher
author_rank: Frequent Contributor
timestamp: November 22, 2015, 10:13:58 am
content: Quote from: Simon on October 25, 2015, 09:08:03 amwe have an lpkf milling machine, qfp and even qfn is possible with lots of strong fluxbut yeah i much rather get the boards sentout

author: KL27x
author_rank: Super Contributor
timestamp: November 22, 2015, 09:23:06 pm
content: Milling leaves a rough surface that might be stickier to solder. I have found essentially no difference in soldering TQFP on regular etched board with or without mask. You always need enough flux.