Beginner SMD design (copied) - Page 1
Forum section: electronics / beginners

author: Hughieandrolf
author_rank: Newbie
timestamp: October 15, 2015, 04:00:15 pm
content: Hi.I posted this in the Pro (non-beginner) section - Sorry!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: janoc
author_rank: Super Contributor
timestamp: October 15, 2015, 04:51:58 pm
content: Hi and welcome!Quote from: Hughieandrolf on October 15, 2015, 04:00:15 pmNope, don't do that. N/C meansdo not connect, not that the pin is not connected internally. It could be just left floating (dummy) but it could also be internally connected to ground, used for some test/internal signal, whatever. Only the manufacturer knows which case it is.Quote from: Hughieandrolf on October 15, 2015, 04:00:15 pmThe track should attach along the longer axis of the pad, at least that is the recommended practice. It is not a religion, though. Just be aware of some nasty gotchas, such as this one:http://hackaday.com/2015/10/13/fail-of-the-week-marginally-documented-pad-shorts-to-maskless-pcb/Quote from: Hughieandrolf on October 15, 2015, 04:00:15 pmSure, just be aware of the design limits of the board manufacturing process that you or your board house can manage! That you can sneak a trace between two pins of a 0.5mm pin pitch QFP in your CAD doesn't mean that such board can be actually manufactured as well (or it would cost insane amount of money).Also keep in mind electrical constraints - sometimes this sort of thing could be undesirable because of impedance or EMI reasons (e.g. a clock or power trace under a sensitive amplifier). This can be more of a concern with SMD parts because of the generally thinner traces and smaller parts as opposed to through-hole designs, thus the "crosstalk" effects have more chance to bite you.Quote from: Hughieandrolf on October 15, 2015, 04:00:15 pmSure, why not. I wouldn't recommend it in general, but it has more to do with esthetics of the board than anything else. And sometimes you simply don't have a choice - e.g. putting that QFP down at 45deg angle could be the difference whether your design will or will not fit on the board.

author: Hughieandrolf
author_rank: Newbie
timestamp: October 15, 2015, 05:48:47 pm
content: Thank you janoc.I need to make the PCB as small as I can while still keeping it possible to manufacture at home by hand. I've set the TQF at 45 degrees, but I think I'll have to abandon trying to avoid laying signal tracks on the bottom layer.You have been a great help.I hope I have attached the PCB top layer the hell of it.

author: janoc
author_rank: Super Contributor
timestamp: October 16, 2015, 03:31:54 pm
content: The image doesn't seem to have anything on the second layer, only the green vias. However, there are a few unrouted tracks (air wires) - upper left corner has two purple lines, one is towards the center near the decoupling caps (?) and one via.The vias are odd, they don't seem to be connected on the other layer?Why didn't you put the copper pour over the entire board and only the right side? It will save you both etching time and routing work - use the copper pour as ground.Your board doesn't have any mounting holes? How are you going to install it in your project?I am not sure about the clearances on your board - the two pads for the cap (or whatever that is) are actually shorting to adjacent track, some vias are too close to the tracks and will short. Did you run DRC on this?DRC is mandatory- otherwise you are very likely to produce a board with short and incorrectly connected traces. Run the DRC checker and fix every error it will show. That should give you a reasonable chance that the board will be okay.

author: Hughieandrolf
author_rank: Newbie
timestamp: October 19, 2015, 08:59:16 am
content: Thanks for your comments, It's just a screen shot of where I'm up to at the moment to demonstrate placing components on the angle and routing pad connections along the long axis of the component. The bottom layer was hidden at the time of the shot, and I'm still adjusting component positions. The board is told be held in clips formed in a custom 3D printed enclosure.Thanks again.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 19, 2015, 09:28:39 am
content: Don't connect using a line inbetween tqfp/qfn pads. Route away from the pad first before connecting to the other pad.If you don't you'll think there is a short each time you look at it, since the solder will always bridge.Before you continue. Run DRC. You've most likely not set the correct rules yet. Do that first.Get a rules file from a pcb manufacturer (eurocircuits, itead whatever) and run DRC.It also looks like the trace going to the tqfp pads is larger than the pad. I would change that.