Critique my first board! - Page 1
Forum section: electronics / general

author: paolo
author_rank: Contributor
timestamp: November 28, 2012, 05:24:06 pm
content: So, day 2 of being a pretend PCB designer - and I have something where my 'fix list' has dried up. So here's your chance to ruin any sense of satisfaction I currently haveFirstly a brief word about the overall layout. The requirement for this is for it to be narrow. I've got maybe an inch and bit to play with on the Y axis.Secondly I wanted to make it breadboardable. So the interface pads at the edge will have header pins. Maybe right angle if I can find more space in the enclosure, otherwise straight. Easy to pop into a breadboard for testing / prototyping a bigger system.Electrical stuff... the outputs on the two 12 pin headers will each be sinking up to 30ma per pin. That means the common ground might be returning up to 720ma per board. So I decided to use a spare pin on the bus headers and bi-wire it. The VCC won't be carrying anything like that (there's a seperate source for the high current driver need, exposed via the VLED jumper. It doesn't interact with the circuit on the board).Everything else is 'just' logic carrying SPI type stuff. The fastest signals will be about 1MHz tops.So, err, *blushes*, here it is... (DipTrace btw)The board is here:https://docs.google.com/open?id=0B3a3xsH0-EKabVZ6VTRWNjJhQVEThe schematic is very slightly out of sync with the board, but it gives you the general idea.https://docs.google.com/open?id=0B3a3xsH0-EKaTzBRWXYyNHBnVkE*fingers in ears*------------------ETA... The board top and bottom as PNG files:https://docs.google.com/open?id=0B3a3xsH0-EKaSmVlX24wdXpvNm8https://docs.google.com/open?id=0B3a3xsH0-EKaN0xMNkRhMk0xMnM

author: mtkaalund
author_rank: Contributor
timestamp: November 28, 2012, 05:27:32 pm
content: Is it possible to get a pdf or a picture of it?

author: 8086
author_rank: Super Contributor
timestamp: November 28, 2012, 05:27:38 pm
content: PDFs or images would be nice for those of us without diptrace.

author: paolo
author_rank: Contributor
timestamp: November 28, 2012, 06:48:05 pm
content: Ah, ok. Will do some PDFs when I get home.(Anyone mega-curious, there's a freebie version of DipTrace that's downloadable).

author: paolo
author_rank: Contributor
timestamp: November 28, 2012, 11:29:19 pm
content: So, here's the top and bottom as PNG files.https://docs.google.com/open?id=0B3a3xsH0-EKaSmVlX24wdXpvNm8https://docs.google.com/open?id=0B3a3xsH0-EKaN0xMNkRhMk0xMnM

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 29, 2012, 12:12:23 am
content: Quote from: 8086 on November 28, 2012, 05:27:38 pmdiptrace is as fee as any other pdf viewer, with more toolsjust 60MB download.

author: paolo
author_rank: Contributor
timestamp: November 29, 2012, 12:41:00 am
content: Hmm. Those screen grabs didn't show any solder mask.Here's a 3D render of the top, as a PNG:https://docs.google.com/open?id=0B3a3xsH0-EKaVkF6WjR3cDc0Vnc

author: free_electron
author_rank: Super Contributor
timestamp: November 29, 2012, 01:01:01 am
content: don't need any mounting holes ?

author: 8086
author_rank: Super Contributor
timestamp: November 29, 2012, 01:34:48 am
content: Quote from: paolo on November 28, 2012, 11:29:19 pmPut thermals on the resistor and capacitor pads. Otherwise looks alright.Quote from: Mechatrommer on November 29, 2012, 12:12:23 amI don't think I will ever need diptrace, so 60MB and install to never use it again...well, no thanks

author: paolo
author_rank: Contributor
timestamp: November 29, 2012, 02:00:31 am
content: Quote from: free_electron on November 29, 2012, 01:01:01 amGood point.I don't need mounting holes for this project, but I should allow for that in case the boards get repurposed. To avoid growing the board size they'll have to go on that spare edge, which wouldn't be brilliantly stable, but better that than nothing. Good tip.

author: paolo
author_rank: Contributor
timestamp: November 29, 2012, 02:17:48 am
content: Quote from: 8086 on November 29, 2012, 01:34:48 amAh.... glad you mentioned this. It was something I was going to ask about.When I was designing this, I was using an 'official' TI demo board for inspiration. I saw that they didn't have any thermals. I kind of scratched my head and thought, hey, they must know better than me. But I dunno. Maybe they had reasons I don't understand.Their thermal-less board design is here:http://e2e.ti.com/cfs-file.ashx/__key/CommunityServer-Discussions-Components-Files/192/3872.TLC5947_5F00_Layout_5F00_PCB.pdfAny guesses as to why they ended up with that?I think I saw something on one of their forums about maximising copper around decoupling components. Then again maybe I imagined that - will have to go back and check.

author: 8086
author_rank: Super Contributor
timestamp: November 29, 2012, 02:26:14 am
content: Quote from: paolo on November 29, 2012, 02:17:48 amA lot of example layouts, particularly for things like switching power supplies, don't have thermals. That's fine on paper, but it makes assembly more difficult and less reliable and repeatable. I'm not entirely sure why they like to forget thermals but you have to take what you read and what you know, and put them together to achieve a good solution. Thermals are almost always a good thing to have.

author: paolo
author_rank: Contributor
timestamp: November 29, 2012, 02:33:45 am
content: Quote from: 8086 on November 29, 2012, 02:26:14 amOK, that's sold it to me. Thermals it is then.Assembly is my next challenge and I'd rather not make things any more difficult than it will inevitably be.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 29, 2012, 04:24:06 am
content: i put 1mm thermal cross on my recent (and the first too) pcb, its a bit PITA to solder using my low power (and temp controlled) iron. my imagination tells me that thermal is not necessary if i have a bigger iron which works fine with my hakko "near infinite" power iron.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: November 29, 2012, 07:46:16 am
content: One thing to remember when asking for help...the easier it is to view a schematic or board layout, the more people will look at it. Word .DOC, Eagle, Diptrace, any proprietary format is going to limit how many people look at a file. PDFs and jpg or png files are standards that most people have no problem opening.Even if there are free viewers for a proprietary format, most people aren't going to take the time to install something new.It's also most convenient for people who may try to help to attach files to posts here. This prevents broken links and difficult to access files.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 29, 2012, 11:57:47 am
content: noob here - what's a thermal?

author: 8086
author_rank: Super Contributor
timestamp: November 29, 2012, 12:31:28 pm
content: Quote from: dr_p on November 29, 2012, 11:57:47 amhttp://en.wikipedia.org/wiki/Thermal_relief