Layout review for PCB, LED signage - Page 1
Forum section: electronics / projects

author: 128er
author_rank: Regular Contributor
timestamp: June 05, 2015, 11:44:16 am
content: Hi!In the past weeks I was working on a little LED signage project. It would be great if more experienced users could do a review of my PCB layout (If i have done any big mistakes).This is my first PCB which will be professionaly made (not etched by myself). I have no experince with that. But it would be great if it could work at the first try. Did I missed something, that could go wrong at production? Don't worry about silkscreen over pads on the pictures. That is not a final silkscreen.Some problems that I see at the moment . . .-I think the LEDs are way to powerful for backlighting such a small area (Roughly 130mmx20mm). These are the LEDs I want to use (not only red):http://de.farnell.com/osram/lse67b-t2v1-1-1-z/led-smd-rot/dp/1226388?ost=1226388And I think the lighting could be much more even, when I choose more tiny LEDs instead of a few big ones. Maybe I could add footprints for smaller LEDs on the module PCBs, to have the choice of changing them.-At the moment I'm not sure which PCB house to choose. Finaly I have to check the design against there designrules. I already checked my design with the OSHpark designrules. And all seems fine, apart from a clearance issue at C14 and C15 (decoupling at MCU).-What should I do with respect to the seperate boards? Panelize them? I would prefer the cheapest way.Thank you for your time and sharing your knowledge!

author: 128er
author_rank: Regular Contributor
timestamp: June 05, 2015, 11:49:47 am
content: Layout

author: 128er
author_rank: Regular Contributor
timestamp: June 05, 2015, 11:50:33 am
content: More layout and enclosure

author: RR
author_rank: Contributor
timestamp: June 05, 2015, 12:25:19 pm
content: Hi iam not expert so just some obvious details-T1-isnt that holes too small?-C14,C15-i would add more vias separate and not just share one-CS3,C5-you forgot to connect one via to gndWhat is the purpose of that extra diode on led modules?Did you do math about cooling for 7805?On digital inputs wouldnt just resistor be sufficient?

author: 128er
author_rank: Regular Contributor
timestamp: June 05, 2015, 01:25:23 pm
content: Quote from: RR on June 05, 2015, 12:25:19 pmHoles should be good. Maybe the padsize could be a little bit larger.Quote from: RR on June 05, 2015, 12:25:19 pmThanks, even two vias at CS3 are not connected!Quote from: RR on June 05, 2015, 12:25:19 pmReverse protection. Could be omitted, because the driver provides that.Quote from: RR on June 05, 2015, 12:25:19 pmD'oh! You got me. How could I forget about that. Total noob fail.A quick check says, that it will die.Power dissipation with roughly 20 mA for the 5V rail should be:(24V-5V) * 20mA=380mWMax. power dissipation for 78L05 in SOIC 8 package:ThetaJA= 180 °C/WTjmax= 80°CTamb= 50°CPdiss= (80°C-50°C) / 180°C/W =166 mWAm I right about that?Quote from: RR on June 05, 2015, 12:25:19 pmSure. Bu at least a diode for reverse protection should be there. The zener diode thing is, to get a threshold. I could delete that.Thank you so far for your input!

author: schwarzrmsu
author_rank: Contributor
timestamp: June 05, 2015, 02:00:00 pm
content: I would double check your part to part clearances for your SOIC's. I would be a bit more conservative and space the diode/passives from the end of the SOIC's a bit more just to be safe. Plus you have a lot of room on the board.Example:Also may want to double check the thermal relief for parts as well. I see you added thermal relief for certain parts connected to certain planes, but it seems like you have some parts connected to large planes with no thermal relief.

author: 128er
author_rank: Regular Contributor
timestamp: June 08, 2015, 11:53:47 am
content: Quote from: 128er on June 05, 2015, 01:25:23 pmWould the UA78L05CPK from TI suit my needs? It comes in a SOT-89 package with ThetaJA=52°C/W. With that I calculated a max Pdissof 577 mW for the SOT-89 type.http://www.farnell.com/datasheets/1848990.pdf

author: richard.cs
author_rank: Super Contributor
timestamp: June 08, 2015, 02:47:02 pm
content: A SOT-89 package is definately better thermally. Watch out that the pins are reversed from the way you might expect.Another option is to add a resistor in series with the input to drop most of the volts. You then dissipate the power into a cheap part with a high Tmax.

author: 128er
author_rank: Regular Contributor
timestamp: June 08, 2015, 04:01:38 pm
content: New approach to 5v supply

author: ovnr
author_rank: Frequent Contributor
timestamp: June 08, 2015, 05:53:29 pm
content: I'd go for a DPAK instead, and add tons of via stitching to your groundplane. You have plenty of room after all.Edit: Also, you have thermal isolation cuts on the main pad on your SO89. That's completely counterproductive when you want to distribute the heat to your copper pour.