PCB Design, trace thickness and signal lengh - Page 1
Forum section: electronics / beginners

author: Jackster
author_rank: Frequent Contributor
timestamp: January 22, 2017, 08:48:28 pm
content: Hi all.Two quick questions.Trace thickness for power. I have VIN (5-7v), 5v regulated and 3.3v.It defaults to 0.254mm in CircuitMaker. I am guessing this is fine for signal traces but on these power traces I should enlarge them?On the 3.3v I plan on drawing around 250mA and on the 5v around 50-100mA.The 3.3v runs around 45mm and the 5v is just over 30mm.In regards to signal trace length. I know high speed stuff like motherboards need traces for RAM and PCI to be the same for timings.But for something like a NRF24L01 transceiver. I am fine having them a few mm longer or shorter than the others?Sorry for the newb questions, but got to start somewhere.ThanksJack,

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 09:19:26 pm
content: I have my personal preferences based on many years of PCB design (which doesn't mean I've found the holy grail here, but just what I've found works well)I never go below 1 mm on power/ground traces.1 mm is fine for up to 100 mA2 mm for 200 mAThere is no science or calculation behind this, just that every PCB artwork I've designed has worked from the start and decoupling was unproblematic.

author: Zbig
author_rank: Frequent Contributor
timestamp: January 22, 2017, 09:29:11 pm
content: You might want to check this out:https://www.saturnpcb.com/pcb_toolkit.htm

author: ovnr
author_rank: Frequent Contributor
timestamp: January 22, 2017, 09:57:45 pm
content: Quote from: Benta on January 22, 2017, 09:19:26 pm10 mm trace width per amp is incredibly conservative. If you have room for it, sure, it's fine - but it's not necessary. 1 mm per amp is fine for most purposes, and I wouldn't be afraid to go lower.Here's an online calc:http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/As a general rule of thumb, I'll route normal digital power traces with 0.3mm to 0.5mm traces. If it's a sensitive (... to voltage drop) circuit, I'll go a bit higher if there's room. Pretty much all my signal traces are 0.16mm for digital, 0.25mm for analog, mostly because they stand out a bit so you know to keep noisy crap away from them.Trace length: For the SPI bus on a NRF24L01, you can pretty much do whatever you want. I mean, don't go out of your way to make it into a spaghetti factory, but it's way off from being critical. I'd probably get worried if the length mismatch exceeded 100 cm, and that's more about your sanity than the signal integrity...

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 10:07:41 pm
content: QuoteYou're absolutely right, and I forgot to mention that I mostly do analog designs, where quiet power lines play a role.Your recommendations are good and realistic, I'll keep them in mind for future designs.

author: SPCBD
author_rank: Newbie
timestamp: January 23, 2017, 01:07:24 am
content: You don't need to length match the NRF control / data traces. A 25-35mil wide trace will be fine for that low of amperage.Ken

author: Jackster
author_rank: Frequent Contributor
timestamp: January 23, 2017, 11:02:19 am
content: Thanks for the help everyone!I increased them anyway by a factor of 2 (compared to the default 0.254mm) as I have the board space. Can't harm...Another question I have is about mounting another PCB onto the board directly.I have this buck converter that has four 2.54mm though holes.I need this to be mounted, pretty much on my board because of limited space (as in I can't have 2.5mm of header then then board).Could I just have 4 large pads on the surface of my board and solder it directly on?I have seen it done when they have the through holes in half, like on transceiver boards.But would soldering though the whole be okay? I could always put solder on the pads and out it in the oven?TaJack,

author: ovnr
author_rank: Frequent Contributor
timestamp: January 23, 2017, 01:38:46 pm
content: Put through hole pads on the board, as though you were soldering it on via a header. Then just lay it on top of the holes and solder through both. You can also insert a copper wire through the holes for extra robustness.If you're planning on moving it around a lot, I'd suggest adding some glue underneath it. Also, avoid having any traces underneath it, and if you do, no open pads. Flooding the whole area where it'll be mounted with silkscreen is also not a terrible idea.

author: b_force
author_rank: Super Contributor
timestamp: January 23, 2017, 01:44:32 pm
content: How thick your trace can be also really depends on how long the trace will be.5A trough a small trace is not a problem if it's only for half a mm or so.If that trace is 10cm long it's a different story.Because you're talking about power supply voltages, I assume you're talking about DC (or low freq) currents.High frequency current is again a whole different story (with a lot of headaches)

author: james_s
author_rank: Super Contributor
timestamp: January 24, 2017, 06:25:34 am
content: There is science and calculation with this, there are formulas that can calculate resistance, voltage drop and temperature rise for various trace widths as long as you know the copper thickness. Generally for power and ground traces you want them as wide as they can reasonably be made and for most hobby stuff I don't bother to calculate unless I know I'm pushing it. For signal traces I normally use 10 mil in my designs although sometimes I go thicker when there is space, especially if I want mechanical strength at a pad. Until you get into the several tens of MHz range you generally don't need to worry about matching trace lengths. Even in the kHz though you have to be careful running a signal trace over a ground plane as it forms a capacitor, the effect of which is more significant the higher the frequency.