PCB Traces and Widths Between - Page 1
Forum section: electronics / general

author: Falcon69
author_rank: Super Contributor
timestamp: March 21, 2015, 03:58:34 am
content: Hello Peoples.I am thinking of redoing my PCB Boards to make them a little smaller.The PCB Is mostly consisting of CMOS Logic Gates, Some MOSFETS and Transistors. At the End of each circuit (of eight individual circuits) there will be an OPTOMOS.My questions are.......If I use 0.15875mm tracks with the same spacing between tracks, at what point do I need to start worrying about signal or voltage jumping the tracks?Would that same space (0.15875mm) be okay between the pads of the resistors/caps as well?This is not a sensitive circuit, meaning, it is only used for a switching application using Hall Effect Sensors and LED's, that's it.I will also plan on updating to 0402 components vs. the 0603 I'm using now.The problem I have with the boards right now is that I am using .2mm trace width and space between, but on a 0.15875 grid (2.54/16), the space between ends up being like 0.26mm and it just doesn't layout good at that.

author: Rerouter
author_rank: Super Contributor
timestamp: March 21, 2015, 04:10:58 am
content: there are tons of calculators out there for the minimum gap to withstand a voltage, but as the gap gets smaller there will always be an increase in leakage current, its only really something to worry about if you have anything with a very high impedance signal, e.g. greater that 1Meg,as you shrink your component sizes, pay attention to the power being dissipated, and if you want to shrink furthur, see how you can simplify your circuit.

author: Falcon69
author_rank: Super Contributor
timestamp: March 21, 2015, 04:16:01 am
content: Yes, I realize about the power when scaling down the components.I found a calculator. According to it, a trace width of 0.15875 can maintain 630mA in an external trace. I will only have about 100mA max through these traces.The calculator says nothing about the space needed between tracks for leakage current.Do You have a link to one that does?

author: Rerouter
author_rank: Super Contributor
timestamp: March 21, 2015, 04:19:37 am
content: http://www.smps.us/pcbtracespacing.html

author: Falcon69
author_rank: Super Contributor
timestamp: March 21, 2015, 04:25:07 am
content: I found this as well...ClearanceThe required spacing between conductors is determined by the voltage difference between them and the amount of leakage current you can tolerate. Leakage current is primarily associated with surface contamination of the PCB (e.g., residual flux, as well as accumulated dust, moisture, etc.).One guideline comes from safety testing services such as UL, which requires a creepage distance of 5mm per kilovolt for circuits that are supposed to be "isolated" from each other (material group I, pollution degree 2 from UL840).... By that logic, 5mm/1000v=0.005mm/1vSo if I have 5.32volts going through the traces, then that is equal to a space distance between traces of 0.0266mm. That would be WELL within the 0.15875mm I am planning.

author: Falcon69
author_rank: Super Contributor
timestamp: March 21, 2015, 04:39:24 am
content: Okay, thank you guys.According to those links and information you provided, at the 5.32 volts, low mA, and a space between widths of 0.15875mm, I should be okay. Even if the Fab House is off alittle.The Calculator link that Rerouter supplied conforms with UL, and it says that 0.1mm width between is what is needed, even for voltages 3 times what I will have.

author: Rerouter
author_rank: Super Contributor
timestamp: March 21, 2015, 06:15:49 am
content: another thing to keep in mind if you have fast rising edges on your signals is crosstalk, as the gap is smaller, the parasitic capacitance is bigger,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2015, 09:50:25 am
content: I'd keep 20 mil spacing (0.5mm) in the power supply section, around the higher voltage stuff. This is overkill for 24V. Everywhere else, you are more than safe to use the fab's minimum width/space rules, which are usually 6/6 mil (0.1524mm).Mind the trace width for power, and ground if applicable. And also trace length, since you'll have a lot of power running back and forth, and you don't want much voltage drop.Ground should be filled on both sides, so current capacity shouldn't matter much, but do make sure to inspect that a thick and very redundant ground path is available to all components. Power should be preferentially routed around the heaviest loads first (which I guess will be your LEDs and plug-in dongles, IIRC?). Don't forget bypasses every inch or two (not necessary per chip, but you don't want too much distance between, and again, make sure there is a clear path for ground as well).Tim

author: Falcon69
author_rank: Super Contributor
timestamp: March 21, 2015, 04:05:37 pm
content: Thanks Tim.The whole circuit gets supplied with a walWart of 7.2-24volt, depending on which walWart is on hand at the time. From there, it goes into a reverse Polarity Protection MOSFET circuit, and then to a Voltage Regulator to bring the voltage to 5.32volts. The entire circuit with all the Hall Switches activated and LED's, won;t see more than 2 amps total. That would be it for the main power trace. I plan to keep about .5mm spacing for that.