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Questions about Vias and Traces

samman1 , 08-17-2022, 06:17 AM
I'm trying to understand vias and traces and there seem to be minimal forms/groups out there that are active when it comes to help for beginners on Orcad Capture/PCB Editor 17.4. I watched your 17.2 and 17.4 videos and have updated Orcad as older versions were missing 3D Mapper and other things shown in the videos.

Is there a trace width calculator that you follow with regards to how much current is flowing through? If the current is 5A, then a trace of 0.012 is too small and will likely burn up like a fuse. Also, inner layers need trace size twice as big compared to external layers. Also what is the general rule for vias in respect to trace size?

As you can see here I have a signal carrying 0.8A. Originally the entire trace had 12mil trace and 12mil vias. Since the inner layer is generally double the size, I beefed up the entire trace to 20mil. Now do I only need to change layer_1 to 20mil or the entire trace? Also my vias are at 12mil. Do I need to change them to 22mil or keep it at 12mil? I have two vias in my padstack which are 12mil and 22mil.

Also, I have this trace for Color 2 Input that carries 5A. Again, originally these had 12mil traces and 12mil vias. I put a trace of 40mil that goes through the bottom layer (Signal, Layer_1 GND, Layer_2 VCC, Signal). When viaing to the bottom layer, one .010 via is not sufficient to pass 5A of current. How do I use multiple vias and a copper area to do this?

  • Traces should be sized to carry current at operating/full range of temperature. It is suggested to calculate current needs to determine trace sizes. Does the noted 5A trace (and same for the 2A traces) actually require 5A? The trace (N1126286) from U31p3 to U35p4 and U35p8 and Color 2 Input trace to U31p2 are to carry 5A, then the trace is too small at 0.012 and will likely burn up like a fuse. The recommended size for a continuous 5A external trace at 60°C is 0.050 (universal) for a 1 oz copper board or 0.025 for 2oz copper board on an external copper layer. If you via to the bottom layer, one .010 via is not sufficient to pass 5A of current. Use multiple vias/larger vias to satisfy current needs. EX: dual .030 vias, five .010 vias, etc. You can use multiple vias and a copper area to accomplish this. I am not certain how you will get a trace this size to the placement of D10. If this is a 50% duty cycle feed line with no overlap of “all heads on” then the trace can be smaller as .050 is a baseline for 5A continuous current. If on the inner layer, the trace needs to increase to at least 0.100 to carry 5A.
**NOTE: Internal layers require much larger traces. **
  • For calculations, I used a 0°C starting temperature, 55°C rise (I don’t know what average temperature the PCB reaches as it will operate at all different ranges). This calculator yields slightly different values than the “universal”; but, is still sufficient for use. There are many others available. I just use this one because it is very simple and once you know the trace length, it will show you voltage drop and power loss if you need to know it… https://www.4pcb.com/trace-width-calculator.html
  • The vias and traces for each LED line (all traces as well) should be sized to handle the current to each head from the LED driver with temperature variation. Verify the current in the strings of LEDs is less than .5A. Some traces carry current to two LED groups which puts those traces at double the LED current. If they are border-line acceptable, increase the via size & trace size. A 0.012 via (0.010 drill) is only able to carry 1.1A at a temperature and will choke down the current if trying to run more. If the LEDs are driven over .5A, you should probably increase the trace size and via size as well. If they are under .5A, the 12mil traces will be sufficient on outer layers; but, all traces on the inner Layer 1 need would need to increase in size proportionately to the current. At 25°C the inner layer traces should be at least 17.6mil. 18mil or 20mil size would be better.
robertferanec , 08-18-2022, 08:54 AM
Saturn PCB Toolkit is a free software with number o useful calculators, including maximum track current. This video may help: https://youtu.be/fJCRrEf_IH8
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