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What are the implications of routing the same signal on the top and bottom of a PCB?

I have a situation where it would save me a via to do this just for about 3mm. I'm routing an I2C signal to a connector on the bottom side whilst my pull-up resistor is on the top side. Components are constrained to the top side, and there are other top-side signals that make it impractical to route the I2C signals on top.

I2C signal on top and bottom

When SDA is low, current will flow from the pull-up resistor to the connector pin and most of it (minus the slave's leakage current) will flow to ground through the master device (green trace on the PCB). Therefore the current on the top and current on the bottom will be flowing in opposite directions. Using the right-hand rule I believe the fields in the traces will cancel between the traces and everything will be peachy (i.e. EMI emissions won't compound). But my signal integrity chops aren't too sharp -- am I overlooking something?

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    \$\begingroup\$ I2C is so low frequency I wouldn't spare a moment worrying about it. \$\endgroup\$ Jan 11, 2017 at 17:23
  • \$\begingroup\$ Agreed, I don't think there's really anything to worry about \$\endgroup\$
    – DerStrom8
    Jan 11, 2017 at 17:30
  • \$\begingroup\$ The trace is so short, there is no problem with an I2C signal, frequency is low and rise time is not extreamly short. \$\endgroup\$
    – Uwe
    Jan 13, 2017 at 8:53

2 Answers 2

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It is not unusual to run the same track over multiple layers to produce a high-current track. I have done this to route 100 A across a PCB.

The I2C signals do not have sharp edges and so the maximum track frequency will be relatively low. They should be fine.

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What you propose to do is fine and normal. IIC is limited to rather low frequencies, so a few mm trace length difference is inconsequential.

Your connector is apparently thru-hole, so the signals are available in all layers. All you're doing is wiring a resistor to one of them. The fact that there are other things wired to other layers of the same thru-hole pad (acting like a via in this case) doesn't matter here.

You are way over-thinking this.

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