Any problem parallel traces on both sides of board carrying same signal?

[Michael Noone]

Hi - I'm working on a board that will be switching a fairly high current (I
think maybe 5A) 120V AC line. I'm trying to design the board to handle as
much current as possible. I'm making all of the traces carrying this signal
as wide as possible. I was struck with an idea though - would it be OK to
mirror all these traces? In other words - have a duplicate of each trace
directly below each trace on the bottom of the board? Being that they're
the same signal I don't think there will be any issues - but I thought it'd
be best for me to check first. Thanks,

Michael J. Noone

 

[Chris]

Hi - I'm working on a board that will be switching a fairly high current (I
think maybe 5A) 120V AC line. I'm trying to design the board to handle as
much current as possible. I'm making all of the traces carrying this signal
as wide as possible. I was struck with an idea though - would it be OK to
mirror all these traces? In other words - have a duplicate of each trace
directly below each trace on the bottom of the board? Being that they're
the same signal I don't think there will be any issues - but I thought it'd
be best for me to check first. Thanks,

Michael J. Noone

Hi, Mike. There's no potential between the traces if you mirror them
so that should be safe. Try less expensive solutions first, though.
Specify 2 oz. copper and solder coat. These are a lot cheaper than
getting a double sided board, and will boost your safe current rating.

Obviously you're increasing the trace heat sink surface area, so you'll
be increasing power dissipation for a given trace temperature.
However, if you've got significant heating with both traces, you'll
tend to hotbox the circuit board between, causing discoloration and
tending to delaminate the traces from the board. That might be a bit
of a problem.

I would tend to have some doubts about your solution being that much
better, but it should help some. I would guess the positives would
somewhat outweigh the negatives, especially if there's a fan or it's
not buried in still air. Mounting the board vertically rather than
horizontally will also help if there's no air circulation.

Sorry that doesn't give you a good answer, but if you give more
information, you might get a better answer than yes and no. What's
your trace width, and how long does it run? Are you expecting elevated
environmental temperatures, or is this running in free air at room
temperature?

Good luck
Chris

 

[mike]

Hi, Mike. There's no potential between the traces if you mirror them
so that should be safe. Try less expensive solutions first, though.
Specify 2 oz. copper and solder coat. These are a lot cheaper than
getting a double sided board, and will boost your safe current rating.

Obviously you're increasing the trace heat sink surface area, so you'll
be increasing power dissipation for a given trace temperature.
However, if you've got significant heating with both traces, you'll
tend to hotbox the circuit board between, causing discoloration and
tending to delaminate the traces from the board. That might be a bit
of a problem.

I would tend to have some doubts about your solution being that much
better, but it should help some. I would guess the positives would
somewhat outweigh the negatives, especially if there's a fan or it's
not buried in still air. Mounting the board vertically rather than
horizontally will also help if there's no air circulation.

Sorry that doesn't give you a good answer, but if you give more
information, you might get a better answer than yes and no. What's
your trace width, and how long does it run? Are you expecting elevated
environmental temperatures, or is this running in free air at room
temperature?

Good luck
Chris

Don't forget to worry about how you get the current into and out of the
trace. For thin copper, the current density right at the connection pin
can get high enough to cause long-term heat problems. Might need to
think about multiple input points on the trace. I see a lot of power
supplies and TVs where the solder joint cracked over time.
mike

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[John Larkin]

Hi - I'm working on a board that will be switching a fairly high current (I
think maybe 5A) 120V AC line. I'm trying to design the board to handle as
much current as possible. I'm making all of the traces carrying this signal
as wide as possible. I was struck with an idea though - would it be OK to
mirror all these traces? In other words - have a duplicate of each trace
directly below each trace on the bottom of the board? Being that they're
the same signal I don't think there will be any issues - but I thought it'd
be best for me to check first. Thanks,

Michael J. Noone

No problem. But 5 amps isn't a lot of current for a pc board.

John

 

[Michael Noone]

No problem. But 5 amps isn't a lot of current for a pc board.

John

Hi - the traces will be 35um copper (1 oz) - I think about 3/8" of an inch
wide. Is there a good way to calculate how much current that can safely
take, and if I need a dual trace on the bottom of the board? Thanks,

Michael

 

[Michael Noone]

Don't forget to worry about how you get the current into and out of
the trace. For thin copper, the current density right at the
connection pin can get high enough to cause long-term heat problems.
Might need to think about multiple input points on the trace. I see a
lot of power supplies and TVs where the solder joint cracked over
time. mike

The relay that I'm using has two pins for each contact, so that should help
somewhat with this. I also thought I'd make the pads very large and then
use a much larger than normal amount of solder, giving more surface area to
the connection.

-Michael

 

[Michael Noone]

Hi, Mike. There's no potential between the traces if you mirror them
so that should be safe. Try less expensive solutions first, though.
Specify 2 oz. copper and solder coat. These are a lot cheaper than
getting a double sided board, and will boost your safe current rating.

Unfortunately I don't have control over the copper thickness - it will
have to be 1 oz. For other reasons (primaryily due to a RF receiver
needing a groundplane) the board has to have two sides anyways, so
making it two sided is a non-issue.

Obviously you're increasing the trace heat sink surface area, so
you'll be increasing power dissipation for a given trace temperature.
However, if you've got significant heating with both traces, you'll
tend to hotbox the circuit board between, causing discoloration and
tending to delaminate the traces from the board. That might be a bit
of a problem.

I would tend to have some doubts about your solution being that much
better, but it should help some. I would guess the positives would
somewhat outweigh the negatives, especially if there's a fan or it's
not buried in still air. Mounting the board vertically rather than
horizontally will also help if there's no air circulation.

It will be mounted vertically

Sorry that doesn't give you a good answer, but if you give more
information, you might get a better answer than yes and no. What's
your trace width, and how long does it run? Are you expecting
elevated environmental temperatures, or is this running in free air at
room temperature?

I believe trace width will be about 3/8". I'm making the final layout
later today so I'll know then. It will be run inside a home, so normal
environmental temperatures for the most part. It will not have much air
circulating over it though, and a fan is not an option, unfortunately.

Good luck
Chris

Thanks,

Michael

 

[Chris]

Unfortunately I don't have control over the copper thickness - it will
have to be 1 oz. For other reasons (primaryily due to a RF receiver
needing a groundplane) the board has to have two sides anyways, so
making it two sided is a non-issue.


It will be mounted vertically


I believe trace width will be about 3/8". I'm making the final layout
later today so I'll know then. It will be run inside a home, so normal
environmental temperatures for the most part. It will not have much air
circulating over it though, and a fan is not an option, unfortunately.


Thanks,

Michael

Hi, Mike. There are trace width calculators all over the net. Here's
one I've used before:

http://www.circuitcalculator.com/4pcb/trace_width_calculator.php

It says that you should be OK with single side, 1 oz. copper at over
..108" outer trace width (10 degree C rise in temp over ambient being
the unofficial standard for the start of deep doodoo). As long as you
don't have an internal trace, that is.

Take these results with a grain of salt -- your results may vary.
However, I can unequivocally guarantee you have nothing to worry about
conducting 5 amps with a 3/8" wide 1 oz. single copper trace. You
won't need to double up, for sure.

It's great you're being cautious about these things -- that's a good
sign, and you're headed in the right direction.

Good luck
Chris

 

[John Larkin]

Hi - the traces will be 35um copper (1 oz) - I think about 3/8" of an inch
wide. Is there a good way to calculate how much current that can safely
take, and if I need a dual trace on the bottom of the board? Thanks,

Michael

http://polysat.calpoly.edu/documents_cp1/systems/trace_capacity.pdf


John

 

[colin]

Hi, Mike. There are trace width calculators all over the net. Here's
one I've used before:

http://www.circuitcalculator.com/4pcb/trace_width_calculator.php

It says that you should be OK with single side, 1 oz. copper at over
.108" outer trace width (10 degree C rise in temp over ambient being
the unofficial standard for the start of deep doodoo). As long as you
don't have an internal trace, that is.

Take these results with a grain of salt -- your results may vary.
However, I can unequivocally guarantee you have nothing to worry about
conducting 5 amps with a 3/8" wide 1 oz. single copper trace. You
won't need to double up, for sure.

It's great you're being cautious about these things -- that's a good
sign, and you're headed in the right direction.

Good luck
Chris

I would be surprised if you run into problems with 5 amps at 120v, i worked
on smpsu designs ages ago wich handled 15 amps on single sided pcbs,
although in such cases its not too clever if you have long runs as cross
regulation becomes an issue, i measured the voltage acsross a wire link once
at 50 mv.

Colin =^.^=