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Running large current through vias: large vias or many small vias?

M

Michael

Jan 1, 1970
0
Hi there - I'm working on the layout of a PCB that will be carrying
~30A or so. I'm running some parallel traces to increase the current
carrying capacity of the PCB. I would like these traces to be
connected with vias. My question is this: Is it better to have lots of
really small vias, or just a few really large vias? Also, should these
traces be connected just at either end of the trace, or should they
have vias connecting them throughout the trace?

Thanks!

-Michael
 
M

Marra

Jan 1, 1970
0
Hi there - I'm working on the layout of a PCB that will be carrying
~30A or so. I'm running some parallel traces to increase the current
carrying capacity of the PCB. I would like these traces to be
connected with vias. My question is this: Is it better to have lots of
really small vias, or just a few really large vias? Also, should these
traces be connected just at either end of the trace, or should they
have vias connecting them throughout the trace?

Thanks!

-Michael

A large via will have quite a big surface are but so will many small
ones.
You can use either approache.

Personally I would go for one large via so there arent so many tracks
running around to confuse matters.
 
L

legg

Jan 1, 1970
0
Hi there - I'm working on the layout of a PCB that will be carrying
~30A or so. I'm running some parallel traces to increase the current
carrying capacity of the PCB. I would like these traces to be
connected with vias. My question is this: Is it better to have lots of
really small vias, or just a few really large vias? Also, should these
traces be connected just at either end of the trace, or should they
have vias connecting them throughout the trace?
You can do relately simple resistance estimations on printed wiring
traces.


Copper Resistance: Rc= r . L/HW

r = 1.7241WcmE-6 or 6.786WinE-7

Tempco Tc= +.039%/°C

R = K L/W

K @ 1oz (H=.0014 or .035mm) = 4.84E-4
@ 2oz (H=.0028 or .070mm) = 2.43E-4
@ 3oz (H=.0042 or .105mm) = 1.61E-4
@ 4oz (H=.0056 or .140mm) = 1.21E-4
@ foil .01 = 6.79E-5
@ foil .02 = 3.39E-5
@ foil .05 = 1.37E-5
@ bar .10 = 6.8E-6

Note: K is traditionally expressed as ohms/square,
as L/W of the 'printed' square is 1.

The resistivity of 1oz copper can therefore be considered as 0.48
milliohms/sq.

L/W 2oz 3oz 4oz

1 .243 .16 .121 mohms
10 2.43 1.61 1.21 mohms
100 24.3 16.1 12.1 mohms


- and a 1oz Plated-Through-Hole .062L .0017H

dia. 0.02 W=0.0613 R=0.48mohms
dia. 0.04 W=0.1257 R=0.24mohms
dia. 0.06 W=0.1885 R=0.159mohms
neglecting solder fill.

Individual squares (with individual L/W ratios) along the length of a
complicated trace-width are simply additive, like series resistors
would be. Parallel traces can be treated as parallel resistors.

Self-heating of the trace and the surface temperatue rise resulting,
in free air, can still be first order approximated as 1deg rise for
every mW dissipated from a cm^2 surface area.

So depending on what you're trying to do, there's no reason why you
shouldn't be able to check for effectiveness, potential hotspots,
efficiency effects and overkill.

RL
 
J

John Larkin

Jan 1, 1970
0
You can do relately simple resistance estimations on printed wiring
traces.


Copper Resistance: Rc= r . L/HW

r = 1.7241WcmE-6 or 6.786WinE-7



Or simply 500 uohms/square for 1 oz copper.

Tempco Tc= +.039%/°C


I think it's 3900 ppm/k, which is +0.39%

John
 
M

MooseFET

Jan 1, 1970
0
Hi there - I'm working on the layout of a PCB that will be carrying
~30A or so. I'm running some parallel traces to increase the current
carrying capacity of the PCB. I would like these traces to be
connected with vias. My question is this: Is it better to have lots of
really small vias, or just a few really large vias? Also, should these
traces be connected just at either end of the trace, or should they
have vias connecting them throughout the trace?

If there are AC currents involved, multiple vias works better and you
should join your traces here and there along the length.

With DC currents, conducting area is about all that matters.

A cute trick is to put a through hold test point pin in the design to
do the via. The soldered pin lowers the resistance of the hole.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
THINK this way is it better to have one solid wire as opposed to many wires in parallel? answer that question and you have the solution.
 
Q

qrk

Jan 1, 1970
0
If there are AC currents involved, multiple vias works better and you
should join your traces here and there along the length.

With DC currents, conducting area is about all that matters.

A cute trick is to put a through hold test point pin in the design to
do the via. The soldered pin lowers the resistance of the hole.

I wouldn't put vias "here and there along the length" as the hole
reduces the width of the trace. Along the line of the test point idea,
you can use copper rivets. There are hobbiest size rivets that work
nicely.
 
J

Jim Thompson

Jan 1, 1970
0
I wouldn't put vias "here and there along the length" as the hole
reduces the width of the trace. Along the line of the test point idea,
you can use copper rivets. There are hobbiest size rivets that work
nicely.

Last time I did that the military disallowed it... "solder joint not
inspectable".

...Jim Thompson
 
L

legg

Jan 1, 1970
0
Or simply 500 uohms/square for 1 oz copper.




I think it's 3900 ppm/k, which is +0.39%

You're right.

Those 'W's are also supposed to be ohm signs. Corrected elsewhere but
missed here.
 
M

MooseFET

Jan 1, 1970
0
I wouldn't put vias "here and there along the length" as the hole
reduces the width of the trace.

The hole needn't reduce the width. Normally, a trace bulges out where
the via is placed.
Along the line of the test point idea,
you can use copper rivets. There are hobbiest size rivets that work
nicely.

I wouldn't go that way. This would be a lot bigger than the test
point but may not provide as good of a connection. You cant see into
the gap between the pad and the rivet.
 
Q

qrk

Jan 1, 1970
0
The hole needn't reduce the width. Normally, a trace bulges out where
the via is placed.

If you're running a wide trace, you aren't going to see any bulge
unless you purposely make an oversize pad to suit the situation.
Staking along the run of a trace doesn't do anything electrically.
I wouldn't go that way. This would be a lot bigger than the test
point but may not provide as good of a connection. You cant see into
the gap between the pad and the rivet.
No worse than using a swage terminal post. Lots of power supplies use
them.
 
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