Re: Measuring PC Board Copper Thickness

Ross Herbert wrote:
On Mon, 26 Feb 2007 17:27:00 +1300, Terry Given <my_name@xxxxxxxx>
wrote:


Multi-layer pcb's are a different story. Usually copper layers on
these boards aren't very thick at all. If heavy currents are to be
carried you might parallel tracks on several layers to achieve

greater

overall copper thickness for those connections which require it.

or find somebody who makes heavy-copper multi-layer PCBs


Yep, that's an option if the situation warrants the expense.


If you really could get 8oz or 10oz laminate as you say, the copper
would be more than 1/16" thick.

yep. and looking edge-on, there was hardly any FR4 visible. When Dan gave me a blank PCB, my immediate reaction was something like "Fuck
Me! how much copper is in this damn thing", it felt just like a 100mm
x 100mm x 2mm piece of copper (which it very nearly was)



When 99% of the tracks on most pcb's
don't carry any appreciable current, why would you want tracks say
10mil wide by 1/16" high sitting on the board?

A PCB I designed in early 2002 uses a 1-4-4-1 Oz 4-layer PCB, for exactly this reason. It has TQFP144s, 1206 quad-packs etc, as well as
a planar magnetic transformer and inductor for the 50W 2.8V on-board
SMPS. Admittedly we only built 30,000 or so of them....


That's a viable qty to amortise the cost I think.


it was viable in 10+ quantities. a 1-4-4-1 pcb was IIRC NZ$3 more than a 1-2-2-1 ($12 or so total)

The thicker the copper
on the board the greater the cost and the greater the waste copper
during etching. No laminate manufacturer would make material with
copper this thick.

FFS dont mention that to whomever makes the PCBs for Syncor, lest the
entire company cease to exist!


Yeah, it does depend on the requirement I suppose. I did a bit of
digging and came up with this piece of current news
http://www.pcb007.com/anm/templates/feat_article.aspx?articleid=13951&zoneid=143&v=design

I didn't go into the article too deeply but it seems that at least one
manufacturer has cracked the problem of plating heavy (up to 6oz)
copper for thermal plane application on multi-layer boards.


and why would you want to etch off much of the copper? its there for electrical and thermal reasons.....


Yes, quite true. Depending on track density requirements you may have
to reduce the track width to a minimum in some areas and in others you
might want to keep the tracks widely separated.

the biggest problem with heavy copper is the track-track spacing, followed by screen printing the overlay.


Thermal planes are a
slightly different question but still introduce other problems. There
is also the question of emi/rfi susceptibility - too much copper is
not always good.

I'd love to see you expand on this concept. I guess it depends what you connect the copper to.

I would guess than on many practical boards as much
as 40 - 50% of the total copper volume has to be etched away.

I'd believe that. most PCB layouts I've seen have really sucked from an EMI perspective. A simple test - hold blank pcb up to the light. if you can see thru it, the layouts probably bad.

I try to keep as much copper as possible, after all I paid for it.

the heavy copper stuff I have done tends to have about 80-90% Cu, mainly governed by track-track spacing and voltage creepage/clearnace

This
would mean as much as 4oz per sqft of pcb using 8oz copper laminate
would disappear into the etching bath. That would soon saturate the
etchant so you would need a really efficient means of replacing the
etchant quickly to do a big run of fairly large boards. Automated pcb
manufacturing plants could handle it easily but smaller manufacturers
might find it a bit daunting. Increases the costs dramatically.

Cheers
Terry
.

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