Finished PCB thickness vs finished copper weight requirement...

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Electronics designers,
I am breaking this thread off from a recent discussion of measuring
the temperature of D2PAK devices.  This below thread is meant to
discuss PCB copper weight vs PCB thickness and how to tell what was
delivered to your door.  John Larkin had made this comment and I
respond below...

"John Larkin" wrote:
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In this case my fab drawing said (in HUGE letters), "Board shall be
2oz finished copper."  The fab house claims that they started with 1oz
copper and plated up to 2oz.  I have asked for them to provide in
writing what they did.  I am now wondering how to prove whether or not
they complied with the requirement.  I personally dont think it
"feels" like a 2-oz copper board.  The raised height just isnt what I
was expecting....it seems very thin.  What types of quality control
rules do you use for determining what was actually delivered?  I dont
have an accurate depth gauge, but I suppose I could use calipers to
measure the thickness of an area with no copper (only soldermask) and
compare that to an area with full copper to determine the copper
+plating thickness....then determine what thickness 2oz copper should
be.

A colleague told me that "Copper weight is given in oz / per sq foot.
1 oz 3D% a minimum of 0.001294%- 0.001494% thickness".  So I suppose that
2oz finished would be 0.0024" - 0.0028" thick.  Also, in a 2003 usenet
archive post, Rene Tschaggelar offered these numbers (and I provided
the translation to mils)...
1oz  35um  1.38mils
2oz  70um  2.76mils
3oz  105um  4.13mils
6oz  0.2mm  7.87mils
9oz  0.3mm  11.81mils

For my current situation, the PCB is measuring 0.062" thick in an area
with no copper (only PCB and soldermask) and 0.065" in an area with
copper on both sides.  This implies 3mils of copper on top and bottom,
so 1.5mils of copper on each side.  I dont think this would qualify as
2oz finished copper, but now I have to convince the fab house to eat
1000 pcbs due to their fault...

(PS-My application only deals with automotive 12vdc and 10amps, but I
am mostly concerned with adequate copper for heatsinking my D2PAKs
because of a slooooowly switching Infineon half bridge driver running
as fast as it can go).

Any insightful comments?  Thank you!
frenchy (Steve French)

Re: Finished PCB thickness vs finished copper weight requirement...
"(PS-My application only deals with automotive 12vdc and 10amps, but I
am mostly concerned with adequate copper for heatsinking my D2PAKs
because of a slooooowly switching Infineon half bridge driver running
as fast as it can go)."

If you're mostly concerned with using copper for heatsinking your D2PAKs, the
thickness doesn't matter nearly so much as the surface area.  How much power
are you dissipating in the D2PAKs?  I'd probably only worry about the copper
thickness once it's above 5-10W (although I'd start calculating thermal
resistances and what-not once it's above just a watt or so).

---Joel



Re: Finished PCB thickness vs finished copper weight requirement...

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When it matters, I add test traces and measure the resistance on the
received boards. 1 oz should be about 525 uOhms per square.

Most of the routine "1 Oz" boards I get are actually in the 0.75 oz
sort of range.

Sometimes it's easier to add copper pours on more layers, stitched
with vias, than to fight the board houses over this.

A pound bag of coffee used to be a pound; now it's often 12 oz. And
nobody sells "Pound Cake" any more.

You could do something like this...

ftp://jjlarkin.lmi.net/Copper.jpg

John




Re: Finished PCB thickness vs finished copper weight requirement...
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Extra plating causes "mushrooming" of narrow traces. Try to route
planes on the outsides, signals on the inside.

Re: Finished PCB thickness vs finished copper weight requirement...
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Cut a sample of the copper of known or measurable area from the centre of a pad
and have it weighed on a sensitive balance.


Re: Finished PCB thickness vs finished copper weight requirement...

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Perhaps you could use any of several "narrow" traces as a low value
resistor.  Measure in "the 4 terminal mode".  If they are close to
proper weight the "resistor" will be in tolerance, if not it will be
too high.
  .

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