Hi Scott, I will attempt to make some sort of sense of this subject of copper thickness...
 
1 ounce copper is about .0014" thick (.035mm)
It looks like your customer wants 2 oz. copper thickness for his board based on the dimension of .0026 ..(.0028 would be nominal).
Most board houses put thieving pads around the perimeter of the panel to even out the copper plating over the entire surface of the board... do they not? (comments welcome)
 
From a designer's point of view... the thickest areas are calculated to have the current carrying capacity required by the electrical circuit... which may vary from conductor to conductor... or plane to plane.. that keeps the board from getting to Tg or glass transition temperature... or keeps the board cool enough to not over temp the components that have to operate within a certain temp range...
or keeps the circuit from becoming a fuse... or becoming a glowing ember..... etc. etc...
 
There are usually design tolerances on these features... The designer may or may not have specified these tolerances such that the functionality of the board would be in question if your plating was uneven..... but,
Your processes should be under control enough to keep a relative uniformity of plating across the board... or you should buy 2 oz. material to start with and avoid the process problems...
 
My suggestion ... of course... would be to speak with the designer of the board... he/she will be able to tell you just how critical the thickness requirements are... and how much deviation that he/she can tolerate. QA can only go by the print...
Also, copper thickness is not just used for current carrying capacity... sometimes it's a heat sink. The designer of the board is your best source for resolution of this problem.
Hope that helps a little...

Bill Brooks
Senior PCB Designer - [log in to unmask]
Zoneworx, Inc.                         
40925 County Center Drive, STE 200   
Temecula, CA 92591                   
http://www.zoneworx.com         
Tel: (909) 296-1226 x 1037    
Co-Director / Education Officer / Webmaster 
for the San Diego Chapter of the IPC Designers Council

http://www.ipc.org/SanDiego/index.html

http://home.fda.net/bbrooks/pca/

 
 
 
-----Original Message-----
From: Scott B. Westheimer [mailto:[log in to unmask]]
Sent: Thursday, October 05, 2000 12:04 AM
To: [log in to unmask]
Subject: [TN] Current carrying capacity

I am hoping to get some inputs from both designer and manufacturers  of PCB's on this topic. The issue is when customer specifies finish copper thickness where is the thickness most important. We have several designs with ground area and isolated pads and traces. The customer calls out for finished copper of 0.0026" and does not specify where measurement is to be taken. The total ground area is 15.84 square inches and the total circuit area is 3.36 square inches. Plating up the ground to meet this thickness is almost impossible without the circuits dramatically over plating and holes in this area becoming undersized. I have done some calculations of the current carrying capacity of the trace verses the ground area and the ground can carry 100 times more current carrying capacity than the trace. My question is why would the ground and the trace have to have the same copper thickness? Or does it? My QA department is rejecting everything because the ground does not meet the specification of minimum .0026" while the traces are .0038" to .0042". This is becoming a major problem causing many rejects due to over plate, under etch, traces shorting.
Yes the use of 2oz copper foil would make this easier but have any of you gotten a quote lately on 2oz copper foil. Would appreciate your inputs
 
Scott B. Westheimer