Cal,

I think to make this decision you need to look at your requirements and how they can be achieved.
Firstly, how much copper do you need to properly carry your circuit on the finished PCBA?
Then, do you want to add a little for derating of thermal and current capabilities?
Lastly, do your Board houses communicate well and accept any of the responsibility if there is not enough copper on the finished PCB?

OK, so there are just a few questions and here is some of my reasoning for calling out copper requirements the way I do.

1. If you call out staring thickness rather than finished thickness for your foil or cores you can ease the manufacturing of your product as the fabricator is not required to use any special plating process (different plating processes throw copper differently).
2. You can calculate your current/thermal requirements at design and consider any additional plating towards derating.
3. If you call out hole wall thickness after plating and starting thickness of foil to meet your requirements then additional outer plating will usually result in additional robustness for your design.

4. If you call out all coppers at finished weights than you can leave the "how to achieve" completely to your fabricator.

Items 1-3 are my practice and recommendation, item 4 is the alternative. 
To keep from tying your board house's hands completely do not call out starting copper and finished copper on outer surfaces.
Well hopefully I didn't make this even more confusing for you.
Call me if you would like to discuss further (I think maybe I make more sense verbally).
Hope it helps,
FNK

Frank N Kimmey CID+
Principle PCB Designer
Powerwave Technologies
EDH 916-941-3159
FAX 916-941-3195
CEL 916-804-2491


-----Original Message-----
From: Cal Driscoll [mailto:[log in to unmask]]
Sent: Wednesday, July 28, 2004 5:20 AM
To: [log in to unmask]
Subject: [TN] 1oz Copper vs. .5 oz Copper


Hi All-

One of our PCB designers came to us with a recommendation from our Bare
Board fab house-
We typically use 1 oz copper and the fab house is asking us to use .5 oz
copper and build it up to 1 oz.

Since I am not a designer, so I would like some input from TechNet on this
type of practice.

Facts:
99% of our boards are used in Telecommunications products
Most of our boards have QFP's in the 12 mil range
Typically most of our boards have Gold pads/finish


Thanks,
cal

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