Steve Joy asked:

>I have had requests for the reduction of the 30m" minimum for tab plating
>recently. Where did the 30m" minimum average rule come from?

As soon as I hit send on my reply last night, I thought to look in
IPC-RB-276 "Qualification and Performance Specification for Rigid Printed
Boards", March 1992. See paragraph 3.5.8 Metallic Plating and Coatings, and
Table 3. For IPC Class 1 and 2, 30 uin minimum Au is required; 50 uin for
Class 3. Paragraph 3.5.8.7 adds that gold plating shall be in accordance
with MIL-G-45204 with class and type specified on the drawing.

Nickel is to be 100 uin for class 1, and 200 uin for class 2 and 3. IMHO
this is thicker than most other specs require, and probably overkill to
prevent copper diffusion when used as a gold underplate. Again, this may be
an issue for plating rate versus the gold tab line production capability,
and might affect the price of your PWB's. Bellcore does not require such
thick nickel.

This is probably familiar, but:

Class 1 is General Electronic Products
  "consumer products, some computer and computer periperhals"

Class 2 is Dedicated Service Electronic Products
  "communications equipment, sophisticated business machines...where high
   performance and extended life...is desired but not critical"

Class 3 is High Reliability Electronic Products
  "equipment...where high continued performace...is critical"

If your question was more along the lines of how these criteria were
established relevant to gold coatings on finger tabs, I have no immediate
reference for you, but am certain these have been established with the aid
of real testing.

IMO it has a lot to do with the expected service environment, mating and
unmating cycles, temperature excursions, etc.

We consider our network interface controllers and peripheral controllers to
be communications equipment.


cheers,

Jerry Cupples
Interphase Corp
Dallas, TX
http://www.iphase.com/