Walter,
External conductors are usually thicker than inner layers.  If you use one
oz copper foil internal and external; the external layer is subsequently
plated with .001 to .002 of additional copper thickness.  It is more
difficult to maintain consistent fine line imaging with the thicker copper.
 The inner layers will have a very consistent and thinner copper thickness
making etching a much more uniform process.  In addition external layers
are more susceptible to handling and thermal (i.e. lifted lands) damage
than internal layers; because of this wider is often more physically
durable.
Bill Dieffenbacher
Lockheed Martin Control Systems

----------
From:   Walter Williams
Sent:   Monday, August 25, 1997 2:31 PM
To:     [log in to unmask]
Subject:        [DESIGNERCOUNCIL] Question?

Greetings

I have been going through the "IPC-SM-782A" (OCT 96) manual.
On page 35 (section 3.7.1) The following statement is made:

     The pad cap concept has two yield improvement benefits: 1)
     no fine conductor geometries on the outer layers because they
     are buried in the inner layers where conductor width control
     is significantly easier, ........

My question is,.......  If you are useing the same design rules (line
width and spaceing) and (most likely) the same etch process on the
inner layers as on the outer layers. WHY then would it be easier
to maintain conductor width control on the inner layers than the
outer layers?

Have a good yall.

Thanks
Walter L. Williams, Senior Circuit Board Designer
I-O Corporation, SLC, Utah
Voice: 1-801-972-3558 ex 123
Alternate: 1-801-973-6767

Carpe Diem