Bob, 

Suggest you take a basic PWB fab process seminar.  I have given some short answers below, but am surprised your fab shops did not do a better clarification.  Contact me off line if you want the chapter (under revision) of the IPC 7-23 Process Effects document that may help explain more. 

Denny Fritz






-----Original Message-----
From: Robert Kondner <[log in to unmask]>
To: TechNet <[log in to unmask]>
Sent: Thu, Aug 18, 2011 10:30 am
Subject: [TN] Is Desmear or Etchback Optional?


Hi,
 
I don't fab bare PCB and I know little about the processes. I have a
ustomer that is a little more "Quality Conscious" so I figured I would ask
round about a few things. Most of what I do is prototype related so I
tarted asking question at various proto shops.
 
I could not get very good answers about Desmear and Etch back processes. In
act, it almost sounds like some board houses do not have desmear processes.
 
So, 3 Questions:
 
1.       Is no Desmear possible? Is the copper contact area on interlayer
onnections sufficiently clean to where reliable interlayer connections can
e made without a desmear process?
I cannot imagine a shop that would take a chance on "post separation" 
defects by not either desmearing or etching back.  Drilling would have
to be pristine to not leave any resin on the copper interconnections. 

.       Plasma etch back. Sounds like this is a specialized and highly
mproved way for treating holes so plated through barrels make good
nterlayer connections. How Much Better is it? Cost?
Plasma uses gas phase reaction to remove organics (epoxy resin)that is 
smeared on the copper interconnections.  Cost is gas used, electricity, and
depreciation of fairly expensive equipment. 
3.       Drill to Copper spacing is important as copper wicks into glass
eave spaces. Seems like my existing houses demand 12 mil. Some new places
ay "We can do 10mil". If I am assuming generic FR4 (it that exists) then
ow can someone just kiss away 2 mils out of 12? Should I disregard anyone
ith such a light approach to rules? I assume they are using the same drill
its, material and copper plating techniques. Is that a reasonable approach?
You are describing "Conductive Anodic Filaments" - CAF.  While poor 
drilling CAN crack the laminate between holes, there is a lot more to 
CAF than just desmear/etchback.
The board in question is 5/5 rules, 8 layer, 8 mil via in 18 mil pad.
aterial is .062 FR4 or Equiv. 
 
Thanks,
Bob K.

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