Mail*Link(r) SMTP FWD>FAB: Etchback (PB Etching Characteristics)
Date: 7/26/96 8:51 AM
From: JIM ENNIS
Hi Jim,
In response to your question, IMO your concern is not "etchback" in the normal
terminology, rather your concern is "etching characteristics". The following
comments are based on our 30+ years of experience for "etching" printed boards
and performing photochemical machining of all types of metals and metal
alloys.
>We are having a significant problem with quickturn cards coming in
>overetched, and we're trying to understand some things about the process of
>etching.
>One question that has come up is: Is it possible for etching to be
>greater in one area of the card over another?
Yep -- you bet, even though the etching characteristics of spray etchers have
come a long way over the years. There are a number of factors, such as impact
spray pressure (not nozzle pressure), stratification/puddling of the etchant,
conductor spacing, spray impingment angle, etchant (type, operating,
temperature, concentration, wetting factors), and spray agitation to identify
a few.
When you have closely and widely spaced features (conductor spacings) the
closer spacings are preferentially etched faster due to etchant turbulence.
With close spacings, you have minimum "laminar" film fluid effects at the
interface between the etchant and the metal to be removed -- that is you have
"turbulent" solution exchange. In larger areas, the etchant will tend to
become more "laminar" in fluid (etchant) flow, it becomes more saturated at
the interface layer and the etching rate is reduced. That's why all of the
above mentioned paramaters need to be characterized and controlled.
When you get down to less than about 100 micrometers (0.004 inch), in
particular in the range of 13-50 micrometers (0.0005-0.002 inch) the
viscosity, wetting factors, and gas generation of the etchant becomes
significantly more critical because the etchant becomes stagnant.
Some manufacturer's of etching equipment have somewhat optimized the design of
their etching equipment to improve the etching equipments performance.
>If so, how is this possible, and what are the design considerations for this?
That is why in the IPC's-D-275 it is "suggested/recommended" that the designer
minimize the areas of "close/minimal" conductor spacings and spread the
conductors out when ever possible. Because, IMO, the majority of us are stuck
with CAD systems with an IQ that is barely measurable, the automated design
systems route to minimum in an x or y orientation and most systems do not
"automatically" spreadout closely spaced conductors, when possible, on a
routine basis. Instead, most CAD designers/operators must manually edit the
conductive patterns --- labor intensive and time consuming, and it's seldom
done in today's fast-lane paced work environment. [Comment -- oh!
Ralllllphoooo - are you going to stimulate comments from the CAD community
with those comments].
The key is to do what you can as a DfM (design for manufacturability) and then
find a printed board manufacturer who has characterized and controls their
processes and that will meet your requirements.
Sometimes finding a PB manufacturer that uses a different etchant will help,
because all etchants don't etch the same, even within the same family of
etchant.
>Some here contend that every feature should etch down by the same
>nominal amount, say .002 inch, whether the feature is an .008 trace or a
>large ground plane, since the whole PCB is exposed to etchant at the same
>time. Is there fault in this thinking?
In theory (sometimes in practice) this is true, provided you identify,
characterized and control the 10-50 variables.
>Thanks in advance.
>Jim Ennis
>Adtran Inc
>[log in to unmask]
Hope this helps
Ralph Hersey
e-mail [log in to unmask]
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Subject: FAB: Etchback
Date: Fri, 26 Jul 96 09:35:00 PDT
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