Hi, Rudy,
The boards are ENIG finished, the ENIG plating being carried out
post-solder-mask, which must be after final copper etch, so No, there is no
other metal in the area, unless my understanding of the plating/etching
sequence is all wrong. My understanding (faulty or otherwise) of what the
fab house has done, is that they laminate, drill and plate the layers with
the first suite of blind vias, then laminate, drill and plate for the
second suite of blind vias, then again for the 3rd suite until everything
is stacked up, pressed, drilled and plated for the through-holes. I haven't
quite been able to make sense of it all yet, but I think what I'm being
told is that the plating becomes "stepped" somehow. I have 3 sets of blind
vias from each outer surface (for ease of argument), making 4 plating
sessions on each side by the time the through-hole group is done. The first
areas to be drilled and plated are therefore plated 4 times, while the last
areas to be drilled are only plated once, with the groups in between
getting their proportional share of multiple plating.
Thus at final etch, there is, I am told, a compromise to be reached - how
to achieve the correct widths and spacings on the heavily plated areas
without over-etching the lightly-plated areas. In my simple way, I would
have expected the entire board surface to have an even plating thickness,
however thick, but seemingly not. The compromise hasn't worked for the
number of blind via groups featured in this board, and I need to try to
find out how to address a solution without solely relying on what our fab
house tells me.
In my entire life, I never had to worry about the processes involved in
fabricating PCB's - only that components could be soldered to them, that
they passed test and proved to be reliable in the field. I only ever worked
with good boards - the ones that passed in-coming inspection - in an
assembly environment, so my experience of board design and PCB fab is
limited to the past 8 or 9 months, in a company that has no previous
convictions for designing this type of board, so it's a case of the blind
leading the blind to a certain extent.
Any further help in this area would be greatly appreciated.
Best regards
Peter Duncan
"<Rudy
Sedlak>" To: [log in to unmask]
Sent by: cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
TechNet Aero/ST Group)
<[log in to unmask] Subject: Re: [TN] Over-etch
ORG>
11/16/01
02:13 PM
Please
respond to
"TechNet
E-Mail
Forum."
Peter:
I am not sure if I fully understand the process you are using....so I am
going to ask a dumb question, that may shed some light on the issue...
Is there, anywhere in electrical contact with Copper being etched any other
metal excepting Tin?
This sort of thing, selective over-etch, is classically seen when there is
some other metal, (Nickel, Gold, or???) than Tin in electrical contact with
the over-etched Copper, and also in contact with the etchant.
And if I am completely out in space with this question, please forgive the
intrusion.
Rudy Sedlak
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