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February 1998

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Subject:
From:
Greg Bartlett <[log in to unmask]>
Reply To:
Greg Bartlett <[log in to unmask]>
Date:
Wed, 11 Feb 1998 08:32:29 +0000
Content-Type:
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Hi Matthew,

I can't comment on the brown residue that you're seeing, but we've also
noticed OSPs that exceed the so-called "maximum" thickness value.  We had a
similar situation with the dip tank and the spectrophotometer, yet we had
an analytical lab show measurements of up to 3X the max thickness (1.5
microns).  I seem to remember that the technique used was some sort of
ellipsometric method, but I'm not sure.

I believe that the discrepancies between our measured values and the
published maximum thickness can be explained when you consider that the latter
is based on data
from flat, uniform copper coupons, and not on actual boards.

Regards,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
[log in to unmask]

Matthew Park wrote:
>Entek Plus data spec from Enthone specified a coating
>thickness of 0.35um (min 0.2um to 0.5um max).  My pcb vendor
>follows this thickness req. and performs a UV
>spectrophotometer test as per a batch cycle to measure the
>coating thickness.  (By the way the vendor uses dip-tank process
>for coating Entek-Plus)  I am seeing brown residues on TH
>solder joints (not on smt solder joints after reflow) after
>wavesoldering.
>
>I am drawing a conclusion that the cause of brown residue is as
>the result of much thicker coating in plated TH holes.  I believe
>heat and flux (2% no clean flux) do not have a time to dissolve
>and evaporate the coating in plated TH holes.  Semi-dissolved
>coating in TH holes gets pushed up toward the component side
>by molten solder.  Thus causing spotty brown  residue on TH
>solder joints on the component side of boards.
>
>The reduction of belt speed (3.5ft/min) and some heat increase
>do not reduce this residue.  Hand-soldering of TH holes were
>performed with the same result as per previous e-mail
>message.
>
>I wish I had a damn micrometer to measure that coating
>thickness, or Is there more stuff???
>
>regards
>Matthew Park
>NII Norsat International Inc.
>
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