George,
If you would have asked this question a couple of years ago
I would say your asking for trouble. Being open to change
has continually opened these eyes.
Enthone originally made Entek Cu-56, which is a single pass OSP.
Ok for Type I assembly, but crapped out in Type II and III
assemblies, unless you take it through all heat excursions within
several hours with no dwell times.
Then came along Entek 106A which is a multi-pass OSP. This works
great for Type I, II and III assemblies. The Cu-106A
allows anywhere between 4~6 thermal passes before totally
evaporating. Common with both OSPs, they initially evaporate when
making contact with solder paste or solder (wave). Providing
your paste stencil aperture is less than the land you will have
a copper annular ring surrounding the paste following N2, VPR,etc
reflow. Again the solder will not wet beyond its aperture.
Strick control must be in place for 106A. Long dwell times between
heat excursions may eventually leave voids in the OSP coverage,
in turn Oxidation may hinder solder wetting properly. The OSP
will eventually wear off and then oxidation will manifest itself.
You should not experience any functional problems with exposed
copper, unless ionic contaminates (salts) or similar are present
between SMT or PTH pads.
Which brings us to Exposed Copper......
At my last company, we assembled High Storage Devices for Main
Frames which a good percentage of our shipments would eventually
make their way to Japan (A very high humidity and temperature
change environment).
We performed Environmental Bareboard Testing to mirror such
tropical environments. Our thermal shock (through hole reliability)
and humidity (SIR) tests were very extreme and represented
8-10 years of operational life.
The power supply cards were single sided, 1 side aluminum thermal
clad and 100% SMT. These cards have exposed copper on the
component side. So what we have is Exposed copper, eventual
oxidation, humidity and so on. Based on our testing, no dendritic
shorts or similar were encountered. The cards were relatively clean
based on SEC testing.
I'm in agreement with DMG Engineering (David Gengaro) that
oxidation is a protectant, similar to OSP, but in the Angstroms.
Copper oxidation is self limiting and will proprogate no further
then the surface. Unless you have surface contaminates (including
under soldermask) there is not a concern. If not Bellcore
Compliant or otherwise specified, In-Process SEC testing is not
required from most end users. So instead of taking the chance of
unclean boards with exposed copper, it is frowned upon.
Regards,
John Gulley
Compuroute Inc.
Dallas, TX
> Date: Wed, 7 Aug 1996 09:56:04 -0500
> To: [log in to unmask]
> From: [log in to unmask] (George Franck X2648 N408)
> Subject: FAB/ASM: Pressure Fit Entek
> Good day!
>
> We are considering using SMOBC (tented vias) and Entek on a press fit
> connector backplane. The assembly process will not coat the Entek with
> solder or conformal coat. The boards will go into the field with only
> Entek as an oxidation barrier for the copper.
>
> The use environment is in a non-temperature controlled external buildings
> (shacks), ie ambient temperatures and humidity. Installations are expected
> in high humidity and in arid areas, near oceans, in both hot and cold
> climates, basically any metro area.
>
> Question: What level of protection could I expect from the Entek in these
> environments? Using IPC definitions of Performance Classes (eg
> IPC-RB-276), Would yuo use this for class 2 boards?, Class 3?
>
> Are there any success stories, horror stories, or maybe even data?
>
> I would not be upset to receive a phone call from an Enthone Techie on the
> subject.
>
> Thanks for your help.
>
>
> ====================================================================
> George Franck
> PWB Product Assurance Phone (703) 560-5000 x2648
> Raytheon E-Systems M/S N408 Fax (703) 280-4613
> 7700 Arlington Blvd E-Mail: [log in to unmask]
> Falls Church Va 22046 E-Mail: [log in to unmask]
> ====================================================================
>
>
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