Thanks, David.

On pressfit:
I'm pleased to hear that this can be applied on both ImSn and ENIG.  I've 
learned from the crimp connector technology,  that the bearing  idea is that 
the crimping creates numerous GAS TIGHT contacts, which prevent from 
oxidizing and corrosion. Likewise, if you perform gastight pressfit contacts 
with the copper, with the nickel, with the Silver or whatever finish, these 
joints should be as good as crimp ones, don't they?

On solder mask:
I've understood, that chemical situations can occur, resulting in a corrupt 
solder mask, but also that this problem can be avoided by good process 
control and that you must pay attention to the interaction(s) between the 
immersion tin and the solder mask.

It's well described on page 22 in the below article (written many years ago, 
but I guess it's basically the same today)

http://www.epa.gov/dfe/pubs/pwb/pdf/sf_guide.pdf

The solder leaching on the boards I got for analysis, is so extensive, that 
you see red copper through the solder mask, instead of the typical ImSn 
colour.

My report is sent to both the board maker and our customer.

Again, thanks for the help.

Inge


----- Original Message ----- 
From: "David D. Hillman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, September 08, 2009 3:33 PM
Subject: Re: [TN] Need clever comments


> Hi Inge! Sorry for the these tardy comments (I took advantage of the Labor
> Day holiday here in the US and enjoyed a 4 day weekend of whitewater
> kayaking in Wisconsin) but hopefully they add to the information already
> provided:
>
> 1) Immersion Tin PWB Surface Finish - ImSn is a viable pwb surface finish
> and is used on a wide variety of IPC Class 1/2/3 products. However, ImSn
> does not have the greatest thermal excursion robustness and long term
> solderability shelf life - both issues due to the oxidation of the Sn
> and/or the oxidation of SnCu intermetallic phase (copper and tin diffusion
> interactions). There are a multitude of ImSn plating chemistries on the
> market and they are not all "equal" in terms of oxidation and tin whisker
> resistance. I highly recommend that before using an ImSn surface finish
> the oxidation and tin whisker issues be completely investigated. Correctly
> formulated and processed ImSn chemistries should not have tin whiskers and
> should have a 12 month solderability shelf life.
>
> 2) Pressfit Pin and PWB Surface Finishes - all of the pwb surface finishes
> - OSP, ImSn, ImAg and ENIG can be used with pressfit pin technology. As
> Bev described, very close attention must be paid to the plated thru hole
> dimensions/tolerances to insure that a successful pressfit pin process can
> be achieved. And yes, if no attention to the hole dimensions/tolerances is
> conducted, you get damaged holes and damage laminate. Rockwell Collins
> uses pressfit pins with the ImSn, ImAg and ENIG surface finishes
> successfully on a number of product designs.
>
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
> Hernefjord Ingemar <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 09/07/2009 06:21 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Hernefjord Ingemar <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [TN] Need clever comments
>
>
>
>
>
>
>
> Hi all, need some professional backup regarding MIL quality boards.
>
> Objects: FR-4 Class III double-sided multi-layer boards, populated with
> SOICS, BGAs,and a lot of passive components.
>
> Observation 1 : the non soldered board have lots of Tin whiskers on inside
> of the PTH barrel. My thought is this: if whiskers can grow long before
> the board is assembled, then ain't it likely that even CAF can be
> generated?  See photo 1.
>
> Observation 2:  Copper has somehow penetrated the solder mask. This can be
> found everywhere along the conductor traces. You need a very good light
> microscope and a SEM to see it. See photo 2.
>
> Board data: Copper with 0.8 micrometer Immersion Tin. No nickel barrier.
> Solder mask thickness not specified.
>
> Application: Typical MIL-883 environment
>
> Q1: What is your opinion about that thin Tin directly on copper? I dislike
> the concept. Copper is very mobile at high temperatures, and combined with
> humidity, there can be leakage currents and corrosion issues. Even if the
> boards are CCd, there is a risk with copper .
>
> Q2: I gave  the advice to introduce a nickel barrier, but our customer
> claimed, that they can't because of pressfit connectors and pressfit test
> pins on the board. Furthermore, they had heard that one cannot have nickel
> platings when pressfitting, because the nickel will crack and oxidize and
> cause electrical disfunction. Is this your opinion too? Are there any
> relevant testing behind such statements?
>
> Thanks in advance
>
> Inge
>
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