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September 2009

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From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 8 Sep 2009 08:33:25 -0500
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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
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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
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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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