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]>
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09/07/2009 06:21 AM
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[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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