Inge,
Observation 1: How in the world can you make a connection between tin
whiskers and CAF?!!
Observation 2: If copper is "penetrating" the solder mask, it makes me
wonder what it is doing to the tin overlayer! I would be concerned about
tin/copper intermetallic growing to the surface and oxidizing.
Q2 Popppycock! Companies like Nortel and other put press fit connectors
into ENIG boards for years. Of course it is always a good idea not to try
and put an elephant in a mouse hole. Make sure the pin and hole are
dimensional matched, the pin is put in straight and the proper force, speed
and assembly apparatus are used to do it and that the end results are
cross-sectioned initially to make sure that it is right.
Bev
RIM
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Monday, September 07, 2009 7:22 AM
To: [log in to unmask]
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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