Ted,
Its Mr. Christian, not Ms. Ms. Christian is his wife...
Roger M. Stoops, CID [log in to unmask]
TRIMBLE NAVIGATION LLC
-----Original Message-----
From: Ted Stern [mailto:[log in to unmask]]
Sent: Wednesday, April 03, 2002 10:20 AM
To: [log in to unmask]
Subject: Re: [TN] Tin whiskers and immersion tin coatings
Dear Ms. Christian:
Related to your question, we had a customer test the resistivity of an
unsoldered immersion tin though
hole used to mount the circuit assembly to the chassis and also function as
a ground. I did not receive
a copy of the report, but after a 21 day accelerated age (85 C and 85% RH
atmosphere), the increase in
resistance between the mounting screw and the pad was within acceptable
limits. Additionally, no
whiskers were observed to have formed. (The composition of the deposit was
not tested after the aging
cycle, but I would assume the deposit was comprised entirely intermetallic
phases after 21 days at 85
C.) However, given reports previously posted on this forum of observed
whisker growth under soldermask,
it would seem prudent to solder any exposed immersion tin during assembly.
It is my opinion, albeit limited to the immersion tin we manufacture, that
organic and/or inorganic
codeposition agents incorporated into the tin coating serve to reduce the
rate of intermetallic growth
but not whisker growth. Although codeposition agents may lessen the
propensity, given the proper
conditions (moisture, sufficient soluble ionic contamination, and voltage
differential), whisker growth
may still occur.
Regards,
Ted Stern
Bev Christian wrote:
> Technetters,
> To a certain extent I am aware of the work the NEMI tin whisker task force
> and various connector/component manufacturers are doing about 100% tin
> coatings. But I am still left with several questions whose answers I have
> missed or questions that have not been addressed. So... here goes - the
> immersion tin coating are for SMT pads that are going to be soldered.
What
> are people doing about test points? Are there stencil openings to put
solder
> on these pads as well? Or are these pads left unsoldered? Has anyone
> carried out any testing with regards to the tendency of these materials to
> grow/not grow whiskers? Or do the organics in the coating completely kill
> that possibility?
>
> regards,
> Bev Christian
> Research in Motion
>
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
