Hey All ~
A co-worker of mine was receiving these e-mails in part because we are
trying to learn more about the PB free and IPC stuff. Unfortunately
he has left the company to go back to school but if you could change
the contact information from this name to the following e-mail,
that would be great, because I would really like to stay up to date
on what is going on and he has told me this is a good site.
[log in to unmask]
Thanks Again!!
~ Sincerely,
Denise Finnerty
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of David Douthit
Sent: Tuesday, August 03, 2004 10:53 AM
To: [log in to unmask]
Subject: Re: [LF] Tin Pest in Solder Alloys
Gordon,
The questions that come to mind are;
1. Is the damage to the tin structure reversible (at any time) with
warmer temperatures?
2. Is there some form of chart available showing the rate of change vs.
temperature and alloys involved?
3. At what point (time, temperature, and alloy) will lead free solder
joints fail vibration and stress testing?
There are numerous forms of electronic systems operating in uncontrolled
cold weather environmental
conditions (automotive, aircraft, cell towers, portable devices, ect...).
David A. Douthit
Manager
LoCan LLC
Davy, Gordon wrote:
>Back when the subject of tin pest was a daily topic on this forum I wrote
to Professor William Plumbridge of the Open University in the UK to alert
him to the existence of the forum, in case he wanted to offer any comments.
I know of no one who is following this topic more closely than Prof.
Plumbridge, and so I was glad when he responded to my invitation. I have
copied below his response and a brief letter of transmission.
>
>Gordon Davy
>Northrop Grumman ES
>Baltimore, MD
>[log in to unmask]
>410-993-7399
> _____
>
>Gordon
>Thanks for the invite. I managed to read some of the contributions on tin
pest and my response is attached... I wonder if you would so kind as to put
it on site. I have assumed that the forum is an informal discussion - hence
my natural scepticism comes out!!
>
>Many thanks
>
>Bill
>
>Professor Bill Plumbridge
>Department of Materials Engineering
>Faculty of Technology
>The Open University
>Walton Hall
>Milton Keynes
>MK7 6AA
>
>
>I have been invited to comment upon the contributions to the discussion
group on tin-pest.
>
>While a great deal of common sense is presented, I do detect an underlying
'head in sand' philosophy at times. Given the kinetics of the transformation
in this case, perhaps a more accurate description would be "I'll either be
retired or promoted out of the technological arena when the satellites or
whatever falls apart or malfunctions".
>
>While this human response may be understandable to some degree, some of the
recent observations of the Solder Research Group at the Open University in
the UK seem to indicate that the subject of tin pest merits more serious
attention.
>
>The principal findings are:
>
> The most popular members of the new lead-free generation of solder
alloys (Sn-Cu, Sn-Ag and Sn-Ag-Cu) experience surface distortion and
cracking when kept at -18C or -40C for a prolonged period. Traditional Sn-37
Pb does not.
>
> The most dilute alloy (Sn - 0.5 Cu) is particularly affected and large
bulk specimens may disintegrate completely, ie a cast cylinder of some10 mm
diameter and 60 mm length ends up as a bag of powder.
>
> The process appears to be accelerated by strain - machined surfaces
being more severely damaged than as cast.
>
> The crystallography of the transformed and untransformed regions match
the bct [body-centred tetragonal] and diamond cubic structures reported for
tin pest over fifty years ago.
>
>If I were the manufacturer or user of equipment using electronics (not much
is not these days) which was intended to operate at below 13C for extended
periods, I would be concerned now!
>
>The SRG encountered 'tin-pest' several years ago - as a bi product of its
research on mechanical behaviour of lead-free solder alloys.
>
>Its policy was to store samples, after casting in a freezer at -18C, to
ensure microstructural stability. First indications of surface
discolouration and roughening were observed after about one year.
>
>Our earlier findings have been published.
>
>'Tin Pest in Sn-0.5 wt. % Cu Lead-free Alloy' (W J Plumbridge,Y Kariya, N
Williams and C R Gagg), J of Matls, 2001, 53, 39-41.
>
>'Tin Pest in Solder Alloys' (W J Plumbridge, Y Kariya and C Gagg),
Soldering and Surface Mount Technology, 2001,13, 39-40.
>
>Since then, we have built up a unique and highly valuable collection of
specimens, some with exposure times exceeding eight years! (This is a topic
which cannot be accelerated - however much funding is directed towards it).
This includes the three alloys and tin-lead mentioned previously, in bulk
form together with PCBs containing components.
>
>Examination will reveal the extent and propensity for damage and indicate
the effects of microstructure, prior history, pre ageing, strain (monotonic,
cyclic and creep).
>
>An extensive programme is envisaged if we are to incorporate the 'tin-pest'
factor into structured integrity and performance - let alone clarify its
science.
>
>To do this we require partners, collaborators, funds. So if anyone wishes
to save up to 10 person years (in 10 years time, the proverbial horse could
have bolted!) Please contact me.
>
>Otherwise, we might find tin-pest being rediscovered in disastrous
circumstances.
>
>Bill Plumbridge
>
>
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