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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