Ioan,
Intuition and knowledge combined! You my pointer, really. Sn starvation and
stress, you are absolutely right. Why didn't I see that? The upper alloy
(CBGA) is wetting the ball like dipping donut in chocolate. That layer is Sn
rich and doesn't have same coefficient of expansion as the Pb rich ball.
Mismatch. Where will the mismatch be maximum? Around the equator, of course,
because you have maximum diameter of ball, and thinnest layer of Sn rich
alloy. When the ball 'breathes', the very thin interfacial Sn rich layer
will be stretched or shrinked, depending on going up or down in temperature.
As some Sn alloys are harder than pure Lead, the Sn rich alloy may this way
load force into the Pb rich ball's equatorial surface and build up stress in
the ball 'skin' , the crystal plates will slide against each other or vice
versa, and the famous interstitial atomic phenomenon will start forming
whiskers. Furthermore, there may be additional stress caused by the mismatch
between the CBGA and the PWB. Your theory coincides with the fact that all
625 balls get whiskers equally and all around waist. I'll contact a friend
who is a specialist on ANSYS element 3D stress calculations. Worth trying .
Will let you know.
If we are losing, it'll be together with lying horse and crouching behind
the prairie wagon....he-he
Inge
PS. I'm still confused, but at a much higher level
----- Original Message -----
From: "Tempea, Ioan" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 13, 2006 2:11 PM
Subject: Re: [TN] PbCO3
Inge,
I will risk myself and through in some confusion.
How did you realize you had this problem, in the first place? Are you sure
it could produce failures?
Now the slippery teritory, since it's been years since I took my materials
stress class: if I remember well, the stress class we had was based on
several hypoteses and one of them was the homogeneous material. So the von
Mise's I have studied was based on the same material all over the place.
Here you deal with one alloy for the ball, one alloy for the top attachment
and another alloy for the bottom attachment. Maybe this alters the stress
distribution?
Are the whiskers you see mono crystals? In order for you to repeatedly get
them, it is obvious you have the same stress, at the same place, every time.
Maybe you have a Sn starvation at the equator due to migration towards the
joined zones. Then you might have different stress distributions on the
various zones due to different alloys. And on top of it, the creep. Wouldn't
the whiskers a result of the creep?
If you think I am losing it, just have me recall the message.
Regards,
Ioan
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