Jonathan,

That's a very high melting point solder you're using. Is your 285 deg c the
peak oven temperature or the peak component 'case' temperature? What's your
dwell time at that temperature?

You have to tread a fine line between achieving proper reflow and not
cooking the die under these conditions. If your 285 deg C is peak oven temp
and you have weak bumps, my first suspect would be insufficient dwell time
to permit all the solder material to wet properly. Second suspect would be
the flux you're using in case it's not aggressive enough, or the volatiles
are boiling off before full fluxing action has taken place. Make sure your
flux appropriate for the metals involved and can tolerate this high temp.
Can you describe, or better still post pictures on Steve G's site of, what
the bump and/or UMB surfaces look like after a bump has fallen off?

Third suspect would be oxidised Ni or CuNi intermetallics, though being no
chemist, nor a metallurgist, I'm getting out of my depth here, but will add
my 2 cents worth anyway.

Good luck! There's nought worse than your bumps falling off!
Peter



[log in to unmask]      16/05/2002 10:39 AM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to
Jonathan.A.Noquil

             To: [log in to unmask]
             cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
             Subject: [TN] Solder-UBM Separation








Hello Guys?
Can anybody explain the separation of Solder Bump from UBM after the
Assembly Process?

Solder Bump=95Pb5Sn Alloy
It is plated on a wafer with Under Bump Metal (UBM) of TiCuNi. It then
reflowed to form a bump (And is Fluxed).

The bumped wafer then assembled, after assembly there were weak bumps which
some of them separated from the UBM.
Can you explain this phenomena? would this be intemetallic effects? Is it
possible to have this phenomena that is caused by mechanical vibration plus
the heat applied?
By the way the reflow profile had a peak temp of 285 C.

I can't figure out yet why this happened?
Any inputs would lead me to solve this problem

regards

Jonathan

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