Thanks for your responses.
Why is it that to get a good attach to SAC or other Pb-free alloys that I need to get up to their melting points, but in this case I don't?
Following previous lines of discussion in the TechNet archives, the eutectic will melt first, starting at 183C or so. Then, rather than wetting the higher temp alloy, it will dissolve part of it. This reaction will continue until either the heat is suddenly removed or the melted metals result in a mix that the melting point is the ambient reflow temperature. In various TechNet discussions, these intermetallic layers due to having a non-uniform joint tend to be very grainy/brittle (but of course how grainy/brittle depends on the specific alloy).
Are you saying then, that the speed of reaction of dissolving the eutectic into the high Pb mix is so slow that it actually resembles the wetting process instead of the dissolution process?
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, January 27, 2009 1:36 PM
To: [log in to unmask]
Subject: Re: [TN] C4 attach to organic laminate material
Joyce is correct.
When soldering to Pb95 or the more common Pb90 solder balls used on CGA (ceramic grid arrays) such as PowerPCs, etc. standard 63/37 solder is used both for the ball attachment to the CGA and to the circuit board.
You do not need to exceed temperatures for standard 63/37, as you do not melt the Pb90 or Pb95 solder balls, you simply solder to them. You should not attempt to melt the balls, the components are usually not made to handle those types of temperatures.
So if you wish to attach these types of Pb95 balls to a laminate material, you only need to print standard 63/37 solder paste onto the laminate, place the balls into the solder paste, and send the package through a standard reflow profile, say 217 deg. C. max. Later when you solder the components to the PWB, the same process is used. The Pb95 ball never goes into liquidus, but a small amount is dissolved into the
Sn63Pb37 solder when it is molten, and this provides the solder joint connection.
In the case of the PowerPC-type of components, the reason the Pb95 ball is used is to provide the maximum compliancy between the component and the board to accomodate the delta CTE between the relatively rigid aluminum oxide component body and the standard FR-4 substrate. Pb90 or
Pb95 is a very soft alloy.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Tuesday, January 27, 2009 11:21 AM
To: [log in to unmask]
Subject: Re: [TN] C4 attach to organic laminate material
Why you need to go to 290? The 63 only melt at 183 last time I checked.
--------------------------
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----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Tue Jan 27 10:26:21 2009
Subject: [TN] C4 attach to organic laminate material
I've been provided with a C4 bumped die, peripheral contact pads, 200 micron pitch, and the alloy is Sn5Pb95 (classic C4, melting point approx 290C). Someone suggested attaching this to a BGA carrier substrate (core with low elastic modulus SLC layers to absorb the CTE mismatch) using Sn63 solder paste. Tough printing job, but possible. However, I don't believe we can get this laminate material up to 290C, so the alloy mix will be incomplete, meaning a likely brittle zone in the attach.
Only thought I have is using LTCC or HTCC instead of the organic laminate.
Any other ideas out there? Anyone do reliability studies with this type of attach? Any input would be appreciated!
Wayne Thayer
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