You use the name 'wafer', which I think is what we call 'substrate'. Rigid 
substrates use  to be copper clad with a thick nickel and finally tin or 
gold. If you are lucky to have such ones, soldering is not critical at all. 
If you have substrates with refractory metallisation, then follow Dave's 
advice. And finally, if you have substrates with thickfilm, you have two 
possibilities, gold or silver which belongs to Dave's warning, or ditto with 
a Palladium or other base metallisation, then again, no soldering problem. 
So, before doing anything, you ought to determin what metallisation you 
have.

Hot air...hmmm....hope you can keep the temperature within the correct 
windows for each of the above cases. Metallisations on ceramic substrates 
can be sensitive to overheat.

And, to give comments on Dave's leaching warning, if the substrates are 
leaching aspirants, then a second leaching could cause an issue.  A high tin 
alloy should then be avoided.

What kind of substrates do you work with? Thinfilm substrates or thickfilm 
substrates or high power substrates? Alumina or AlNi or Silicon infiltered 
aluminum etc. ?

Inge


----- Original Message ----- 
From: "David D. Hillman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, October 19, 2010 4:42 PM
Subject: Re: [TN] Soldering a flex to a ceramic wafer


> Hi Graham -  your proposed process will work but make sure to be very
> rigid on the process time and temperatures. Most ceramics have
> metallization that is comprised of fired metallic inks and you risk
> completely removing the pads by dissolution (aka leaching) if you don't
> accurately control the process time/temperatures. Good Luck.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
> Graham Collins <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 10/19/2010 09:17 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> [log in to unmask]
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [TN] Soldering a flex to a ceramic wafer
>
>
>
>
>
>
> Hello Technet
> I've been asked to look at an interesting R&O project.  We have a flex
> soldered to SMT pads on a ceramic wafer, we need to replace some wafers.
> My first thoughts are to use a hot air tool to reflow the solder and
> remove the flex from the old wafer, and then maybe screen some paste on
> the new wafer, align the flex and again use the hot air to reflow it.
>
> Any opinions on this, am I out to lunch or on the right track?
>
> Thanks!
>
> Graham Collins
> L-3 Communications ES
>
>
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