A soldering only solution would have to involve the columns of solder being
flexible enough to absorb stresses. Or strong enough not to break under
them.

Stronger alloys are probably out. Better thermal fatigue properties almost
inevitably go with higher melting points. Possibly an SAC alloy as developed
for PB-free would represent the minimum change in alloy here, but then you
would need to look at board  and component finishes everywhere else. 

So the other possible solder only answer to investigate would be to make the
columns "bendier". 
Either by making them taller. 
Or by using a more "giving" alloy. 
This is a property of In/Pb alloys and there are combinations with similar
melting ranges to Sn/Pb, [or higher/lower]. The difficulty then will be to
selectively apply them  (their cost makes it worthwhile spending time on
this). You could for example blank off the chip pad openings and then
stencil, dot dispense as required with In/Pb, p&p and then go to reflow as
now.
Which ever way you go will need some study and thought.

Regards 

Mike Fenner 
Indium Corporation 

T: + 44 1908 580 400
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F: + 44 1908 580 411 
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Pb-free: www.Pb-Free.com 



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Timo Culmann
Sent: 04 June 2004 08:42
To: [log in to unmask]
Subject: [TN] Ceramic chips on FR 4 CTE Problem


Hello,

we have following problem:
We are using ceramic chips on FR4 due to the temperature cycles we use
during burn in (-40 to +85°C) the chips are sheared of the board. I know
that there is the possibility to solder column interposers onto the chips.
But doing this we would loose the warranty on the chips which we don't
want. We need a soldering solution between chip and board. Is there a
company that offer mezzanie ype of boards with a material that takes the
CTE? A solution converting the BGA to a PGA is not possible due to
vibration in the application.

Any other solution??

Thanks a lot
Timo

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