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June 2006

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Subject:
From:
Mike Fenner <[log in to unmask]>
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Date:
Tue, 20 Jun 2006 13:16:55 +0100
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That's clear Richard thanks,
The only adder I can make is that high lead column devices are not
recommended for use with SAC either.
Essentially the concern is that the high tin content, no lead alloy and
higher process temperature will dissolve/collapse the column and the
resultant alloy is not ductile high lead mix.
Somewhere I have some papers referencing this but if anybody wants a copy
then reply to me off line and give me a few days to find them as I am
travelling just now.



Regards

Mike

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: 19 June 2006 19:12
To: [log in to unmask]
Subject: Re: [TN] Ceramic BGA High Temp Ball

I can understand your confusion, and you are certainly not the only one
confused by this.
I will attempt to clarify.
CBGAs (and CCGAs) that are terminated with balls or columns generally
contain Sn10/Pb90 that is fully homogenous. The high lead content has a
good amount of ductility that provides compliancy between the ceramic
(actually aluminum oxide) component body and the FR-4 pwb substrate,
which have different coefficients of thermal expansion. This compliancy
reduces the stresses between the two during thermal excursions.
The Sn10/Pb90 ball/column also has a liquidus (melting point)
temperature of about 245-250 deg. C. minimum. When soldering to these
parts using Sn63/Pb37 solder paste in a non-lead-free process, the peak
temperature is kept around 220 deg. C. max. With this process you are
not melting the high-temp 10/90 ball, but you are melting the 63/37
solder paste alloy. The molten 63/37 solder paste forms an intermetallic
bond with the 10/90 ball because some of the 10/90 ball is dissolved
(not melted) into the IMF. If reflowed properly, the resultant solder
joint consists of wholly homogenous 63/37 solder that is bonding the
wholly homogenous 10/90 ball to the pad.

BGAs intended for lead-free use, of course, are lead-free. They cannot
have 90% lead like the CBGAs do, so they are formed (typically) with 99%
tin and the other 1% usually is silver or copper or some combination
thereof. This Sn99 ball's melting temperature is about 235 deg. C., and
when you attempt to solder them with 63/37 solder, the process window is
correspondingly smaller. Invariably some portion of the Sn99 lead-free
solder ball actually melts rather than simply dissolving, creating a
non-homogenous mixture of tin with areas of lead-rich intermetallics
mixed in. Because this entire joint is in a molten or partially molten
state when this happens, different parts of the resulting solder joint
cool and solidify at different rates, creating a very weak and brittle
solder joint. (Tin is a very brittle metal, Lead is relatively soft.)

So, while lead-free BGAs and high-lead CBGAs both have higher melting
temperatures than 63/37 solder, they are entirely different animals.





-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Scott Lefebvre
Sent: Monday, June 19, 2006 12:19 PM
To: [log in to unmask]
Subject: [TN] Ceramic BGA High Temp Ball

After reading all the articles on Lead-Free backwards capability with
Lead-Free BGA's I'm confused.   It is stated that Lead-Free BGA's are
not backwards compatible because of there high temp solder balls not
becoming liquidus when using SnPb solder, the interconnect is considered
too brittle and prone to cracking.  Isn't this true for CBGA's having
the same issue?  Why was it acceptable for CBGA's with SnPb solder but
not for PBGA's?



Your thought's







Scott Lefebvre


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