Hi, Wayne
Your terms "wetting" and "dissolution" and "melting" are three very
different things.
For example I can "melt" 63/37 solder at 183 deg. C. in a tin pot or
wave solder machine, or with a soldering iron. I can then dip a
toothpick into the molten solder and have good "wetting" or solder
coverage over the whole toothpick when I extract it from the pot (yes,
that is possible). But unless some of the underlying wood dissolves into
the molten solder, no solder joint is formed. (That is impossible). If
you then cut the toothpick in half, you can easily pull out the wood
from both halves, leaving a shell of solder. No bond ever really formed
between the wood and the solder.
When you bring 63/37 solder up to 183 deg. C. and it is in a fully
molten (liquidus) state, some of the melted solder will "wet" when it
makes contact with the Pb95 solder ball, but the Pb95 solder ball does
not need to be in a molten state in order for some of it to dissolve
into the molten 63/37 solder, thus forming your solder joint.
Many different metals will dissolve to some degree when in contact with
molten solder. Their rates of dissolution into molten 63/37 solder are
all different; silver and gold dissolve very rapidly, copper not quite
so fast, nickel even slower, brass even less so, stainless steel much
less so, etc. And all of these resulting solder joints will have greatly
different mechanical characteristics when cooled. For example, a solder
joint formed between 63/37 and nickel is usually more brittle than one
formed between 63/37 solder and copper. Not necessarily less reliable,
however.
All metals dissolve into molten SAC solder as well. However, copper will
dissolve at a much greater rate at the higher temperature needed to get
SAC to melt (about 230 C), as will Pb95.
All of these metals form different types of intermetallic bonds with the
molten solder, whether the solder is Sn63Pb37, or Sn97Ag3Cu.5.
Their rates of dissolution are affected both by temperature and the
particular molten alloy they are being dissolved into.
Each metal's total amount of dissolution at a given fixed temperature is
somewhat self-limiting when in contact with a certain molten alloy. Only
a certain amount of each metal's "desire for equilibrium" can take place
in a certain amount of time, at a certain temperature.
I can't tell you the fine difference between dissolution and melting or
melding, nor can I describe the differences between that and two
different metals combining when neither is molten (galvanization).
Werner or Vladimer can, as they are metallurgists.
My great-grandfather could, because he was a metallurgist.
His name was Galvan Lunde. With a name like that, how could he not
become a metallurgist?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Tuesday, January 27, 2009 1:23 PM
To: [log in to unmask]
Subject: Re: [TN] C4 attach to organic laminate material
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.
--------------------------
Sent using BlackBerry
----- 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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