Ioan,
You asked:
' Then, I would like to know more about the case of high temp solder
balls. Why a joint will be formed around 210C between SnPb paste and
90/10 balls and the solder would not wet at 246C on a 270C solder
ball?'
It is a matter of total mass too.
When a solder with certain composition has to make a joint on solder
with another composition, there is no speak of wetting but, of
dissolution of one alloy in the other.
The answer on your question is, that the final composition of the
solder alloy that then will be formed must have a melting range where
the solidus is lower than the process temperature. In case of SnPb40
and SnPb10, the final composition of the joint would be around SnPb25
tha has a solidus of about 200dC.
In the case of SAC308 solder and SnCu1.75 ball, the resulting alloy
will still have a solidus above 246dC.
Peer
2006/11/16, Victor G. Hernandez <[log in to unmask]>:
> Ioan,
> Early on the discussion you stated: "Due to pressures from our
> customers we run many lead-free BGAs on leaded paste at 220C. If this
> forms joints, why LF on LF would not form joints at 246C, or let's say
> 230C if I am really off in my temp measurements? "
>
> How does this comply with RoHS?
>
> Victor,
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
> Sent: Thursday, November 16, 2006 8:12 AM
> To: [log in to unmask]
> Subject: [TN] Separation in BGA joints
>
> Thanks to everybody who replied, including the non-metallurgists ;-} I
> have certainly learned things!
>
> I think the secret lies in that 270C melting point. I use a regular
> SAC305 soldering recipe (right semantics, not profile; see, I got this
> one Werner), hence the 240C target peak. Of course this will not move
> the 1.75%Cu alloy.
>
> One thing is that the component data sheet says the alloy is SAC305. But
> it also recommends 260C for peak temperature. I find this very weird,
> since somebody told me we need to solder at MP + 20C, so my 246C should
> be more than OK. Not to mention that many ICs on the board are speced
> per J-STD-020C for max peak of 250C.
>
> Question 1: why this 260C recommendation, when 240C should be OK?
>
> Then, I would like to know more about the case of high temp solder
> balls. Why a joint will be formed around 210C between SnPb paste and
> 90/10 balls and the solder would not wet at 246C on a 270C solder ball?
> The flux cushion is there in both cases. And why the SAC ball should
> melt anyway, if solder wets on 90/10, why shouldn't it wet on other
> higher temp balls? Is it because the Pb goes into solution with the Sn
> whereas the high concentration Sn-Sn interfaces absolutely have to mix?
>
> Thank you very much,
>
> Ioan
>
>
>
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