We think Solidus/liquidus of Pb/5Sn/2.5Ag is 287/296C So notionally about
16C higher than 80Au/Sn at 280C.
I say notionally, as if you review the phase diagram of Au/Sn you will see
it is pretty steep on the Au side ramping rapidly up to the MP of Au. So
small changes in composition - from taking Au from soldered surface for
example - can quickly send you up that side. Hence the difficulty on rework.
IN practice both are reflowed around the same temperatures 330-350C. This is
because in furnace soldering with a reducing atmosphere you need to be at
the high end to get any reduction, and if you are using flux you need to be
at the low end to avoid completely killing the flux. In a vacuum oven you
just need a long time and the lower the peak the longer. If surfaces are
not solderable then pulling a vacuum won't change that.
Usually both alloys wet in place with not much spread, however Au/Sn will
spread handily if you get it into the high end temp sweet spot.
925Pb/5Sn/2.5Ag is probably one of the most widely used die attach alloys in
conventional component assembly.
The usual wisdom on soft solders is that they aren't very strong, so the
more ductile the alloy the less likely it is to break. This conventional
wisdom doesn't really apply to Au/Sn as it is more like a braze with tensile
and sheer about 8x 10x lead alloys.
It is used where high temp with stand and high temp strength is required
(also it has excellent corrosion resistance).
Soldering gold surfaces without reducing atmosphere or flux is possible with
Au/Sn as no surfaces are oxidised/tarnished.
High leads can be used instead of Au/Sn and are clearly much less expensive,
but non gold surfaces do tarnish and a reducing atmosphere/flux then has to
be considered.
Regards
Mike Fenner
Indium Corporation
T: + 44 1908 580 400
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Creswick, Steven
Sent: Thursday, October 09, 2008 8:31 PM
To: [log in to unmask]
Subject: Re: [TN] Need soldering advice ASAP No II
Inge,
I'm thinking of greater power cycle endurance for the Pb'ed version as
opposed to the super 'hard' Au/Sn. To the best of my knowledge many power
FETs are still attached with the high Pb solders to provide some ductility
in the joint between the die and copper slug.
Si to alumina would present less CTE delta so would be more forgiving for
either 'solder'
With the Pb'ed version you could possibly go to a Pd/Ag metallization, etc.
Subject to your reliability constraints, thereby reducing substrate costs -
maybe.
Cheaper since no gold would be involved in the solder.
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, October 09, 2008 3:09 PM
To: [log in to unmask]
Subject: Re: [TN] Need soldering advice ASAP No II
Right, they did not say that the solder liq is 400C, but that the max chip
temp is 400C. That makes sence. Question is what makes this formulae better
than AuSn? Do you have any idea? Better wetting or what? Cheaper, but
better?
Inge
----- Original Message -----
From: "Creswick, Steven" <[log in to unmask]>
To: "TechNet E-Mail Forum" <[log in to unmask]>; "Inge"
<[log in to unmask]>
Sent: Thursday, October 09, 2008 11:51 AM
Subject: RE: [TN] Need soldering advice ASAP No II
Inge,
I could be wrong but I thought that 92.5Pb/5Sn/2.5 Ag was 296°C Liq/289°C
Solidus. Assuming ~350°C peak, you ARE really pushing it too far for the
seal ring....
If you were using 90Pb/10Sn [302/275], or 95Pb/5 Sn [312/308], I don't see
that much difference.
Apparently, you are not using 90/10 or 95/5 ??
Steve :-)
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, October 09, 2008 1:56 PM
To: [log in to unmask]
Subject: Re: [TN] Need soldering advice ASAP No II
I got an advice from IXYS's support. They recommend quite another solder for
their large power MOSFET 's :
92.5Pb/5Sn/2.5Ag. Has anyone of you used it?
Will increase the peak temperature from our today's +300C to +400C.
They ought to know. But that will melt the sealframe of the package,
because it's soldered with AuSn.
/Inge
----- Original Message -----
From: "Inge" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, October 09, 2008 10:41 AM
Subject: Re: [TN] Need soldering advice ASAP No II
> Thanks for the support of the linearing idea. I had a feeling that this
> could help.
> You make me think of placeing the connector on the boards before HASL.
> Hmmm...what will happen in the wave soldering of the boards later?
>
> You started the machinery in my old, gaggy brain....what about applying a
> solder mask right over? Is it realistic with that fine pitch? Will the
> solder ignore the solder mask and bridge?
>
> /Inge
>
>
> ----- Original Message -----
> From: "Thayer, Wayne" <[log in to unmask]>
> To: "TechNet E-Mail Forum" <[log in to unmask]>; "Inge"
> <[log in to unmask]>
> Sent: Thursday, October 09, 2008 10:19 AM
> Subject: RE: [TN] Need soldering advice ASAP No II
>
>
> Killing the protrusion should linearly improve the process. If you have
> cut them to negligible protrusion and still have trouble, then you are
> stuck with flux/temperature/time games, or some kind of hot air blow-off
> like out of the HASL machine.
>
> Wayne
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
> Sent: Thursday, October 09, 2008 12:31 PM
> To: [log in to unmask]
> Subject: Re: [TN] Need soldering advice ASAP No II
>
> Also need advice concerning soldering of a 120 pin, three row, right
> angle, through hole edge connector. The pitch is 1 mm, and the through
> hole copper collar is such, that the space between them is just a
> fraction of a millimeter. We tried to use our selective soldering
> machine, but whatever we do, the SnPb overbridges all the pins! I'm not
> surprised, because the solder surface tension will not allow the solder
> to withdraw pin by pin.
> However, such connectors are made and sold plenty of, so they must be
> solderable, question is with what. The selective soldering machine's
> 'wave pot' is 10 mm across and the minimum wave top is 3 mm. Does it
> help to cut the pins so that they are even with the board ? Today, the
> pins protrude
> 0.3 mms . Anyone who knows nothing?
> /Inge
> PS. Will send a photo to Dr Gregory.
>
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