Besides the increased temperature for Pb-Free you also hav
Dave,
Besides the increased temperature for Pb-Free you also have an increased
Sn concentration (95% for Pb-Free and only 60% for near-eutectic SnPb
solder) which can "suck-up" lots of Ag so it shouldn't matter if the Ag
concentration is 1% or 35).
Regards,
George
George M. Wenger
Senior Principle FMA / Reliability Engineer
Wireless network Solutions
Andrew Corporation, 40 Technology Drive, Warren, NJ 07059 (908) 546-4531
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Thursday, February 01, 2007 8:31 AM
To: [log in to unmask]
Subject: Re: [TN] Soldering to Pt/Ag terminations
Hi Peter! The original intent of implementing the Sn62 solder alloy was
exactly as you described - using the 2% silver content of the alloy to
alter the solder joint dissolution characteristics (the idea that the
introduction of Sn62 alloy was a plot by evil metallurgists to takeover
the electronics industry solder processes is just a myth started by Doug
Pauls!). The use of SAC solder alloys families would seem to be a
improvement for reducing the "leaching" effect due to the 1-4% silver
content the alloys possess but also keep in mind that the melting
temperature is going up by approximately 34C. We can't revise the laws
of physics and the temperature impact outweighs the benefits we get from
the increased silver contents. And yes, double sided reflow just adds
another layer of complexity to the issue. Pbfree soldering has a much
tighter process window than SnPb soldering. And your headaches probably
don't stop there - depending on the alternative termination finish
choice you might be trading leaching concerns for tin whisker concerns.
Isn't Pbfree soldering fun!
Dave
Peter Barton
<[log in to unmask]>
Sent by: TechNet
To
<[log in to unmask]> [log in to unmask]
cc
02/01/2007 03:05
Subject
AM [TN] Soldering to Pt/Ag
terminations
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
Peter Barton
<[log in to unmask]>
Many thanks to all who responded to my question on the above subject, a
couple of which confirm my own theories.
It seem that parts with Pt/Ag (and for that matter others with a Silver
content) terminations are susceptible to Silver leaching from the
component
metallisation into the bulk solder and that this is exacerbated by
higher process temperatures.
Now I am not a metallurgist so correct me if I am wrong - we used to
employ
62Sn/36Pb/2Ag solder on hybrid assemblies to mitigate this type of
problem back in the good old non-RoHS days. The theory being that the 2%
Ag in the solder slowed the dissolution rate. As we are now using a
solder with 3.5% silver should this not improve the situation, or do the
higher process temperatures used just cancel this out?
Dave Hillman has said that using these parts with SAC305 CAN be done
with careful attention to the soldering process profile. We have done
our best to minimise the TAL and the peak temperature, mindful of
getting all the other
components on the assembly reliably soldered also. The problem parts
concerned are on both sides of a double sided assembly and those on the
side that is reflowed twice are definitely much worse so I guess this
confirms that they are not really suitable.
Bit of a problem really as there is not an alternative part. Oh well -
As a responsible contractor we will now inform our customer and await
their response.
-----------------------------------------------------------------------
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan. CF40 1XX Wales
Tel: 01443 425275 (direct)
Fax: 023 8048 4882
International Tel : +44 1443 425200
International Fax : +44 23 8048 4882
Website/URL: www.acw.co.uk
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