There are too many variables in your questions to give dimensioned
responses.
Mixing Sn100C with SAC 305, assuming homogenous joint, would give a SAC
alloy with Ni and Ge modifiers/dopants, the precise mix of which is unknown.
Such alloys are available so you might take some comfort from that.
Mixing lead into the alloy changes the picture completely, before you were
looking at mixing alloys from the same or closely related series, some close
enough that the distinctions are sometimes only visible to Patent lawyers.
Now you are mixing alloy types. Here I suggest you look at the literature on
this subject, of which there is lots, as advised by Dave. This might be a
tad confusing but essentially you will see that as a minimum you must have
an homogenous joint and for that it needs to have been processed as for the
lead free alloy. If you don't have uniformity in the joint then it is more
likely to fail prematurely at the discontinuity. If you do then it is still
an unknown complex mix with properties not as good as the original separate
alloys. Certainly more difficult to understand and predict - unless you can
think in more than 3 dimensions.
Regards
Mike
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tom Burek
Sent: Monday, February 16, 2009 9:59 PM
To: [log in to unmask]
Subject: Re: [TN] Lead Free solder compatibility
Hi Denny,
Currently the specification for SN100C as the PCB finish is on the FAB
drawing. I am told by the supplier it is applied in the same manner as
HASL. The compatibility concern with the SAC solder is in a hand solder
operation of a component after the board has been processed through
reflow. The technician is using a SnAgCu (SAC) solder to hand solder the
component. Excluding for the moment the Lead Free issue, how would 63Sn -
37Pb do in this hand solder operation?
I have read on this forum concerns about IMC layers affecting reliability
when mixing solder types and was pursuing that path with my question. To
clarify my earlier statement about similarity to SnPb, I guess I meant to
say that most PCB suppliers I have spoken with prefer to use SN100C as the
HASL replacement for SnPb.
Thank you all for your responses.
Tom Burek, CID+
[log in to unmask]
02/16/2009 02:19 PM
To
[log in to unmask]
cc
Subject
Re: [TN] Lead Free solder compatibility
Tom, I am not sure you understand the composition of SN100C - here goes:
SN100C（99.25Sn-0.7Cu-0.05Ni＋Ge） that is 500 ppm nickel and something
under 100 ppm Germanium
SAC 305 is (96.2Sn- 0.5Cu-3.0Ag)
Normal tin lead is (63Sn - 37Pb)
Can you see that all the "Lead-Free" compositions are at least 96% tin
compared to eutectic solder at 63%?
Your statement "most similar to standard SnPb HASL finish leaves the door
wide open - do you mean:
In appearance - yes, SN100C is more shiney than other SAC alloys -
resembling tin lead
In melting point - no, eutectic is at 183C and all the SACs are 220
or more. I think, without the silver, SN100C is something like 228C.
In wetting - 99.3Sn-.7Cu has been used a standard wave solder
lead-free for several years. SN100C is a modification of that well known
alloy. Germanium is particularly supposed to help wetting, I understand.
In low copper erosion and improved drop shock resistance - I
understand that SN100C does improve those properties, back toward
performance by SnPb. However, I have no comment on the th ermal cycle
resistance, until the NASA/DoD test work is finished - as mentioned by
Dave Hillman.
There is no exact drop-in for eutectic. How is your SN100C applied to the
boards - by a converted HASL machine?
Denny Fritz
SAIC, Inc.
-----Original Message-----
From: Tom Burek <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, 16 Feb 2009 2:53 pm
Subject: Re: [TN] Lead Free solder compatibility
Thank you for your response. Is it a much different situation if the mix
was SN100C and SnPb? Also, is it not true that SN100C is most similar to
standard the standard SnPb HASL finish that the SAC finish?
Thank you again.
Tom Burek, CID+
Werner engelmaier <[log in to unmask]>
02/16/2009 12:34 PM
To
[log in to unmask], [log in to unmask]
cc
Subject
Re: [TN] Lead Free solder compatibility
Hi Tom,
The reliability of SAC 305 is understood; the data for SN100C are scarce.
You are creating some mish-mush alloy, but they are all in the same family
with the same Solidus and roughly the some Liquidus. So you are not
getting the same situation you get when you mix SnPb and SAC.
Werner
-----Original Message-----
From: Tom Burek <[log in to unmask]>
To:
[log in to unmask]
Sent: Mon, 16 Feb 2009 1:28 pm
Subject: [TN] Lead Free solder compatibility
Hello technetters,
I have a lead free solder question. If I have a PCB with a SN100C finish
and hand solder a component using a tin Silver solder (SN96.5AG03 Cu.5),
is there a possibility of creating a joint that may be less reliable,
perhaps more brittle?
Thank you
Tom Burek, CID+
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