The conventional wisdom is that (speaking generally) high Pb solders don't
crack they bend, that's one of the reasons that people use them. Cracks etc
are usually found to be poor solderability or mechanical failures /plating
failures etc induced by the very high processing temperatures associated
with their use. You may also find on close examination that the cracks are
actually surface imperfections, or slightly worse, caused by spent/charred
flux included in the joint. The reflow temperature of high leads is right on
the edge of the flux envelope. So Dave's suggestion for microscopic
investigation is a good one.
On more practical/pragmatic grounds
High leads do not flow as well as high tins so joint areas may receive an
extra battering from the operator as they probe around with the iron.
Meanwhile everything is in a somewhat more vulnerable state because of the
temperature and the flux is charring nicely.
Make sure that your vendor is supplying you with a heat stabilised resin and
if you are using flux core consider applying additional flux (a very high
solids heat stabilised resin is preferred). Consider flushing nitrogen into
the work area or using a hot nitrogen jet instead of an iron. This will
reduce the oxidative degradation of the flux, reduce the burden of extra
metal oxidation on the flux and simplify removal of the flux after
soldering.
Make sure that your background heating is adequate, if you are not using
background heating then that will simplify matters enormously.
It is possible that the wire you are using is actually quite old so the
surface could be dulled, thereby using up your precious flux. High Pb alloys
are not fast movers. Some companies ship from stock. It is better to use a
make to order vendor. If the wire you are using is flux cored then the
probability of it being old is high as the min make on FCSW is large.
Alternatively you may have stored it a long time before use as you probably
don't rattle through a lot either! Pull the wire through a piece of folded
(sulphur free) paper or wipe.
Hope this helps.
Regards
Mike Fenner
Indium Corporation
T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]
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Pb-free: www.Pb-Free.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Thursday, September 16, 2004 12:55 PM
To: [log in to unmask]
Subject: Re: [TN] HMP Solder Joint Cracking
Hi Mike! Sorry, I have only dealt with high Pb solder alloys on a couple of
occasions so I have minimal experience to apply to your question. And
plated thru hole soldering is typically a pretty robust technology so if
you are getting cracks you have something that is definitely skewed. If you
have the opportunity, get a microsection of the crack solder joint(s)
completed. The microsection will show you if the crack is in the solder
joint or the solder joint/component lead interface. Cracks in the bulk
solder joint may be related to weaknesses in the solder microstructure
(e.g. floating intermetallic phases, segregated zones due to dendrite phase
formation, shrinkage voids, etc.). Cracks in the solder joint /component
lead interface may be related to problems with the formation of the
intermetallic phases. The solder joint microstructure can give clues to the
heating/cooling of the soldering process which may help determine the root
cause of the cracks. Good Luck.
Dave
Mike McMonagle
<mmcmonagle@SYSTRON To: [log in to unmask]
IX-INC.COM> cc:
Sent by: TechNet Subject: Re: [TN] HMP
Solder Joint Cracking
<[log in to unmask]>
09/16/2004 06:17 AM
Please respond to
TechNet E-Mail
Forum; Please
respond to Mike
McMonagle
Mike and David,
Any ideas of the possible mechanisms that can cause fractures in hand
soldered through-hole parts using Sn5/Pb93.5/Ag1.5 HMP alloy? Pretty
much the opposite of Bryan's Sn96/Ag4, we're lead-rich and tin-poor....
Mike
Good thought. The slow cool down associated with larger parts will give
what are effectively shrink holes (like in a solder pot) especially with
Sn96.
[On hand soldering parts with poor solderability operators tend to put
more solder on as well which exacerbates the problem.] The solutions
offer themselves, but you could also consider using a SAC alloy. The
different structure of the solder on solidification should help. If
nothing else the "cracks" will be more rounded. A final thought to
double dot and cross the Ts and Is. In the unlikely event the turrets
are brass they should have a NI or Cu barrier plate to prevent the
solder sucking out Zn. This would certainly lead to a cracked appearance
if not cracks. Regards
Mike Fenner
Indium Corporation of Europe
T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]
W: www.indium.com
Pb-free: www.Pb-Free.com
Hi Bryan! One possible root cause to explore is that the "cracks" you
have observed are actually shrinkage voids/cavities that intercept the
solder joint surface. There have been several studies (NIST, NCMS) that
have show that the high tin content solder alloys can form small
shrinkage cavities that terminate at the solder joint surface. These
shrinkage cavities could easily be mistaken for cracks. I recommend you
microsection one of the suspect solder joints to see if you have a crack
or a shrinkage cavity. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
The cracks occur immediately on alloy solidification - I've watched it
happen under a scope. Can't be sure that some don't occur later but that
is a worrying thought which could affect reliability.
Cheers
Bryan
When are the cracks occurring? Ate the assemblies thermally stressed
after the terminals are soldered? Do the cracks occur after a wire is
soldered to the turrets?
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]> 631-595-5879
Thanks Jim
We are seeing some of the edge solderability issues that you
mention however
there is definite cracking as shown in microsections done by
ourselves.
Another batch of pins has better solderability at the edge
however we still
get cracks occurring.
Cheers
Bryan
We've seen this problem in the past. The main culprit, in our
case, was the
solderability of the terminal on the corner of the swaged area.
What looks
like a crack may be an unsolderable line around the periphery of
the swaged
area. Try dipping the swaged end of a terminal into a solder
pot and see if
the corner edge wets.
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
Hi
We are fitting a swaged turret terminal and soldering
using 96% Tin
4%
Silver alloy and have experienced cracks running around
the edge of
the
swage solder joint - between the pin and the solder.
Re-soldering
gets rid
of it in most but not all cases. I have thought that
perhaps some
stress
relaxation is occurring in the swage area (brass base
material)
during
soldering resulting in these cracks.
Can anyone shed any light on this problem ?
Cheers
Bryan Kerr
Principal Quality Engineer
Process and Materials Laboratory
AMS Hillend
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