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March 1997

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Thu, 20 Mar 1997 12:58:53 -0500 (EST)
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Since the NASA report about 30 years ago about embrittlment of solder joints
by the formation gold/tin intermetallics and the other 300 articles that
followed, gold has been many times unjustly blamed for many solder joint
failures.  I am aware of one assembler who soldered HASL coated boards who
accused his solder supplier of sending hin solder contaminated with 4% gold.
  Those of us who have been around the industry for a while and look at these
thing have see intermetallic related failures with copper, nickel, silver,
gold and palladium; together they form about 18 intermetallics with tin and
several more with lead.  Most of these are brittle and are concentration and
soldering temperature related.  Several modes of failure are seen: (1)
Intermetallic crystals form in the bulk of the solder and form brittle
structures or initiation points where cracks originate and spread (2) A layer
or several layers of different composition intermetallics forming at the
interface between the solder the underlying metal (copper, nickel, gold,
silver, palladium).  Cracks may form in one of the more brittle
intermetallics or between intermetallics of different compostion. (3) As the
underlying metal which often has occluded or codeposited organics, bacteria
or other elements moves into the solder to form the intermetallics, it does
not have a  place to put these other materials as it forms the nice pure
intermetallic crystals; therefore, it kicks them out into a weak debris layer
between the intermetallic and the base metal or in some case between two
intermetallic phases.  These debis layers have been shown to  be composed of
compounds containing sulfur, carbon, iron, aluminum and other miscellaneous
crap.

All three mechanisms are related to intermetallic formation and may cause
solder joint cracking.  They may be formed during soldering or by solid state
diffusion during operation.  At room temperature the reactions are very
minimal, but at boiling water temperature or under-the-hood temperatures or
in the passenger compartment of my car in Arizona in the summer time the
reaction is greatly accelerated.  I know that many of you have seen parts
that left the assembly floor in great shape and then fail 18 months later.
 Analysis of the solder joints on these failed units showed that the strength
of the solder joint the the intermetallic layer had gone from super to zip
and that any movement caused by mechanical or thermal stress caused solder
joint cracking. 

So much for the mechanics, gold/tin forms "one" of the more brittle
intermetallics  
and gold should be minimized as saturation ( about 4%) of the gold in the tin
may occur readily in localized areas or in layers at the interface.  If all
of the gold in the platings could be mixed in the solder the likelyhood of
reaching saturation is minimal.  As you may have seen people smearing around
gold plated lands with a soldering iron and a small amount of solder, they
are diluting the gold with the solder.  What the maximum plating thickness
can be to prevent the concentration at the interface is debatable.  Wave
soldering often washes away or mixes the gold sufficient to prevent
saturation,  hand soldering is also a way of causuing good mixing and
diffusion in a liquid state.  Reflow or oven soldering produces the minimum
of mixing or diffusion and therefore the gold must be thin.  For reflow
ovens, some people have a 0.2 micron (8 microinches)  number; others have a
0.5 micron (20 microinches) number.  All of this depends on your peak
temperatures, dwell etc.  For hand soldering a maximum number of about 1
micron (40 miroinches) has been used sith good success.

Again going back to the theory try, try and try again.  I would solder one
sample your normal way, take another soldered sample and put it in the oven
for about 16 hours at 150 deg. C and a third sample I would strip the gold by
dipping or wicking before soldering .  Try to pop the componet from the board
with sharpened orange stick a small screw driver or if you are more wealthy
use a Instron, pulling from one edge.  If one or both or all is easily
removed or the joint strength seems to be different go look for the cause.  A
typical acceptable one often  breaks the leads or pulls the land from the
board.  A weak one pops clean, often with you thumb nail pressure.  There are
many articles written on sophisticated test methods, if you feel squeamish go
do some of these.

However, in closing do not automatically give "gold" a hit when you have
cracked solder joints, I find it is only to blame in about 20% of the
investigations that I have done.

Phil Hinton
Hinton "PWB" Engineering  

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