Cristina,

The cracking of solder joints on electroless nickel and immersion gold finish
is being more often reported as more people are using it and multiple
soldering operations are being employed.  The problem is probably not the
gold which is only about 5 microinches thick, but an interaction of the
nickel and tin in the solder.  In a study that I am familiar with, they
determined that boards that were plated with electroless nickel and immersion
gold, soldered and then underwent a burn-in cycle of about a week at 150 deg
C experienced considerable solder joint cracking.  The joints failed at the
solder/nickel interface leaving a clean land with no solder retained.  The
metallurgical mechanism indicated that as the board was thermally aged, the
nickel moved into the solder forming tin/nickel intermetallics, and left all
the phosphorus behind.  The layer underneath the solder jont was initially
about 6-8% by weight phosphorous or about 25 % by volume and after aging the
immediate layer jumped to about 16-20 percent phosphorous by weight or about
60% by volume and the joint became very weak and the parts fell of  during
shake, rattle, and roll.  Multiple solder operations without the benefit of
the burn-in also produced the same cracxking but it was considerably more
random and some did not fall off after five solderings.

The tin/nickel intermetallic grows in solid state and increases as
temperature increases, this intermetallic growth also increases as more time
at soldering temperature is added by more soldering operations.  It must be
noted that the intermetallic growth itself probably is not a bad thing, but
what is left behind is probably the culprit.  A comparison test with
electrolytic nickel gold of similar thicknesses did not produce the same
joint failure.  The study was unpublished and I can not send you a copy.

Above is one scenario or the nickel/gold solder failure.  Another is fairly
common wherein the supplier found that all of the lands did not plate in the
electroless nickel process and so he gave it light scrub, and replated it,
the adhesion between the multiple plates is not great.  The joints having
double plating will often crack after a little stress and often exhibit gold
at the crack.  A microsection of the joint will determine if this is the
cause of failure.

A third failure system that I have seen with EN/IG is that there is too short
of a dwell time at the peak in the soldering profile.  The failed joints will
often exhibit a gold color on the cracked joint, and it appears that the
solder soldered to the gold but not the nickel and it peeled off of the
electroless nickel   A small increase in peak soldering temperature or a
longer dwell time can eliminate the problem.  There are some out there who
have decided that the profile for HASL coated boards must be used for all
finishes, I and some others do not agree with this.

Hope this has some relevance to you questions about nickel/gold solder
joiints.

Phil Hinton

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