From: Patrick A. Stanton             PKSN
 
Subj: Electroless Ni-Boron
 
Please advise of any known issues related to the random presence of microcracks
involving the use of electroless Ni-Boron with Au/Sn braze.  These cracks
appear randomly at the braze joint interface of the Sn and Ni and are visible
only in component x-sections viewed under high SEM magnification.  The
materials being brazed together are used in what is supposed to be a hermetic
housing for the electronics industry:
 
       - Alloy 48 housing electroplated with 50u" minimum of Au over 100u"
         minimum of Ni.
 
       - Ceramic substrate plated with 80u" minimum of electroless Au over
         100u" minimum of electroless MacDermid Niklad 752 Ni-Boron.  Note that
         the Ni is plated in a 2 step process with a sintering step (approx.
         800 degrees C) between 1st and 2nd Ni and also after 2nd Ni.  These
         sintering steps are intended to drive off Boron, but may also be
         negatively impacting Ni ductility.
 
Brazing takes place in a reducing atmosphere belt furnace that has been
profiled specifically for Au/Sn --- i.e. other, non-electroless Ni-Boron plated
substrates braze together crack-free with Au/Sn when using this furnace
profile.  Alternatively, the use of Au/Ge and its specific furnace profile also
yield crack-free results with the above listed materials.  Note that the
microcracks can be detected immediately after furnace brazing via the use of
standard tracer gas leak test procedures for hermetic housings.
 
My suspicion is that the microcrack root cause is the formation of a brittle
Sn/Ni intermetallic that forms during brazing and subsequently cracks during
the latter stages of the braze furnace profile.  The odd part is that it
appears to be fairly random in nature --- some ceramic substrate lots are good
while others fail miserably.  This obviously indicates the presence of gremlins
in the process, but despite "best effort" attempts at process control the
problem still occurs.
 
I'd like your input on the following:
 
1. What are you suggestions for key process parameters to focus on and monitor
for potentially significant variations? (ie: Boron content on Ni surface)
2. Is this combination of materials/processes/etc. the proverbial "round peg in
the square hole?"  If we're doomed for failure as such then we need to reverse
course ASAP.
 
Please keep in mind these constraints:
1. Processes changes on the order of changing to electroless Ni-Phosphorus
plating or using Au/Ge braze are not allowed.
2. Plating thickness limits MIGHT be allowed to change, but only to the extent
that no loss of functionality was incurred to the housing.
3. Braze and/or sintering furnace profile changes are allowable.
 
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