High Phosphorous Nickle platings are difficult to impossible to solder to
without the use of acid fluxes. my recollection is that the low phospher
plating bath should be < 3-4% Phospher for successfull soldering ability.
Further given a low Phospher formulation Nickle oxides readily and again
will not be readily solderable with Organic acid fluxes. To prevent
re-oxidation of the Nickle the plater should thoroughly clean following
the Nickle bath and then immedialty overplate with 50-100 micro inches of
soft Gold I.e no brightners, hardners etc. The alternative is to hot
solder dip the Nickle to prevent re-oxidation of the lead.

since you are already having trouble intermittently sometime being able
to use a Rosin flux and sometime requiring a OA flux I assume the leads
already have a Gold finish as is the customary practice. solderability
problems are common with Gold finishes if not properly performed ,
causing the Gold to be porous and thus allowing the Nickle beneath it to
re-oxidize. The presence of organics due to poor cleaning will usually
evidence itself by forming pin-holes when soldered.

good luck

Jess

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