Fabien,

Where are you getting the 10% spec?  IPC-4552 does not give a specific P content, as that is entirely process specific to the nickel bath.  It varies by manufacturer, but 10.5% is by no means an indicator of a problem.  P content around 10% has become the norm, as it drastically reduces hyper corrosion that leads to "black pad".

Ask your PCB fabricator if they are using a high-P nickel bath.  If they are, don't worry much about your 10.5% measurement as a reason for your joint problem.  Depending on manufacturer, it might even be on the low side.  If they are using a low-P bath, then yes, that's too much P (should be <7).  In that case, the gold thickness may be low, as parameters for the gold bath are based on the intended amount of P in the nickel bath.  If the intetntion was low-P, and you have that much P, it's an indicator that your PCB vendor ahs poor process controls.  Which means almost anything could be wrong with your fab.

But, if you do have 10.5% P, by design or otherwise, it does reduce the margin of error during reflow somewhat.  Poor profiling could be causing P enrichment at the joint, creating a brittle interface that could be opening at cold temperatures.

Which brings us to what you need to do next.  You need a good cross section and SEM analysis of the actual soldered joint, your EDX of the plating only shows a rather normal P content.

Pete

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