I am working on a magnetic sensors design and noticed that my measured magnetic field bias shifts over time. I have studied the bare PCB and measured that it can be magnetized with a strong magnet.
I tested two blank (unpopulated PCBS). The first PCB I tested had an ENIG plating and could be slightly magnetized. I degaussed the PCB and confirmed that the magnetism was removed, and repeated the experiment to confirm. After a while I realized that the nickel in the ENIG may be playing a part in that.
So I tested a PCB with an immersion silver plating, but it also exhibited the same behavior. Once again, I degaussed it, confirmed the magnetism was removed, and re-magnetized it to confirm that the behavior I was seeing was real.
I don't think it should matter, but the ENIG and immersion silver PCBs were on Isola P95 substrate.
I then tested two PCBs with HASL finish, and they could not be magnetized. These PCBs were on FR-4 substrate.
Any ideas?
Best Answer
ENIG plating translates to "Electroless nickel immersion gold" (for neophytes).
This means gold plated copper surfaces using nickel as intermediary layer between copper and gold.
Then, if your PCB is slightly magnetized, could be the nickel plating, because nickel is a ferromagnetic material.
https://terpconnect.umd.edu/~wbreslyn/magnets/is-nickel-magnetic.html
EDIT with more info: With Silver Immersion, the type of silver used is silver sterling: http://www.multicircuits.com/assets/content/files/immersion_silver.pdf
This kind of silver mainly contains silver alloy with copper (7.5%), but there is no industry standard saying that sterling silver has to be this specific alloy, some metal processors will use small quantities of other metals that could be magnetic.
Hope it helps.