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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?

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    \$\begingroup\$ Wow, interesting find! I'm looking forward to any answers :) \$\endgroup\$
    – bitsmack
    Jan 29, 2018 at 17:04
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    \$\begingroup\$ With a sensitive enough magnetometer, almost anything that's not designed to be totally free of magnetisable stuff may contain enough ferromagnetic impurities to do what you're seeing. I had some magnetic brass once! PCB material only has to be 'good enough' to be processed, to be insulating, to be strong. There's no specification, unless you need it and pay money for it, for it to be non-magnetic. \$\endgroup\$
    – Neil_UK
    Jan 29, 2018 at 18:07
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    \$\begingroup\$ Wonder if maybe they nickel plated, then did the silver immersion. \$\endgroup\$
    – JRE
    Jan 29, 2018 at 18:09
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    \$\begingroup\$ There is a process with silver above nickel plating too, see. \$\endgroup\$
    – Uwe
    Jan 30, 2018 at 17:51
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    \$\begingroup\$ You need to change only one variable at a time. Try HASL with Isola P95 to isolate if it is the ENIG. \$\endgroup\$ Jan 30, 2018 at 18:20

1 Answer 1

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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.

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  • \$\begingroup\$ This sort of misses the point. He KNOWS that ENIG has nickel and is therefore magnetic. The question is why the immersion silver plated boards are also magnetic. \$\endgroup\$
    – JRE
    Feb 14, 2018 at 12:58
  • \$\begingroup\$ etsy.com/forums_thread.php?thread_id=6190245 Pure silver would not be attracted to a magnet, but with sterling the other 7.5% can be any undisclosed (legal) metal. There is a high possibilty that your silver was simply an alloy of silver and a ferrous metal. This would explain the attraction to a magnet. It is still sterling silver as there are no specific rules stating that sterling silver HAS to be a mix of silver and copper. \$\endgroup\$ Feb 15, 2018 at 16:31

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