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Why can my bare PCB be magnetized?

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#79667

Why can my bare PCB be magnetized? | 29 January, 2018

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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Rob

#79677

Why can my bare PCB be magnetized? | 31 January, 2018

Hi Fluxgate,

Doubt it's the laminate unless you have embedded components.

Nickel is ferromagnetic (can be magnetised),so I would expect some level of magnetism in ENIG, but not silver. Are you 100% sure it was immersion silver and not something similar looking? Also years ago saw papers on ENIS, but don't know if that made it out into the wild.

Rob.

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#79682

Why can my bare PCB be magnetized? | 31 January, 2018

Hi Rob,

I did check with the board house, and the confirmed that there was no Nickel plating, but that the silver went straight onto the copper in their immersion silver process.

Interestingly I've even tested an unplated sample from that board house and even that can be magnetized. I wonder if it's some other contaminant or residue in their process that is causing the issue.

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Rob

#79687

Why can my bare PCB be magnetized? | 1 February, 2018

> Hi Rob, > > I did check with the board house, and > the confirmed that there was no Nickel plating, > but that the silver went straight onto the copper > in their immersion silver > process. > > Interestingly I've even tested an > unplated sample from that board house and even > that can be magnetized. I wonder if it's some > other contaminant or residue in their process > that is causing the issue.

Possibly ferric chloride residue from the etching? or if the PCB is multilayer it may be acting as a capacitor.

Can you magnetise it if you earth the ground plane?

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