Letter 0004

Making Current Shunts Solderable

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Hi,

Our company has had serious problems with solderability of small (0.5") alloy wires (10 AWG to 24 AWG) we use as current shunts for sensing overload and short circuit conditions in our switching power supplies. A temporary solution has been barrel plating a copper strike followed by .0003 -.0005 tin plate. This works until you get to the thin wire gauges. Can you suggest any other solution? The alloy is called Alloy 90. It does not solder well in a no-clean process.

Thank You,

P. Lopes
power systems

Do you need to use a no-clean flux for your process? This is one of the more difficult to use processes, and consequently will require a very solderable surface. But to the meat, is Alloy 90 a 90/10 Nickel/Iron alloy, as the name would imply? If so keeping this gem solderable could be a challenge. Probably some sort of plating is mandatory. But it is difficult to plate very small wires.

Would you consider immersion plating? If so, we have recently put together an immersion plating bath for Palladium that seems to work well on pure Nickel. If the alloy is what I suspect, this might work well for you. If the alloy is as I suspect, we would be happy to run some samples for you.

This finish is inherently not the easiest to solder to in the world, but we have had some no-cleans work well, and the good news is, that if it works well fresh, it will not change solderability characteristics over the next year.

Rudy Sedlak
Mountain View, California

-Is it 90 Sn / 10 Pb, or other?

Mike L [name deleted for privacy by editor]
Coopersburg, Pennsylvania

Alloy 90 is a copper nickel alloy 88 Cu/12 Ni. The 90 comes from the resistivity of the material (90 ohm-cmil/ft).

Regards,

Todd D [name deleted for privacy by editor]
fuse mfgr. - Des Plaines, IL

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