On 1/16/22 4:21 PM, Roland Turner via groups.io wrote:
(apologies for the previous post; there was some fat-fingering on my part :-) )
On 17/1/22 00:21, Jim Lux wrote:
The other problem is at lower frequencies, where the spray on shield is
too thin, relative to skin depth.? This is why nickel is often used -
it's magnetic so the increased mu makes the skin depth shallower, so a
thinner material can be used.
I didn't know this, thanks.
That bites microwave designers a lot - you have a gold plating over nickel, which "sticks" a lot better to many things, and doesn't have a diffusion problem like gold or silver directly on aluminum or copper. If the skin depth is greater than the thickness of the gold, then current flows in the nickel, which is lossy and the skin depth is shallow, so the current is entirely contained within the nickel.
(you'll see ENIG on PCBs - Electroless Nickel Immersion Gold - 2.5- 5 micron nickel with 0.5 to 0.23 micron gold - it's RoHS compatible, easy to solder to, etc.?? At 10 GHz, skin depth in gold is about 0.8 microns, in Nickel, about 0.1 micron, so pretty much none of the RF actually flows in the copper trace underneath)
Keeping this discussion more VNA like - you could probably measure this by looking at loss vs frequency. Skin depth goes as the square root, so you can look for S21 or S11 that is 5 dB/decade.
