Signal Integrity Effect of Magnets Glued to 400-800 MHZ PCB
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Post subject: Signal Integrity Effect of Magnets
glued to 400-800 MHZ PCB Posted: Fri Oct 19, 2007 2:11 pm
Joined: Thu Oct 18, 2007 8:47 pm
I'm a Process/Fabrication guy working with assemblies
that contain a Ceramic(LTCC) PCB operating in the 400-800 MHz (Digital)
Clock range. No Analog(Yet.) Signal lines may be 4inches total length,
multilayer. But there are no ICs, its just a big fanout. Maybe a
few 10050 or 0102 caps. Now I want to stick another ceramic circuit
card onto the surface using small embedded micro magnets about the
size of a grain of salt but there will be hundreds. Some of them
will be glued directly over signal lines but all will be electrically
isolated by dielectric layers. Their positions will also be fixed
so there will be no possibility of moving magnets relative to conductors
like in motors. Now my question for you Signal Integrity Experts
here at RFCafe, because nobody I talk to yet can give me a straight
answer on this: Will these isolated fixed magnets influence the
Integrity of typical 400-800 MHz digital signals nearby? If so,
HOW? For example, You hook this assembly without magnets up to your
TDR and to other expensive analyzers and give me a $30,000 SI report.
Now I place the magnets and you re-run the exact same test. Can
you see any difference? Can you derive the presence of fixed magnets
inside the assembly from this data? HOW? I only seem to get nebulous
answers to this, and I ask "Experts." I'm a CHEM Engr, not an EE,
so don't go PhD on me. But if someone can educate me on this topic
I would be most grateful.
Post subject: Posted: Mon Oct 22, 2007 8:15 am
Joined: Fri Feb 17, 2006 12:07 pm
Basically, any effectsd will be due to
a 3 part mixture of dielectric material into the RF electro-magnetic
field around the clock line ceramic lines.
and permeability of the magnetic material comprising the magnets
will be frequency dependent. Not knowing the magnetic material you
are using makes it hard to give a definite answer, but I suspect
the molecules of material will not be able to follow the changing
EM field, whereas the dielectric material of the adhesive possibly
will. Hence I predict the major effect will be due to the permeability
of the mixture of magnetic material and adhesive. If the magnetic
material is mainly a Barium Titanate type this has a high permittivity
and loss factor. It will be mixed with a low permittivity relatively
low loss adhesive, of which again we do not know the properties.
When one knows the permittivity of both and the relative concentrations
of each, one can compute the loss and permittivity of the mixture
using Lichteneker's Mixing Formula (basically a logarithmic relationship).
To predict qualitatively what effect the mixture would have,
we would need to have detailed info on the various materials comprising
the magnets and adhesive, the percentage of each in the adhesive
mixture,and how these behave with increasing frequency through the
UHF spectrum. If the k value remains significant compared to the
substrate material in the UHF range, it will show up as high refelction
coefficient. If the magnetic mixture has high loss at UHF, there
will be a greater loss through the power divider,
Post subject: Posted: Mon Oct 22, 2007
Joined: Thu Oct 18, 2007 8:47
Thanks so much.
If I understand you, the
magnet is no different than any other metallic object despite its
magnetic field and particular permittivity. Its effect is simply
capacitative... So to abate this effect am I correct in concluding
that I could do 2 things: 1. shield the signal lines. 2. Route the
lines a bit out of the way and open the dielectric above a ground
plane and maybe use a conductive epoxy and glue the magnets down
onto ground plane to render them continuous with GND? Seems this
would kill the cap effect. But am I forgetting anything?
Post subject: Posted: Tue Oct 23,
2007 6:22 am
Joined: Fri Feb 17, 2006
Location: London UK
those precautions would be appropriate.
I had (probably wrongly)
assumed the magnets were there to interact with an RF device, but
seemingly that is not the case. Therefore keeping the magnets away
from or shielded from the trace line on the substrate will ensure
If they must be within a close distance of the
trace, say a few millimeters, then using a low density of magnet
grains mixed in an adhesive using polystyrene instead of epoxy should
also have minimum effect.
Epoxy is quite lossy at UHF, but polystyrene
has low loss characteristics by comparison. Melted polythene is
also OK so long as it is not so over-heated that it chars.
luck with the project