[SI-LIST] Re: References on electrical properties of dielectric materials?
- From: "Oluwafemi, Olufemi B" <olufemi.b.oluwafemi@xxxxxxxxx>
- To: "'scott@xxxxxxxxxxxxx'" <scott@xxxxxxxxxxxxx>, "larry.zu@xxxxxxxxxxxxxxxx" <larry.zu@xxxxxxxxxxxxxxxx>
- Date: Thu, 22 May 2014 16:39:21 +0000
I agree with Scott.
In addition though, dielectric losses also occur because, energy of the applied
EM field is used up to align disorganized polar molecules in the direction
of the applied EM field, not just because they are colliding with each other.
So with increased temperature comes increased disarray: As a result, dielectric
losses increase with more collision and energy been used up
to align in the direction of the EM field.
Femi
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Thursday, May 22, 2014 9:20 AM
To: larry.zu@xxxxxxxxxxxxxxxx
Cc: Loyer, Jeff; si-list (si-list@xxxxxxxxxxxxx)
Subject: [SI-LIST] Re: References on electrical properties of dielectric
materials?
Larry,
Technically you are incorrect. The resonant frequency of water dipoles is in
the Terahertz region. The frequency for microwave ovens was chosen to be in a
band unused for radio communications. It is known as an Industrial,
Scientific, Medical (ISM) band.
Heating of water molecules with EM energy is simply a manifestation of
dielectric loss heating. The dielectric losses occur when electric dipoles
(polarized molecules) bump together when they "wiggle" under the influence of a
time-varying EM field. Dielectric loss is a measure of both the
disorganization of the molecules and the relative polarity. Teflon, for
instance is non-Polar. As a result, it is about the slickest substance known
to man, and has one of the lowest dielectric losses. Oxygen is also non-polar.
Increased in dielectric loss with temperature in a material is due to two
effects. First, as temperature rises, the amount of molecular disorganization
may increase. When it does, there are more collisions, and therefore more
loss. This is usually a secondary effect. Second, however, is water
absorption. Water is a highly polar molecule, and highly disorganized in the
liquid state. As a result, as temperature increases, the amount of
disorganization increases, and therefore the dielectric loss increases. This
is seen as an increasing loss tangent with temperature rise. Finally, the
resultant dielectric loss of a composite material is quite complex, since it
forms what is known as an electromagnetic mixture.
When humidity is absorbed by a material, the dielectric losses increase in the
mixture, but where the water is absorbed is quite important, and changes the
relative difference seen in both dielectric constant and dielectric loss.
Water can be absorbed in two places in common PCB and packaging materials: it
can be absorbed between the fiber filaments, or within the dielectric matrix.
With exactly the same amount of water absorbed, the dielectric constant (as
seen by a trace) in the mixture will be different.
To understand what is going on with dielectric constant and dielectric loss in
the presence of humidity and temperature you need to:
Make sure that the material has been saturated. Which means that it has to be
placed in a humidity chamber for extremely long periods of time, to allow the
moisture to diffuse into the material.
Measure the weight of the material before and after on a precision balance to
determine the exact amount of water that has been absorbed.
Perform precision measurements that are both de-embedded and temperature
compensated, so that the impact of measurement method does not influence your
findings.
When you do this, you will generally find that the increase in loss and
dielectric constant of a dielectric with humidity is a function of where the
water is absorbed, % of water that is absorbed, and the temperature.
(would it be surprising that the Dk and Df of water changes abruptly at
0C?) Because water is a polar molecule, it is attracted to other polar
molecules. Materials that are hygroscopic are polar materials that like to
bond with water,absorb humidity and tend to have higher electric polarity.
Essentially they have unbound sites that are polar, where water molecules can
bond. Most low-loss materials (but not all) are well-organized, and have low
water affinity. Ester-based materials, as are used in semiconductor package
have high water affinity.
--
Scott McMorrow
Teraspeed(r) Consulting a XXXXXX company
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
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