[SI-LIST] tolerance on components
- From: jan.vercammen1@xxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 29 Oct 2010 14:39:09 +0200
hi si-list,
I am looking for information on tolerance of SMD capacitors, to be
more specific the: the (probability) distribution of component values.
I remember - maybe incorrectly - that this subject has been already
discussed on the si-list. I have searched the archive, but without succes.
I have been working on a RF matching circuit with lumped components.
The tolerances on the inductors and capacitorshave been selected as 5%
From simulations I know that certain capacitors values are more senstive
than
others and I use multiple parallel capacitors to improve on the tolerance.
(the frequency is low enough to allow for parrallel parts).
All capacitors used are generic 0603 generic series 50V, NPO or COG,
were one can select the the tolerance from 1, 2 or 5%. However, not all
values
of 1 or 2% are readily available and we use 5% as a standard in our
database.
Our boards are assembled by an external partner and the components
are acquired from various sources - buying a generic part.
However, I can imagine that capacitor manufacturers select 1 and 2%
tolerance
parts and sell the rest as 5% parts (discarding parts or as 10%) . Which
means that
the distribution is not necessarily normal, but maybe normal with a gap in
the middle.
Is my reasoning correct? Has anyone ever observed this kind of
distributions?
Met vriendelijke groet,
Kind Regards,
Jan Vercammen | Agfa HealthCare
EMI specialist | HE/A&D / ELEC2
T +32 3444 6233 | F +32 3 444 6268
Agfa HealthCare NV, Septestraat 27, 2640 Mortsel, Belgium
http://www.agfa.com/healthcare/
R.O.: Septestraat 27, B-2640 Mortsel, Belgium | RLE Antwerp | VAT BE
0403.003.524 | IBAN Operational Account BE81363012356224 | IBAN Customer
Account BE20375104592856 | ING Belgium NV, B-1000 Brussels
Click on link to read important disclaimer:
http://www.agfa.com/healthcare/maildisclaimer
__________________
hi si-list,
I need help on the resistivity of copper tracks of a 2-layer PCB
It concerns a RFID reader with a loop antenna operating at 13.56MHz.
I have measured the loop impedance as 2uH + 7Ohm
I have simulated the loop with FastHeny as 2uH + 4.6Ohm using w=150um and
t=35um and sigma=30e6
I have been using for the resitivity of copper 34nS/m, the double of the
textbook value of 17nS/m
of pure copper (or 50% of the textbook conductivity)
A reference (manual LC Cray) quotes the PCB conductivity as 30-70% of the
conductivity of pure copper.
If I repeat the simulation with sigma=20e6 (or about 30%) then the result
match better or about 6.4Ohm.
Next I measured (optically) the track width (there are 4 loops, w=150um
and separation = 125um):
- the two outer tracks are consistently thinner, about 120um (at
the top)
- the two central tracks are about 140um (at the top)
- they seem to be trapezoid, the top is less wide than the base,
the base seems to be right on 150um for the inner tracks
and about 140um for the outer tracks
- registration is very good
With this information and using thinner tracks (average of about 130um)
the resistance turns out about 6.7Ohm
I assume that the trapezium shape could have more loss than a rectangular
bar ...
So can I assume that the conductivity of PCB tracks is about 30% of that
of pure copper or is the variation
much larger (between PCB manufacturers)?
As the skin effect is completey developped at 13.56MHz I would assume that
the loop resistance would be proportional to
the sqaure root of the resistivity because the skin depth is proportional
to the square root of the resistivity and the resistance
is proportional to resistivity/skin_depth ... or is this argumentation
incorrect?
I am asking this because I have to match the RF generator to the loop and
reading range is very sensitive
to good matching of both the inductance and the resistance ... the latter
is a problem because it seems more prone
to variation
There is also a temperature effect. Can I use the coeff of pure copper as
an approximation?
Met vriendelijke groet,
Kind Regards,
Jan Vercammen | Agfa HealthCare
EMI specialist | HE/A&D / ELEC2
T +32 3444 6233 | F +32 3 444 6268
Agfa HealthCare NV, Septestraat 27, 2640 Mortsel, Belgium
http://www.agfa.com/healthcare/
R.O.: Septestraat 27, B-2640 Mortsel, Belgium | RLE Antwerp | VAT BE
0403.003.524 | IBAN Operational Account BE81363012356224 | IBAN Customer
Account BE20375104592856 | ING Belgium NV, B-1000 Brussels
Click on link to read important disclaimer:
http://www.agfa.com/healthcare/maildisclaimer
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