From: Chon Wong
Date: Fri, Jan 31, 1997 3:14 PM
Subject: Re: current in screened silver circuits
To: Andrew P Magee
Andrew P Magee wrote:
>
> FYI, from the IPC TechNet.
>
> From: Ralph Hersey
> Date: Mon, Jan 27, 1997 3:49 PM
> Subject: Re: current in screened silver circuits
> To: Andrew P Magee; cbyxbee
> Cc: TechNet
> [log in to unmask] wrote:
> >
> > A related question to current carrying capacity in circuits:
> >
> > Need a method of calculating current carrying capacity in screened silver
> > polymer traces on 0.007" thick polyester membrane circuits. Resistance of
> > the polymer per supplier data sheet is 0.015 ohms/square at 0.001" applied
> > thickness. Typical width of screened trace is 0.025", and the actual
> > thickness applied in most real designs is 0.0005". Length of the trace
> > varies with application.
> > Is there a formula for determining how much current can be carried by the
> > circuit? Voltage is typically 5 VDC.
> >
> > ***************************************************************************
>
> Dear cbyxbee--
>
> I'm glad someone is manufacturing "true" printed circuits, using a printed
> condu
> ctive
> pattern process as well as having electrical requirements. The terms and
> acron
> yms for
> PCB's (polychlorinated biphenols???) and PWB's are consistently mis-used by
s
o
>
> many in
> our industry, perhaps some day we'll read, understand, and use the preferred
> ter
> ms in
> the IPC's-T-50 -- so off the soap box of my hang-ups and to address your
> inquiry
> .
>
> You mentioned the sheet resistance of you silver (Ag) loaded polymer is 15
> mohm/
> sq for a
> conductor thickness of 25 micrometers (per the manufacturers spec. sheet).
Yo
u
>
> also
> indicated you experience a conductor thickness of 13 micrometers, the design
> wid
> th of
> the conductor is 0.64 mm, and is applied to a polymeric film base material.
>
>
> IMO, you've got one main unknown -- the acceptable "thermal rise" above
ambien
t
>
> to
> identify.
>
> A very good approximation can be derived by using (preferredly) the "flex",
or
> a
> s
> an alternative the rigid printed board current carrying capacity and thermal
> ris
> e
> above ambient charts with appropriate compensations and interpretations.
>
> The Key -- we have to assume you've got good quality "in-process" control on
> the
> screened contuctor width, thickness, and composition of the Ag paste (in
other
> words you've got good control of the conductor's cross-sectional area, AND
> CONDUCTANCE / RESISTANCE OF THE DEPOSITED CONDUCTIVE MATERIAL), if not all
bet
s
>
> are off.
>
> As a rule of thumb, the conductivity of silver is slightly better than
copper.
>
> Therefore, the available tables/charts for "copper" conductors may be used
wit
h
>
> minimum
> risk.
>
> For convenience, we'll use Figure 3-4, on page 10, in the IPC's-D-275, and
> unfor
> tunately
> the chart is in customary, USA dumb units, oz/sq. ft, etc. So to make it
> easier
> to use
> the chart, the cross-sectional area of your conductor is about 12 sq. mils
> (inch
> ) (25 X
> 0.5 mils).
>
> Using Figure 3-4a, (we don't need Figure 3-4b because we know the
> cross-sectiona
> l area)
> we approximate the 12 sq mil area on the ordinate axis and go up to the
desire
d
>
> "thermal
> rise" above ambient curve in the chart. The allowable current for a thermal
> ris
> e of 10
> deg. "C" above ambient would be about 0.75 A. I suspect that because you're
> usi
> ng a
> "polyester" base material you'll want to limit thermal rise to 10-20 deg. C
> abov
> e
> ambient.
>
> If you do not have a method of transferring heat from the conductor to
> something
> ,
> then you may want to "derate" the current by approx. 50% (like a MLB) and use
> Figure 3-4c instead. Then again, if your printed board is mounted in good
> therm
> al
> contact with a cooling sink then you could increase the current density.
>
> I assume you are aware of the possibility of "electromigration" of silver (I
> bel
> ieve
> this has been mentioned a few times on "technet").
>
> If you need some more info. or want some more comments/thoughs, feel free to
> contact me as indicated below.
>
>
> --
> Ralph Hersey,
>
> Ralph Hersey & Associates
> PHN/FAX 510.454.9805
> e-mail: [log in to unmask]
>
> ---------------------------- Forwarded with Changes
--------------------------
-
> From: Ralph Hersey::(HERSYR) at ~FABRIK
> Date: 1/27/97 3:49PM -0800
> To: Andrew P Magee at Rogers-MCD
> *To: CBYXBE at ~FABRIK
> *cc: TECHNT at ~FABRIK
> Subject: Re: current in screened silver circuits
>
------------------------------------------------------------------------------
-Dear Andrew:
Sorry for returning your answer late. If you look at the flexcon 96
Proc
eeding
on page 230, I have a table for the PTF ink and on the 19th row from the top, I
have
everything similar to your request except for the line width. The current
capaci
ty for
that parameter is 0.316A. I have also calculated the 25 mils width and
obviously
the
current is larger and it can now carry 0.395A. The power per square has
changed
from
0.00150 W/sq to 0.00234 W/sq. The criteria is not power per square but Power
per
square
mm which is kept constant. The value is still at 0.00581 W/sqmm. I can fax the
s
pread
sheet for you. THe essential calculation has been described on page 201 of the
proceeding eqn 11 & 12. The Power per sq mm is obtained empirically and so
there
can be
no derivation. I have tried with heat equations and there is a general trend
but
does
not match the empirical values very closely so I did not use it. I can fax the
t
wo pages
of my spread sheet to you next week.
Have a nice weekend.
Regards,
Chon Wong
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---------------------------------- Forwarded ----------------------------------
From: Chon Wong::(WONGCAY) at ~FABRIK
Date: 1/31/97 3:14PM -0800
To: Andrew P Magee at Rogers-MCD
Subject: Re: current in screened silver circuits
-------------------------------------------------------------------------------
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