Michael,
you should start with asking yourself what will the maximum temperature of
the conductor be? To get an answer on that you need calculate the
resistivity. If you got voltage, resistivity and current, it's just to use
Ohm' law as you know. (It may be little more complicated if you have pulsed
current with certain duty cycle.) Then how to get the resistivity? Lazy as I
am, I use a Siemens nomogram with conductor width on Y axis and resistivity
on X axis. The curves show the recommended copper thickness for various
temperatures. I think it's simply from a Deutsche Industri Norm, but I don't
have the actual number,
I'll scan my copy and send it offline to you later this day.
/Inge
----- Original Message -----
From: "Forrester, Michael (H USA)" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, July 20, 2010 5:34 PM
Subject: [TN] SMT PAD Current Capability
I'm a bit confused on how to calculate the current capability of a SMT
pad. We have a 2220 size chip bead that has 8A flowing through it.
Our pad geometry is built per IPC specifications but the manufacturers
datasheet calls out for a PAD twice the width (part max is 10A).
So the question came up, with our current pad geometry (to IPC), what is
the current capability? I understand that it is based on the
cross sectional area of the copper. The current density of copper is
4.02 A/sq-mm. For a 1 oz copper trace the area is
1.4 mils x trace width. But what do I use for a PAD? I have an width,
length, and thickness. For instance I have a SMT pad that
is 1.5mm X 6mm using 1 oz copper with multiple traces coming off of the
pad. What is the current carring capability of that pad?
Or am I thinking about this wrong? Thank you.
Best Regards,
Michael Forrester
Sr. Product Engineer
Siemens Healthcare Diagnostics
101 Silvermine Rd.
Brookfield, CT 06804
PH: (203) 740-6452
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