Hi Michael,
Current carrying capability of the 'copper traces' is actually a
function of temperature. The weakest link in the system is the board
material... (typically) so we try to protect it... also the
semiconductors have temp limits that must not be exceeded in order to
protect them as well.
I suggest that you treat the footprint land the same as a external
trace... calculate the max temp rise over ambient compared with the
capability of the board material and do not exceed 50% of it's rated
limits and you should be good. Take into account ALL heat sources into
the trace... heat from the component and heat from the current in the
trace, and heat from nearby components and other heat sources... Plot
the stabilized temp of the complete system and compare it with the most
temp sensitive item in the system and de-rate the limits to get
reliability.
Claton's comment is a good point because the copper in the pad and
traces is also a heat conduit to lower temperature items in the system
so it acts like a heat sink to the board material, other traces and
planes, other components and a heat dissipater to air, etc... Not a
simple problem if you don't know how to figure the thermal analysis...
you might want to get some assistance from someone that can model it.
Bill Brooks | Datron World Communications, Inc.
PCB Designer/Engineer | Office: 760-602-7004| Fax: 760-597-3777 |
[log in to unmask]
1808 Aston Avenue, Suite 230, Carlsbad, CA 92008 | www.dtwc.com
Performance You Require. Value You ExpectTM
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wrobel, Clayton
(SA-1)
Sent: Thursday, July 22, 2010 7:28 AM
To: [log in to unmask]
Subject: Re: [TN] SMT PAD Current Capability
Are you sure the pad size is due to current capacity and not due power
dissipation of the component. Many components require more area on the
PCB to dissipate the rated power.
----- 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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