It is my understanding (I wish I could show you the math that I was once
shown) that if a thermal spoke width is not less than the gap between
the pad and plane area it is spanning, any inductance is negligible.
Rule of thermal relief design: thermal spoke width=plane gap to pad.
What you get is a conductor whose length and width are equal.


Denis Lefebvre, CID+
PCB Designer
Finisar Corporation
(408)542-3832
(831)449-3850

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Toby
Carrier
Sent: Tuesday, April 13, 2010 12:37 PM
To: [log in to unmask]
Subject: [DC] SMD Pad Thermal Reliefs

Hi All,

I am reviewing a PCB (after just passing the CID exam Sunday), and have
found that there are no thermal reliefs on some surface mount components
that are attached to a conductive plane. I have told the designer of the
board to add them but they are worried that they will cause EMI problems
do to the inductance of the spokes. I do believe this to be an issue
since we are talking about a low current low frequency circuit (below
2MHz fundamental and below 1.5A current), but they are still refusing to
believe my take on the matter. 

So what does the community think about this problem and have there been
any good documents published about thermal reliefs causing EMI problems
in low frequency applications?

Thanks,

Toby Carrier
IPC Desingers Council CID

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