Hi Michael,
There is an additional consideration—what is called 'local expansion mismatch' [see pp. 39-40 in IPC-D-279] between the solder and the material(s) to which the solder is attached.
The CTE of solder is about 24 to 26 ppm/C, FR-4 and Cu are not much of a problem, but soldering ceramics, Alloy 42, Kovar, Thermount could create reliability issues if the pads become too large.
Werner
-----Original Message-----
From: Forrester, Michael (H USA) <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, Jul 20, 2010 11:34 am
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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