Allen and other responders,
I have read all of the responses to this thread, and excepting a fer specific issues
dealing with a QFP and a Flip Chip, nobody deals with the real issue, or should I
say, "question"?
What is the size of the smt pad, what are youu hooking it to, and are there any
adjoining pads that are closely spaced.
The issue you appear to be specifically referencing, is called a "thermal relief",
and depending specifically on the size of the pad and just what that pad (either
thru hole or smt) is connected to, you may or may not need to provide a "thermal
relief" for the pad in question.
Generally, the "thermal relief" is provided when a pad connects directly to a large
"bus" or a "plane".
The real critera, is just what will be needed in your specific instance, in terms of
manufacturability.
While a few examples of why a trace should be necked down have already been given in
a few of the previous post, I am going to give an example of where NOT to neck down,
for design reasons.
In some work I have done for one engineer, he would design his stackup such that all
of his exterior 50 ohm traces in a 2.4GHz design would be 20 mils wide, exactly the
width of his 0402 component pads, such that he could seamlessly enter and exit any
components, such as a coupling capacitor between stages. In such an instance, you
absolutely would NOT want to neck down for a "thermal relief", no matter what (even
at the risk of the expense of hand soldering if necessary).
So back to where I began, just what size smt pad are you talking about, what is it
connected to, and what are you connecting it to?
JaMi Smith
----- Original Message -----
From: "Allen T. Maddox" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, July 27, 2005 11:47 AM
Subject: [DC] Trace width ratio to pad
Hey Folks,
I was always told to NOT exit a pad, especially a SMT pad, with a trace the same
width of the pad for the reason of dissipating the heat off the pad and creating
soldering issues.
I have an engineer who doesn't believe me and wants proof. Does anyone have any
articles that support what I've been taught, or have I been mislead all these years?
Thanks,
Allen Maddox C.I.D.
Sr. PC Board Designer
GAI-Tronics, Corp
610-796-5854
PO Box 1060
Reading, PA 19607-1060
[log in to unmask]
www.gai-tronics.com
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