Good Point. Mostly what I am talking about are QFP with a 20mil pitch connecting to 0805. The concern is for the fast edge of the device, not necessarily the clock speed. What bothers me is that technique this is used across the whole board, PS, analog, and digital sections. Even when the 0805 are connected to each other, a trace the width of the resistor pad is used. Sometimes this means that a smaller trace is used on one side of the resistor or cap than the other side.
Someone had mentioned that an oven profile could be set up to compensate for this, but to tell you the truth, it's not being done. Mostly because our facility is old and it takes time that the manufacturing engineer cannot commit to. We do a lot of small runs, so things need to be made similar in order to diminish set up times. The board that I'm working on does not fit into what 80% of what our boards are. This needs special handling and is causing a headache down the line. I need to find the compromise so most of my "customers" (i.e. engineering, manufacturing, board vendor) are somewhat satisfied.
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
>>> [log in to unmask] 07/28/05 06:14PM >>>
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
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