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July 2000

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Subject:
From:
"Olson, Jack" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 10 Jul 2000 15:35:34 -0500
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Just out of curiosity, does a PCB fabricator care whether the teardrops are created using traces or flashes? It seems like the teardrop shapes created with draws would baffle a netlist checker seeing all the extra connections, whereas the snoman or keyhole method (using round flashes) wouldn't, but I have never heard anyone mention that. Does anyone care?

Jack

                Regarding:
                A quick comment:
                Adding teardrops (aka snowman pads, doubleflash) is very easy for a
                fabricator to add with a CAM station (global edit). From a design
                perspective it may be more difficult. Some software (CAD) requires the
                teardrop placement to be specified with respect to the pad location.
                If added to an edge or corner of a pad it may not end up on the
                location where the trace connects to the pads. (It's getting better
                though. Some design software packages are getting smarter. :-))

                Now on the electrical performance side.....Be very careful if the design
                is for RF! (Such as telecommunications) If the designer has tweaked
                and peaked his design for HF and has modeled his design from
                an EMC/EMI perspective, adding teardrops to a short trace
                can greatly change the net capacitance to the point to where
                it may not perform as designed. : /
                It's always best to ask the customer up front or get a blanket approval.

                IPC-2221 has great (suggested) designer guidelines showing teardrops
                and fillets.

                Dave Hoover (aka, Groovy)


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