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Subject: teardrop SMT footprints ?

Date: 11/13/00 at 11:26 AM
Posted by: David Cary
E-mail: d.cary@ieee.org
Message Posted:

Should I teardrop my surface-mount (SMT) component footprints, or not ? Why ?

I've just upgraded (finally !) to a new printed wiring board (PWB) layout program. One new feature I noticed is that it can automatically generate teardrops on selected components.

Original pad:

+---+
|pad|
| |
+-+-+
|
|
trace

After activating teardrops:

+---+
|pad|
| |
\ /
\ /
|
trace

I'm getting mixed signals about whether I should actually use this feature.

Teardrop everything:
* because the sharp inner right angle can trap etch chemicals even after the board is pulled out of solution, leading to
over-etching in those areas.
* because the sharp corner causes higher mechanical stress. Flex circuits break apart there much more rapidly than if there was a smoother transition. (That's not an issue with the ordinary FR4
fiberglass boards I'm designing).
* because pads often lift with reworking and repairing a board. It's much better to lift a pad and part of the trace as well, than for the trace to stay on the board while the pad tears right off.

Don't teardrop anything:
* because some people recommend necking down wide traces to 10 mil just before entering a SMT land, exactly the opposite of what adding teardrops does. Why ?
* because SMT footprints are specially designed to give just the right amount of solder. Adding teardrops makes each pad effectively larger. This makes the solder spread out more thinly over the larger area, not leaving enough to connect to the component pins.
* because teardrops pull heat away from the pin-pad connection, interfering with the bonding process.
* because teardrops make the pads effectively larger, giving small 2 or 3 pin components more room to "swim" around and get out of position during re-flow.

So do I add teardrops or not ? (Knowing why would also be nice).

--
David Cary

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