| Thank you all for your replies. Thanks to your replies and backup from the IPC-SM-782A (section 3.6.3.2) and James Blankenhorn's "SMT Design Rules & Standards," the designer has agreed to provide adequate clearance between the via and the pad. | | I will note, however, that I became aware of some other products that we process where vias actually are connected to the edge of the pad. In looking at these under the microscope, some solder did get sucked through the holes. The resultant solder fillet, although steeper than those without vias, still looked "adequate." | | | Is anyone aware of some guidelines regarding through-hole in SMT pads? One of our designers wants to add through-hole leads in some SMT pads for an inductor. The size of the hole is 0.032 inches and takes-up approximately 25% of the pad area. This SMT component would be reflowed. I am concerned about the paste volume being sucked down the hole. | | | | Any input will be welcome. | | | | Thanks, | | Mike Demos. | | | | Mike,
It's still the same old story. R/F type designers and engineers need closely coupled devices for performance. They have difficult times realizing compromises needed to effect solder joint, component, and product reliability.
I've been down this road hundreds of times. About 30% of the time I get engineers to recalculate and effect design rules ensuring maximum benefit for the compromised elements.
Neckdowns, in the last instance you mention, are quite adequate for performance, when the numbers are recalculated, and ensure solder joint integrity - the first time - during reflow. There is no need for touch up after solder sucks down the via. Therefore, chip device types do not crack under excessive hand soldering touchup thermal excursions.
You're definitely on the right track with IPC and Blankenhorn (best in the business). To prove it all, create some sample boards utilizing all design criteria thought to be needed and that from industry as discussed. Then, simply assemble the boards and derive objective evidence concerning performance, solder joint quality and reliability, and component quality and reliabillity - during and after reflow, and after touch up.
And on it goes.
Earl Moon
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