Re: Through holes in SMT pads

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.

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| 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.

Sorry Mike, but I'm confused about yor question. Are you trying to do a paste-in-hole for mounting a through hole component on a surface mount assembly? If so see the thread titled "paste in hole" starting on 6.24.99 Or is this a vacant hole (via) in a SMT pad? Why? Sounds like a bad idea. On a standard thickness board with an 8 mil stencil you could accommodate all of the deposited paste in less than half the hole volume. Of course some would be pushed down the hole, but... I'd bet on uneven melting and component movement or tombstoning among other nasties. Disregarding that the designer would want to put a TH and an SMT in the same place, please come back if I've completely missed the point. John Thorup |

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| 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. | You have several choices - slim and none. That's only two? Can't remember the other. What's new?

You can do unreliable, to this date, micro-vias (.003" vias sort of) or you can do hole in pad as long as it's capped. That is, you can drill a normal size hole in a normal sized SMT pad and cap it with plating. It's a little involved but I, and some others, have been doing it for years.

Your only other real alternative is to use neckdown features as described in any good design standard. For R/F people, I've managed to prove this both reliable and performance effective - when they listen and do the math.

Enjoy the adventure,

Earl Moon

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| | 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. | | | You have several choices - slim and none. That's only two? Can't remember the other. What's new? | | You can do unreliable, to this date, micro-vias (.003" vias sort of) or you can do hole in pad as long as it's capped. That is, you can drill a normal size hole in a normal sized SMT pad and cap it with plating. It's a little involved but I, and some others, have been doing it for years. | | Your only other real alternative is to use neckdown features as described in any good design standard. For R/F people, I've managed to prove this both reliable and performance effective - when they listen and do the math. | | Enjoy the adventure, | | Earl Moon |

I agree with the 'several'choices above. Short of using the more elaborate via's mentioned above, take a note from Nancy R and 'just say NO!'.

We have had conventional vias 'sneak' into designs, and sure enough,we prove that the solder goes down the hole.

I guess another option to offer the designer is reliable rework on each pad, factor the cost of rework into the quote to help them understand the impact.

I want to encourage you to stick to your guns and make them design something manufacturable. Enjoy!

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| 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. | Hey Mike: The previous folks are correctomundo ... the paste on your pad is going to try to spread evenly over the pad, though hold and component termination surfaces when you reflow the solder paste.

So why can't this designer do the traditional thing? Ya know, SMT pad, trace covered with solder mask, and then through hole and annular ring? What makes him/her so special?

OK so it's your boss's prince (or princess as the case may be), can you reflow solder the through hole component?

Ta

Dave F

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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. |

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| 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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