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Thread: Vias on top of SMD capacitor solder pads?

  1. #1

    Default Vias on top of SMD capacitor solder pads?

    Hey guys. As many of you probably know, I am a big dummy and I don't really know what I am doing most of the time.

    So I've been laying out a 4-layer PCB with some SMD ICs. Not much experience with SMD layout. I'm usually a through-hole guy.

    Anyway, I have the inner two layers acting as Vcc and GND planes. I have some SOIC ICs on one side, and I am putting 1206-size (easier to solder!) bypass caps on the other side of the board.

    I've been just plopping the capacitor footprints down with one pad right on top of the Vcc or GND pad of the IC on the other side of the board. Then I've just been plopping vias down right on the solder pads, so as to connect everything up to the supply planes without having to run any traces.

    Is it considered bad practice to put vias on top of solder pads? Or am I doing the right thing in keeping the bypass caps literally right on top of the supply pads?

    Thanks guys.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): Tandy 1000 EX/HX power supply, Mac IIci hard drive sled and one bottom rubber foot, Multisync VGA CRTs, Decent NuBus video card, PC-era Tandy stuff, Weird Old Unix Stuff, Aesthetic Old Serial Terminals (HP and Data General in particular)

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    Can you upload pictures?

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    "Then I've just been plopping vias down right on the solder pads"

    I'm not following. To me, a via is an interlayer device connecting two layers of a PCB. Do you mean something else?

  4. #4

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    Sounds to me like you're doing "via in pad" layout. Try googling that phrase for some detailed pros/cons of doing that, but in a nutshell I believe the issue boils down to the solder paste wicking down into the via during reflow and starving the pin to pad connection of solder - leading to a bad solder joint there. The 'fix' for that is to have those vias filled (with copper) during manufacture - which can add considerably to the PCB cost.

  5. #5

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    Quote Originally Posted by bladamson View Post

    Is it considered bad practice to put vias on top of solder pads? Or am I doing the right thing in keeping the bypass caps literally right on top of the supply pads?

    Thanks guys.
    I think you are meaning to say Component solder pads on top of vias ? After all a via cannot go on top of anything except the pcb surface.

    That is ok, the via would get filled with solder when the surface mount part, or even through hole part got soldered in, that doesn't normally cause any trouble. Though it might be customary to have a separate pad for the component nearby and the via as a discrete entity.

  6. #6

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    I agree with kizmit99's assesment.

    I did the via-in-pad for two .016" vias / 0603 pads on a dense prototype board due to space issues. I was only hand soldering and these pads definitely took a LOT more solder. While this worked for prototyping, I would have serious reservations for solder paste and reflow without going to plugged vias.

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    Ah, I think I understand now--you're placing a via in a component soldering pad. I wouldn't do it--vias are formed by a electroless plating process and are, therefore, less robust than regular circuit copper.

  8. #8

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    Yeah, vias on component solder pads is what I meant.

    I'll be hand soldering all of this, so I don't think solder paste/reflow will come into play. It's all SOIC and 1206 size stuff, too. Big stuff, for ease of hand soldering.

    The vias use the same intra-layer plating as through-hole solder pads though, right? Or do they not?

    I'll try to redo some things and remove as much of the via-in-pad as I can, but yeah, this board is turning out to be very dense. Indeed, I wasn't doing via-in-pad to begin with, but I started doing it so I'd have more room to route my traces.

    This is with the copper fills on the inner layers turned off, so if it looks like there are vias going nowhere that's why. Also I haven't finished routing everything yet.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): Tandy 1000 EX/HX power supply, Mac IIci hard drive sled and one bottom rubber foot, Multisync VGA CRTs, Decent NuBus video card, PC-era Tandy stuff, Weird Old Unix Stuff, Aesthetic Old Serial Terminals (HP and Data General in particular)

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    Quote Originally Posted by bladamson View Post
    Yeah, vias on component solder pads is what I meant.

    I'll be hand soldering all of this, so I don't think solder paste/reflow will come into play. It's all SOIC and 1206 size stuff, too. Big stuff, for ease of hand soldering.
    I understand your motivation to be conservative but I think once you have stuck a few down you will find 1206 hilariously large and that 0805 is totally manageable.
    Quote Originally Posted by bladamson View Post


    I'll try to redo some things and remove as much of the via-in-pad as I can, but yeah, this board is turning out to be very dense. Indeed, I wasn't doing via-in-pad to begin with, but I started doing it so I'd have more room to route my traces.
    If you dropped own to 0805 it might give you space to bring up the power from the inner layers to an area outside the footprint. However I wouldn't worry about it being 'bad practice'. A lot of the rules around PCB design are not relevant to one-off hobby projects that are hand-soldered.

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    Don't let the small caps faze you--I've hand-soldered 0603s with no problem. Just takes a little practice is all. With a hot-air tool, it would be even easier.

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