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So I'm designing a 2-layer PCB consisting of a bunch of ICs (op-amps, OTAs) and passive components. I would like to have my decoupling caps (little 100nF axial) as close to my op-amps as possible, so I've have the following thoughts.

My op-amp sockets are mounted on the front-layer of the board and will be soldered from the back layer, as per normal. My first thought was to mount the decoupling caps directly under the op-amp socket, but on the back layer (with pins poking through the front-layer), then solder them from the front layer.

But after some more thinking I don't think this will work, since the soldered pins of the caps would be sticking up directly under the op-amp socket, and might not make the header sit flush or flat on the front-layer, depending on the final soldered pin height.

Here's my solution to this. Mount the caps on the back layer and solder them from the back layer, then just pre-cut the cap pins so they don't poke through into the front layer. Two questions about this however:

  1. Do PCB makers (JLCPCB in my case) put soldering pads on both sides of a through-hole board or just the opposite side of where a component is mounted?

  2. Is it good practice to solder passive components on the same side they are mounted?

For reference, here's a Kicad pic of what I was thinking, in terms of mounting (TL074 on front side, caps on back side):

Op-Amp with reverse decoupling caps

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  • \$\begingroup\$ Will the PCB always be hand soldered? \$\endgroup\$
    – Mattman944
    Mar 10 at 15:48
  • \$\begingroup\$ Yes @Mattman944 \$\endgroup\$
    – JTaft121
    Mar 10 at 15:49
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    \$\begingroup\$ PTH gives you solder pads both sides. Soldering component side is mechanically awkward, but OK electrically and thermally. Using a socket for the IC negates half of your otherwise laudable effort to get the decouplers close. With something like a TL074, it shouldn't matter too much, it would for a faster opamp. \$\endgroup\$
    – Neil_UK
    Mar 10 at 15:52
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    \$\begingroup\$ Pick a socket where the plastic doesn't touch the PCB, then you don't need to cut the leads really short. Some sockets are mostly hollow on the bottom. \$\endgroup\$
    – Mattman944
    Mar 10 at 16:11
  • \$\begingroup\$ Or you could just use surface-mount capacitors. \$\endgroup\$
    – Hearth
    Mar 10 at 16:24
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Plated through-holes have metal not only on the far side and near side, but also on the inner edge of the hole all the way through the board. Only on single-sided boards or boards made by unconventional methods (such as milling) will you see pads that aren't plated on the inside of the hole.

Of course, this is all assuming your drill files specify the holes as plated. If you specify a hole as non-plated, then they won't plate it. This is down to how you use your CAD software, but unless you're doing things very wrong with making footprints, all holes where electrical connections are made should be plated through.


From the image you gave, it looks like you're using KiCAD; in KiCAD every through-hole pad is either a PTH pad (standard through-hole with plating on it) or an NPTH pad (which is referred to as "NPTH (mechanical)" in the pad editor, I believe), and this is a property of the pad that you select when making the footprint. All the footprints included in a standard KiCAD installation have the pads appropriately selected; PTH for all the ones that need an electrical connection or where you solder in a mounting pin, and NPTH for all the ones used solely for mechanical mounting with rivets, screws, plastic bumps, or whatever else you may need for a given package.

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  • \$\begingroup\$ Thanks for this information Hearth, it will definitely help in me deciding what to do! \$\endgroup\$
    – JTaft121
    Mar 10 at 23:55

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