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I have a batch of beta PCBs which we have to assemble in our lab. We have an APS manual pick-and-place machine and a tabletop reflow oven so I thought assembly tomorrow would be easy and straight forward until our technician bought up a point.

The PCB to be assembled has a mix of both through-hole and SMT parts. We'd planned to place and bake all of the SMT components first, and then hand assemble the through-hole parts. But out tech is concerned that in reflowing the SMT parts some or all of the through-hole footprints might close up. This is our first attempt of an in-house build so we are looking for how others may have approached a mixed build.

If the through-hole parts can all tolerate the reflow heat curve we can add them to the PCB and reflow all of the parts at once. The through-hole parts would essentially keep the holes from filling, but if they did no harm would be done. Of course, the through-hole parts would still be hand soldered after reflow. Is this a reasonably good approach?

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Both approaches are valid.

Hand assembly would require that the paste mask for the SMT devices prevents putting any paste on the thru-hole pads at all. Then you use wire solder as usual during the hand assembly phase.

Thru-hole devices can be soldered in the reflow oven; this requires a technique called "paste-in-hole" — but it doesn't work with all components, and the PCB has to be designed for this from the start.

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  • \$\begingroup\$ "Paste-in-hole" seems to be more often referred to as THR (Through Hole Reflow). \$\endgroup\$ – le_top Sep 23 at 0:08
  • \$\begingroup\$ Dave & SunnySkyGuy, There are three 0.05" headers which look small and close.I think tomorrow we'll try both (SMT only and fully populated boards) and see what the outcome is like. The next rev of this board will probably go to all SMT. Will post results later this week \$\endgroup\$ – Doug12745 Sep 23 at 0:27
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You are worrying needlessly. SMT reflow followed by hand (or wave) solder for the through-hole components is the normal way of doing things. (Speaking as someone who used to run an in-house PCB build shop)

If you have SMT pads very close to PTH holes on the same track / plane, there should be a solder-resist barrier to stop the solder flowing down the holes. The only possible issue could be if the boards are HASL finish, and have not been levelled properly leaving excess solder around the PTH holes.

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  • \$\begingroup\$ I'm beginning to understand this better. Thanks all for the tips. Part of my confusion was that I mistakenly believed that PT Holes were "tinned" with solder during PCB production. It makes sense the PTH would not fill as there is no solder on them. \$\endgroup\$ – Doug12745 Sep 23 at 16:32
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Usually your paste mask would not expose the through holes. The reason I didn't elaborate further was because I interpreted the question as not being as nuanced as some of the others who chose to answer.

Elaborating (by popular demand), when you design a PCB in virtually any modern Electrical CAD package (Eagle,KiCAD, Mentor Graphic, Altium, Cadence, Zuken, etc.), the last step(*) before sending your board off to be produced is to generate the "layer artwork" (aka Gerber files) which the PCB manufacturer can use to construct your board.

One of those layers will be the Paste (aka Cream) layer for the top (aka Component) side and one for the bottom (aka Solder) side of the board. Those two layers are of no value to the PCB Manufacturer. They are, however, of interest to whoever is doing your PCB Assembly, as they will typically use these files to make a stencil over which solder paste can be dragged to deposit solder paste on all the exposed pads where components will be placed (often by a Pick and Place machine, but also can be by hand for small designs / volumes) prior to going through a temperature profile controlled Reflow process. Almost never (in 2019), I would say, are the through-hole pads (where your through-hole components will eventually be populated) exposed in this solder stencil, and therefore no solder will be deposited on those pads, and there is very little risk of solder flowing into them or their associated holes during reflow.

The risk of reflow is further mitigated by another one of those Gerber layers, called the Solder Mask layer. During PCB manufacturing this layer acts kind of like another stencil to define where not to apply a layer of film that is solder-phobic (it won't bond to it). Usually both through-hole pads and the surface mount pads are exposed through the solder-mask layer, and the solder mask exposure is a tiny bit bigger than the solde so that you can weld the components to the PCB with applied solder paste and hand soldering.

Because of these two factors, the chances are you will very likely not experience any problems whatsoever first doing SMD reflow and then subsequently populating and soldering through-hole parts.

(*) Many manufacturers these days will accept a native design file from these tools (e.g. the .brd file from Eagle) and synthesize the Gerber artifacts themselves. I find it a good idea to do this for myself anyway and review the Gerbers for myself in a Gerber viewer, but depending how much you trust your manufacturer, that might be considered "old school."

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    \$\begingroup\$ Please add some more explanation, why this should fix OP's problem. Maybe they are using HAL-process? \$\endgroup\$ – Ariser Sep 23 at 5:33
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Not all through-hole components can withstand reflow, but you've mentioned that.

If you have ENIG boards the holes will not fill up unless the design is really bad (eg. a big SMT pad next to a through-hole pad with no solder mask between).

If they're HAL they should not cause problems but perhaps if they're very tight you might have issues. Usually the datasheet recommendations for through-hole parts leave plenty of slop.

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Is not a problem at all. Solder paste stencils will not holes where THT parts are and the solder paste will not be applied there.

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    \$\begingroup\$ this is essentially a repetition of my answer, is it not? \$\endgroup\$ – vicatcu Sep 23 at 12:45
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I would reflow the SMD parts first. If some few holes are closed with solder they may be opened easily using solder sucking wick. Then assemble the through hole parts and solder them conventionally. There are wicks of different wides, select one fitting to hole and pad size. Just hold the wick over the closed hole and apply heat to the wick and the pad with a solder tip of suitable diameter. The capillary forces work very well in sucking the solder out of the hole.

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Baking the boards should not bridge the unfilled thruholes.

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You can also put kapton tape on the through hole holes to avoid them filling up

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