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I'm familiar with the manufacturing process of surface mount boards. Chips come on reels (very high volume smaller parts), trays, or tubes. They are loaded into pick-n-place machines. The PCB, with solder paste applied, is put into the machine as well, and the pick-n-place (PnP) will populate the board.

I was wondering how this used to be done/is still done with through hole components. Our classic ¼W resistors and such come with their leads sticking out on a form of tape. How are these put on PCBs in a mass production environment? Is this all manual labour, or are there machines for this as well? I've never seen or heard of machines for this, and I can see their round shape causing headaches for the suction cups on the PnP.

We like to say that "through-hole is dead" and "everything is SMT nowadays", but open up most cheap power supplies or even many audio amplifiers (which often have a handful higher power resistors, transistors, ...) and you have plenty of through-hole parts! Hence I was wondering, just how are these populated in high-volume production?

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    \$\begingroup\$ Through-hole is not dead. Whenever you have to have large components like transformers or big electrolytic capacitors, there is a wave soldering process anyway. So why not also mount other components in the same through-hole process? Sometimes, you end up with a single-layer board and THT only. Sometimes, you assemble some parts in a surface-mount reflow process and everything that needs to be large (power resistors!) or can't handle the heat of a reflow process (some electrolytic capacitors, some wound components) gets added later and runs through the wave soldering process. \$\endgroup\$ – zebonaut May 2 '17 at 6:41
  • \$\begingroup\$ I'm not sure through hole will EVER be totally dead. Not only for the reasons mentioned by @zebonaut, but also because SMT parts suffer from a mechanical issue at the solder joints that will cause them to fail eventually under high thermal cycling and mechanical loading / flexing of the board. Boards designs that must tolerate such conditions need to be designed with thru-hole parts so that the mechanical bond it much, much, stronger. \$\endgroup\$ – Trevor_G May 2 '17 at 8:42
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Mass production THT parts are either done with cheap people, or with axial insertion machines. You can find lots of videos on youtube:

https://www.youtube.com/watch?v=CCBST-qk2t8

https://www.youtube.com/watch?v=P17J5RzbsgQ

They pull the part off the bandolier, cut the leads, bend the leads, insert the part, and bend the leads on the opposite side in one stroke.

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  • \$\begingroup\$ Interesting! I found a related video where you could see the tape go into the machine, but it appeared that the tape was a "custom" one, with all the different components on it (so it would be resistors of various types, diodes, ...). I wonder if this "custom" tape would be produced in house or by the suppliers. Thanks for the replies, seems like this is one of those things where once I know what to look for I see results popping up everywhere! \$\endgroup\$ – Joren Vaes May 2 '17 at 6:06
  • \$\begingroup\$ There usually is a standard for those part reels, just like for SMT parts. \$\endgroup\$ – alphasierra May 2 '17 at 6:11
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    \$\begingroup\$ I didn't mean the dimensions, but the components on it. It was not a set of multiple reels, each with there own component - but a single "reel" that had all components on it, in order they were needed (so like, reistor, resistor, capacitor, diode, resistor, diode, ...): This is the video in question: youtu.be/ktCJ6y-WN2U?t=28 \$\endgroup\$ – Joren Vaes May 2 '17 at 6:15
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    \$\begingroup\$ A really lovely detail about these machines, at least for me: If you assemble a variety of parts, the individual bandoliers get re-taped into one bandolier, which is then fed into the machine (at least on some systems). This bandolier now holds all the different parts that go onto the PCB and it looks like a really neat physical representation of your entire through-hole bill of materials. So much nicer than a spreadsheet. @Joren Vaes: Usually done in house. \$\endgroup\$ – zebonaut May 2 '17 at 6:16
  • \$\begingroup\$ Oh neat. I always figured they were able to pick from different bandoliers which were fed in. \$\endgroup\$ – alphasierra May 2 '17 at 6:19
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There are through hole assembly machines!
https://www.youtube.com/watch?v=2sm_qdeconI

But not all parts are rigid enough to be handled this way. Most of the through hole parts are assembled by hand before wave solder. Or also soldered by hand. This depends on other conditions, like dual loaded boards, or quality requirements.

Some through hole components can be placed with regular pnp. These are most found in short pin connectors. You can even reflow solder these by using the pin-in-paste method.

Most through hole parts remaining today are high power silicons, LEDs, electrolytic capacitors and connectors. Those will never die.

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  • \$\begingroup\$ Yeah, I've seen connectors get placed with PnP, but those are usually connectors that are nice and flat. I didn't know they required special care in terms of solder paste tho. Looks like I found another thing to look into: the pin-in-paste method. \$\endgroup\$ – Joren Vaes May 2 '17 at 6:07

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