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During large-quantity PCB assembly/soldering, are SMD (surface-mount) headers traditionally easier or harder to assemble than DIP (through-hole) ones?

By easier/harder, I'm concerned about differences between the two in terms of assembly time/cost. I'm designing a board where I would like to make a better-informed choice between the two.

I suppose the only way there would be a difference in difficulty between the two is if one of them were manually soldered, so another way to ask the question might be:

Are both kinds of headers soldered by hand to this day, or is one of them machine pick-and-placed with some customized reflow profile so as not to melt the plastic?

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  • \$\begingroup\$ Check with the PCB factory, they may or may not have a capable machine for it. \$\endgroup\$
    – jippie
    Apr 20, 2014 at 16:53
  • \$\begingroup\$ I think it's safe to say that through hole components are more expensive: you need to drill the board and possibly metallize them (is metallize the correct word?). If you have ever made diy PCB you know for sure that drilling the holes is the most time consuming (and boring) operation. \$\endgroup\$ Apr 20, 2014 at 17:01
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    \$\begingroup\$ @VladimirCravero: Yes, but I am concerned more with part-assembly/soldering cost than the raw board manufacturing cost. \$\endgroup\$
    – boardbite
    Apr 20, 2014 at 17:17
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    \$\begingroup\$ @jippie: Will do. Is the "machine" a wave-soldering apparatus, or are there specialized pick and place machines used for positioning and soldering headers/through-hole components in place? \$\endgroup\$
    – boardbite
    Apr 20, 2014 at 17:19
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    \$\begingroup\$ @jippie: Was asking out of curiosity, to familiarize myself with what methods exist. \$\endgroup\$
    – boardbite
    Apr 20, 2014 at 18:39

2 Answers 2

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If you mix SMT with THT , you may need to do wave soldering after reflow soldering for 1sided boards. One way to avoid this is to assemble pin in paste connections by design. Hole size and paste aperture size are selected to reflow the THT header at same time as SMT parts. Consult with Process Engineers for best results.

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  • \$\begingroup\$ Didn't know about wave soldering; I expected hand soldering was the way that headers are traditionally soldered even in large quantities. However, I didn't understand your 2nd sentence: What is "assemble pin in paste connections by design"? How can consideration of the hole size and paste aperture size help with reflowing a through-hole header; won't the plastic on the headers melt off? \$\endgroup\$
    – boardbite
    Apr 20, 2014 at 17:14
  • \$\begingroup\$ I think that plastic can stand high temperatures for a while... smt connector plastic MUST stand it so... \$\endgroup\$ Apr 20, 2014 at 17:19
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    \$\begingroup\$ molex.com/webdocs/datasheets/pdf/en-us/… these headers can withstand 235°C for 5 seconds. That's not too much but it might be enough. \$\endgroup\$ Apr 20, 2014 at 17:23
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    \$\begingroup\$ There is also a process called "selective wave" where a solder fountain is applied only to the areas where THT parts are located. This requires extra clearance between the THT part and bottom-side SMT parts, but it avoids needing to epoxy down all the bottom-side parts. \$\endgroup\$
    – The Photon
    Apr 20, 2014 at 19:00
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You don't necessarily have to do wave soldering for the THT parts. There's a technique called THR (through-hole reflow) that works for certain components like connectors. Basically the holes get filled with paste (e.g. by dragging it into the hole with a squeegee-like device) and then the pin is inserted in the pasty hole. So this method is also called "pin in paste".

enter image description here

There are even detailed guides for caculating precisely the ammount of solder paste that's needed for this kind of THR process. More on THR/pin-in-paste in this Littlefuse appnote, which focused on [their] fuses used that way though.

Yet another name/acronym for this is PIHR (pin-in-hole reflow). Worth mentioning because the guys using/promoting it wrote extensively about it. And I can [partly] answer the question from the comment[s] from there: "The first presentation on the subject was by Racal at Nepcon West, USA in the late 1980s. Unfortunately, the paper was never published." I don't know when it became more widespread though.

Also there are some small scale (still four figures in cost) ovens and stencils (+squeegee) compatible with this THR process; an example I know about is sold by eurocircuits, which also has some tips on using THR with it. Most of the tips would apply to any other small oven used for THR though.

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    \$\begingroup\$ Wow. Is this a more recent innovation? \$\endgroup\$
    – boardbite
    Nov 24, 2015 at 18:49
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    \$\begingroup\$ I don't know when in came about, but it looks fairly common today. \$\endgroup\$ Nov 24, 2015 at 18:50

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