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Why are power supplies almost always made using through hole components? Every computer PSU I've taken apart uses through hole components, though occasionally (not in all cases) surface mount components are found on the bottom. Don't these have to be hand assembled? (before reflow or wave soldering) If so, why are they still doing this, even though labour costs are low in China, it still must cost less for a machine to pick and place SMT stuff... or am I missing something?

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    \$\begingroup\$ would guess that the through-hole stuff is generally higher wattage \$\endgroup\$ – JustJeff Nov 15 '10 at 23:43
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    \$\begingroup\$ And they always use cheap phenolic PCB's instead of FR4! \$\endgroup\$ – cksa361 Nov 16 '10 at 3:10
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Because PSUs use many big lumpy parts that are not SMDable and/or need good mechanical fixing. Also, for minimum cost they like to use single-layer PCBs - TH is a little more amenable to this as parts act as jumpers over tracks. TH parts can be machine-inserted - e.g. http://www.youtube.com/watch?v=eOQ3pZkKX24 (30kparts/hour!)

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    \$\begingroup\$ I wonder if you could mix TH on one side, SMD the other, and wave solder it all in one pass. \$\endgroup\$ – Nick T Nov 18 '10 at 21:21
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    \$\begingroup\$ @Nick T - sure, have a look inside a linear high frequency fluorescent ballast, its very similar to a switched mode PSU and made for low cost. Single sided board, all the SMT underneath, all the through hole on top. \$\endgroup\$ – Martin Nov 29 '10 at 13:33
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One last reason that I haven't seen here (And it's likely the most relevant one):

SMD components are too small.

I mean literally. When you're dealing with high voltage, you have to worry about flashover/board creepage distances, which means that the connections for high voltage have to be separated by a certain amount (There are standards for this, which are required to get UL or similar ratings).

With 240V AC, the distance is (off the top of my head) ~.25", which is far larger then even 1206 parts.
This is also the explanation behind the slots cut into the PCB, which you often see under optocouplers/input filter caps. Basically, to pass testing, the separation on the component's leads is not great enough, so they have to actually mill slots the board. This increases the overall path-length between component pins on the PCB.

Lastly, most of the power devices are through hole because through hole packages can dissipate more power than SMT parts. It's much easier and cheaper to mount a TO-220 package to a cheap extruded aluminium heatsink then have a board with very thick copper fabbed that can dissipate the same amount of power from a TO-263 device.

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    \$\begingroup\$ In which case, why don't they make the primary side with through hole, but the secondary side with SMT? \$\endgroup\$ – Thomas O Nov 17 '10 at 16:59
  • \$\begingroup\$ @Thomas O - well, they do. Have you looked at any SMPS recently? They're almost all mixed SMT/TH. \$\endgroup\$ – Connor Wolf Dec 24 '10 at 6:52
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Half the parts are too big or for other reasons can't be re-flowed (power dissipation, etc) and the cost of doing a re-flow run and a hand process run is higher than just doing it all by hand.

You would be amazed how cheap hand soldering is in Asia.

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    \$\begingroup\$ They don't wave solder them? I'm not sure how much quality you can get from hand-soldering if it's being done en masse, though there are lots of stupidly shaped parts that may not be amenable to machine insertion. \$\endgroup\$ – Nick T Nov 16 '10 at 2:59
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    \$\begingroup\$ they are hand soldered, never seen an Asian CM bother with wave soldering. \$\endgroup\$ – Mark Nov 16 '10 at 7:47
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    \$\begingroup\$ I've been working in power for years. All CMs that I've seen do indeed have wave soldering, not to mention comparable SMD placement capabilities to domestic builders. If you're doing big power with multilayer boards you simply cannot get enough heat into the PCBs to get the solder to properly wet. Cheap ATX supplies are a different animal. \$\endgroup\$ – Adam Lawrence Nov 16 '10 at 14:26
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    \$\begingroup\$ They just have warehouses full of people hand-soldering on assembly lines. I don't understand the economics, but that's how they do it there. :/ They use jigs to hold the parts in place, though, it's not like they're holding each one in their hands. \$\endgroup\$ – endolith Nov 16 '10 at 15:30
  • \$\begingroup\$ Big components, like transformers, caps and inductors must be added by hand, but I suppose small caps and resistors could be machine inserted. \$\endgroup\$ – Thomas O Nov 17 '10 at 16:59
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The power dissipation for a component is much higher for TH. They get better airflow from fans also.

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