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Most consumer appliances/power adapters that I open up have an SMPS providing the power. This SMPS is layed out on a phenolic resin (FR2) board and is usually a one-sided layout. Can anyone explain why power supply boards are designed as such?

My guess is that this board mostly contains through hole components, and is easier to wave solder when laid out on a single side. Since FR4 is used for multilayer boards/through hole plating and vias, it is cheaper to lay the power supply out on this type of board.

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    \$\begingroup\$ There are three reasons: cost, cost, and cost \$\endgroup\$
    – stark
    Apr 18 '20 at 11:46
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    \$\begingroup\$ Let me add another reason: cost \$\endgroup\$ Apr 18 '20 at 11:49
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    \$\begingroup\$ single-sided PCBs are likely to have horrid magnetic fields, because of the poor options available in trace routing. What saves the day is the STEEL SHEET folded into a shield and wrapped around the SMPS. \$\endgroup\$ Apr 18 '20 at 14:48
  • \$\begingroup\$ Another reason is it being cheap. \$\endgroup\$
    – Audioguru
    Nov 11 '20 at 18:11
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Cost. It is cheap, and it is good enough.

"The best is the enemy of the good enough"

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  • \$\begingroup\$ The best is often the enemy of good enough, but not always. There are meny cases where "the best" is cheaper than good enough. The best can lower production costs, maintenance cost, and reduce costs due to malfunction. Good enough generally has a higher risk of malfunction and the cost of failure is not very well assessed most of the time. \$\endgroup\$
    – le_top
    Apr 19 '20 at 12:10
  • \$\begingroup\$ If phenolic more easily becomes a charred conductor, which creates a destructive runaway (and possible fire!), then the solution isn't to use a better material. Instead the solution is to slice a slot in the phenolic! That way, if it turns to charcoal, it still cannot short-out a high-watts circuit node. CLASSIC CHEAP TRICK! (Heh, it really should be used far more often on FR4 boards having few-kilovolt nodes. Carbon paths unexpectedly destroy things, but only after a few-years delay.) \$\endgroup\$
    – wbeaty
    Apr 20 '20 at 2:19
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Why are most consumer electronics power supply boards single sided and made of FR2 (Phenolic resin)?

Cost. The FR2 will be a bit cheaper than glass fibre boards and may be a little easier to machine meaning less tooling cost. Single-sided board will be cheaper than double-sided due to reduced copper cost and processing steps.

enter image description here

Figure 1. XT PC Power Supply PCB - Component Side. Source.

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Figure 2. XT PC Power Supply PCB - Solder Side. Source.

Note that some of the traces are rather circuitous to avoid crossover connections on the other side of the board.

Can anyone explain why power supply boards are designed as such?

The circuits aren't so complex and the board can be laid out without the need for wire jumper links. There are none visible in Figure 1.

My guess is that this board mostly contains through hole components, and is easier to wave solder when laid out on a single side.

Double-sided boards are generally wave-soldered in the same way. The traces on top facilitate jumping over other traces and the through hole vias connect the the two layers.

Since FR4 is used for multilayer boards/through hole plating and vias, it is cheaper to lay the power supply out on this type of board.

Yes. You could get a feel for the cost saving by trying to price, say, 1000 PCBs on one of the online PCB manufacturing companies.

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  • \$\begingroup\$ Hm, curiosity: What are the red wires in the left half of figure 1 if not jumper wires? \$\endgroup\$ Apr 19 '20 at 11:25
  • \$\begingroup\$ @JonasSchäfer - Those red wires appear to come off the transformer on the center left of the board. Those would be more like flying lead connections rather than the type of wire jumpers often found in more complex single sided boards. As an interesting point I am currently making a hack of a USB numerical keypad by adding a miniature Arduino board to it. The keypad circuit board is single sided paper type board. For jumpers the circuit uses lots of zero ohm SMT resistor components. They used large body SMT parts for these so they can squeeze two or three traces under the jumper part. \$\endgroup\$ Apr 19 '20 at 12:09
  • \$\begingroup\$ @MichaelKaras Aha! Thanks. I didn’t see them going into the transformer, I thought they went past it. \$\endgroup\$ Apr 19 '20 at 15:55
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FR2 can be punched instead of drilled. FR4 has to be drilled. When you're making 100's of thousands, it's worthwhile to make one big punch and do a ton of boards in one pass. Drilling is expensive and takes time. So it's "cost" ;)

It's because of the materials they're made of. FR2 is paper phenolic resin. FR4 is fiberglass

Single sided boards are a lot cheaper to work with 'cuz you only have to process one side of them.

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  • \$\begingroup\$ You're right, my bad. Late-saturday-night posting ;) I was thinking of the "V0" etc ratings, which are not tied to material. Post corrected - Thanks! \$\endgroup\$
    – Kyle B
    Apr 20 '20 at 4:19
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One reason for single sided boards in consumer electroniss is they are easily serviceable, and it is easy to service them during the warranty period instead of repairing the whole board.

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The power supply boards having higher voltage, have higher risk of arcing, static and carbon formation making the circuit in-efficient and higher power losses and pose risk of short circuit damaging the psu and could be harmful.

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You will find most PCBs are are made with the copper pre bonded to both sides and it is etched away to make the spaces for the traces. While the designer of the board could specify traces/pad/etc only on the bottom, which will require all of the copper to be removed from one side and hence have little difference in cost to a 2 layer board. I doubt most manufacturers would keep any stock of 1 sided pcb cores. It would be most likely a custom order and if the quality was high enough would maybe justify the saving. 1 sided boards are terrible at containing the return currents for every trace and can lead to crosstalk and high interferance issues. There would be a cost saving not having to plate vias in 2 sided boards, but it is a minimal cost for a better design. If you make millions of units it might be worth the cost saving using a single sided core.

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    \$\begingroup\$ I invite you to check prices online for PCB boards and compare single sided prices to double sided. You will see that single side PCBs are cheaper than double sided PCBs (same size/same quantity) and regardless of the quantity. \$\endgroup\$
    – le_top
    Apr 19 '20 at 12:15
  • \$\begingroup\$ This was not a good start here on EE.SE. \$\endgroup\$
    – winny
    Nov 11 '20 at 17:59

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