Electronic – Cost optimization – PCB Area vs. Double-sided load

costlayout

If one has an opportunity to choose between laying out a PCB as either a single-side load (components all on one side, so it goes through reflow only once and requires only one stencil) or a double-side load but using half the PCB area (so twice the number of boards per panel), which cost optimization tends to be more economical?

I've read this question, but there is no accepted answer and the two answers that are present seem to be at odds with each other. One answer shows that single vs. double-sided for the same board resulting in a small savings ($0.161/board), but that answer didn't include a comparison for board area reduction. It did seem to imply that the single vs. double side load cost different is rather small.

The other answer seemed to imply that single-sided assembly was probably the larger cost driver, but then states that it impossible to know without specifics.

Presume that no components require special treatment (i.e. no gluing or hand-soldering – just a bunch of SMD components).

Best Answer

There are a lot of moving parts when making this comparison, and there's not an easy answer. A double-sided assembly can reduce board area, but can also increase the cost of the board per unit area.

One thing to consider is that a double-sided design doesn't actually give you twice the space:

  • Each via takes up real estate on every layer of the board, which impedes routing efficiency.
  • IC's often cannot be routed "back-to-back", because the via pads from one IC interfere with the lands of the second IC.
  • Even if you can make it work, it is considered bad practice to mount IC's (especially BGA's) back-to-back, as it makes reflow soldering (and also subsequent x-ray inspection) more difficult.

These problems can be overcome by using HDI techniques (e.g. blind/buried vias, microvias), but this significantly adds to the cost of the boards. Not to mention an increase in design complexity.

To compare assembly quotes is easy enough; they only care about how many parts, how many layers, how many reflows. They can quote you for both of these options, before the layout is completed.

With the PCB quotes, on the other hand, it can be difficult to get a board house to make estimates unless you have a completed design to review. I would recommend making two mock-ups for yourself to see how much space you predict you'll need. Also figure out your stackup, your required trace/space, etc, and then call your board house. Be polite, ask for favors, and they may be able to give you an estimate on the two designs :)

Finally, in my experience, it isn't a good idea to choose double-sided assembly for cost optimization. For size optimization, though, a tight, double-sided multi-layer HDI design can be a great (if more expensive) solution.

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