Rule of thumb for surface tension when placing components on the secondary side of a board

Question

I've been working on my first multi-layer PCB design, and have been placing a variety of components on the secondary side of the board (mostly decoupling capacitors and QFN ICs). However, some of these components are fairly large, and I am beginning to wonder whether they would fall off the board during reflow.

• Is there a general guideline dictating the maximum size of passive components allowed on the secondary side of the board? Is 0805 too big?
• At what point does a component become too heavy for the secondary side? Would a ~14mg (0805) inductor be likely to fall off?

Answer 1

0805 resistors are fine without adhesive, in my experience. That's with 63/37 solder paste.

This site contains the following claim:

Most surface mount components will be held in place by the surface tension of the liquid solder alone when run through the re-flow oven inverted. The weight limit of the parts that can be processed on the underside during reflow is related to the pad area. This is approximately 30g per square inch of pad area before the component will actually drop.

My standard (low density) 0805 is 3.22mm^2 for both pads, which is about 0.005"^2, so that's 16mg. An 0805 resistor is 4mg.

You might want to put your 14mg part on the other side if you can. I think 14 and 16 are a bit too close for comfort. A bit of vibration or slightly different processing conditions might be fatal.