Top preheater for wave soldering

Use them always - they help for even preheat of the board. If there is a temp sensitive/melting component on top side, turn them off.

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I would always have top and bottom preheaters in a solderwave machine. Even if you can make it work without them, it's better to heat both sides of a board then just one. Far less stress on the parts you are soldering. Plus you may need it one day for thicker pcb's or pcb's with high thermal mass.

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I used to work for an OEM that made professional audio equipment (studio monitors, mixing boards, etc.). The mixers required precise alignment of LEDs, switches, and pots in the sheet metal so we used aluminum alignment fixtures during wave solder. That precluded the use of top side preheaters since the fixtures would get hot enough to melt some of the switch buttons if we tried to reach 100C on the top of the board.

We had to ignore the IPC-610 requirements for top side fill and write our own because of it.

Long story short, I can't think of an instance where you wouldn't use them unless you had some crazy circumstance where you just couldn't.

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