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Maybe this is a stupid question, but here goes: I just got a piezo buzzer and on the top there is a sticker that says "Remove After Washing".

My question is why would I ever want to wash an electronic part? I have no idea. Is there some manufacturing step where this makes sense?

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Flux removal is one. hackaday.com/2011/06/07/cleaning-flux-from-pcbs-the-easy-way – kenny Feb 2 '14 at 19:09
This reminds me of a story at one of the places I worked. I got called in because they had a new batch of boards with a number of changes I made, and the buzzers much quieter then the previous batch. They overnighted me boards and made a big deal of it. I got the boards, took off the kapton tape on the buzzers, and sent them back. – Connor Wolf Feb 3 '14 at 13:23
@ConnorWolf That is hilarious. – kmort Feb 3 '14 at 14:58
up vote 19 down vote accepted

The industrial PCB assembly process usually leaves residues — mostly soldering flux — on the circuit board. One step in the process is to wash the board (by dipping or spraying) with a solvent to remove those residues for long-term reliability and for the sake of appearance.

Some devices (such as sound or pressure transducers) have openings for their functioning, and their performance would be adversely affected if the solvent or the residues got washed into the opening and lodged there. Therefore, such devices often have a sticker that covers the opening(s) that should not be removed until after the washing.

Removing the stickers adds an extra step to the process, so for really high-volume manufacturing, it is often worthwhile to select parts that are declared "washable" to begin with.

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Many processes use "no-clean" flux today and you can remove the sticker after manufacture. Others use water soluble flux and wash the finished assembled boards in what are basically dish washers.

You can leave it on for a quieter buzzer - and a different frequency. Or remove it for a much louder buzzer plus get the tones intended in the software. (Move your finger around partly covering the hole while a buzzer is running and you will see the many frequency shifts caused by changing the acoustics). A lot of Chinese manufacturers leave them on, maybe not knowing what it says.

They also keep out dust and critters. But for that I would use something more permanent.

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