Antifoam is unfortunately a necessary evil for developing and resist stripping spray processes. I say unfortunate because antifoams are typically petroleum- based products that we do not want on the panel.
However, the surfactants which are in the resist are dissolved into the developer or resist stripper bath and must be controlled. If this foaming isn’t controlled, it can lead to not only a mess, but a loss of spray pressure, due to pump cavitation.
Too much antifoam can cause the redeposit of the antifoam onto the panels by having it sucked-up through the pumps and sprayed through the nozzles. Too little or ineffective antifoam has its obvious issues.
Follow these guidelines for proper antifoam use:
It is very important that antifoam not be added in large amounts in order to avoid the potential deposition onto the panels. I always suggest avoiding manual adds. Start by using a feed pump to intermittently add small amounts of antifoam on a continuous base. Commonly, the feed pump on/off can be activated on a timer run off of the control panel. Low flow metering pumps or peristaltic pumps work well for this function, as they can be set to add less than a liter/hr. The least desirable scenario is having an operator add antifoam directly to a bath from the five gallon container.
Alcohol based antifoams are typically less expensive than antifoam concentrates, but the lasting power is short lived. Alcohol base antifoams appear to do a good job because they are very quick. This is achieved by lowering the surface tension of the bath. However, as the alcohol is absorbed into the solution, there is an immediate need for more additions. I have had customers decrease their usage by tenfold simply by switching to a concentrated non-alcohol based antifoam.
When determining if you are using too much or too little, I use this rule of thumb; a little foam is a good thing. If the feed frequency is set up correctly, there will always be a small amount of foam on the surface. I shoot for approximately a half inch to one inch of a foam blanket across the top of the sump. If this is maintained over time, the usage is correct. Keep in mind that if the resist brand is changed, or loading is increased, the foaming will change accordingly and adjustments will be needed.
I normally recommend floating antifoams for three reasons: they tend to be more effective and last longer, the surface is where the foam is, and I don’t like the thought of spraying antifoam onto the panels along with the process solution.
Some antifoams contain large amounts of silicone and should be avoided. If this silicone is deposited onto the panels it can be very difficult to remove and if this happens in the outerlayer developer, it can lead to plating issues.