|Ask the Experts|
November 20, 2020
Out-gassing and Cleaning
I have created a series of PCBs with no solder mask. I will be soldering 2 spring pins, a 0805 ceramic capacitor and a 0805 resistor to each board. There are 40 PCBs in the stack.
The stack will be installed in a vacuum chamber. Out-gassing from the cleaning solution is a concern.
Do have a recommended process for cleaning the PCBs that will not out-gas when installed into a vacuum chamber?
|Expert Panel Responses|
I believe that vapor phase degreasing can be done with a vacuum purge afterwards. This would help to remove trace solvents before the circuit boards are installed in a vacuum chamber.
Another option is to use a chemical cleaner with high vapor pressure and a fast evaporation rate. This would help trace solvents to evaporate after cleaning.
Regardless of the cleaning method used, the circuit boards could be dried in an oven before installation. Again this would aid in removal of cleaning solvents.
Outgassing from the solvents will not be an issue if you use one solvent, or a blend of solvents, that have a high vapor pressure, for example greater than 300 mm Hg. You do need to be sure that you clean the assemblies well, however, because the solvent that will remove the flux may be entrapped with the flux residue under the components if they have a spacing of under a mil or so.
Water based cleaning residues tend to be more difficult to remove from under components, so I recommend a halogenated hydrocarbon, or hydrocarbon blend. For example, the Fluosolv AP or Fluosolv Deflux, available in aerosol, or similar products from other manufacturers, would probably be your best bet. Look for an aerosol that contains trans, dichloroethylene, a fluorinated carrier, and possibly a small amount of IPA isopropanol, or Ethanol, or even Methanol.
If you can spray under the components, at an angle, it should force out the solvent / flux contaminants from even BGAs. Also, most vacuum outgassing chambers have a "cold finger" to collect condensible contamination. Make sure the cold finger is adequately cold and clean before outgassing / degassing the boards. If you need more info, feel free to call or write.
Due to the nature of the materials you are using, there will be outgassing under vacuum, although you did not define the mTorr value of the vacuum. For analytical evaluations of PWB's under vacuum such as AES or XPS, it is common to pull a vacuum for up to 24 hours before analysis begins because of the sample continuing to outgas.
You should bake the boards post washing for a prolonged time period above 105C to help drive off any volatiles but given the operating vacuum level you still may see outgassing initially.
S T and S Testing and Analysis
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