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SolderTip #44: IPC Guidelines for Flux Selection and Use

Question:  Does the IPC have guidelines or recommendations on flux selection and use?

Answer: You ask a very interesting question regarding the selection of flux or fluxes and whether or not IPC provides information or dictates where certain fluxes can and cannot be used.  The answer is NO, IPC will not provide information as to which flux one should use as it does not know the functionality of the board or the product.  IPC will, however; tell you how to evaluate your flux to meet the requirements of your design and application. Additionally there are many flux suppliers out in the market place and to specifically select one would be a conflict of interest as IPC is an organization, which promotes the total electronic industry.

So where do we begin in selecting a flux.  First and foremost all fluxes are acids and this need has to be kept in mind when the selection process begins, as the acidic activity may or may not be there after the soldering operation. This will guide you as to whether or not you need to have a cleaning process to remove these residues from the surfaces of your product.

Things to consider on your product are entrapment areas where the flux may travel to and you cannot clean it off.  This can be a major problem with surface mount components, especially those mounted very close to the surface of the board, such as micro BGAs, chip caps and resistors.

So where are we: We must select a flux, which will help solder the product, but won’t be a problem once the product gets out into the field.

Basically you want your flux to do three things, prepare the surfaces to be soldered, keep it protected from reoxidizing during the soldering operation and get out of the way when the solder comes along.  Secondly, you need to decide whether or not the residues from the used flux can be left on the board after the soldering operation.

J-STD-004, can be used to evaluate the fluxes, as it discusses corrosion properties and appropriate tests to determine the activity levels of the flux.  Incidentally, J-STD-001 mentions that all fluxes will be evaluated per J-STD-004 for qualification.