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First, can water contamination cause failure? Absolutely! Rainwater contains much higher electrochemically active content than the deionized water used for cleaning PCB assemblies and can lead to corrosion and dendritic growth. If discovered before introduction to the service environment, cleaning may be possible, and the assemblies may be able to be salvaged. Exposure to rainwater in the service environment is a sure path to failure unless the assembly has been ruggedized against such exposures as part of the design.
Second, how do you know if a flux is bad and shouldn’t be used? Flux is a chemical that reacts with oxides that naturally form on metal surfaces. When flux cannot adequately react with the metals that are to be joined, a solder joint does not form properly. Even with fluxes that are properly stored, time can reduce the ability of a flux to perform that intended function.
Third, are there any simple tests for this? A good quantitative test for liquid fluxes is the acid number, and an acid number below the flux’s particular minimum specification is a key indication that a flux should not be used. Otherwise, qualitative testing of a flux’s soldering performance is common to resolve suitability for use after a question. Each form of flux has its own solderability tests that can be performed, but they all boil down to “Does the flux work or not?”
A higher level of defects and reduced soldering performance is to be expected when too-old fluxes are used. Solder joints that form properly are not expected to exhibit reduced reliability. However, a higher number of defects created tends to lead to a higher chance that defective connections escape detection through inspection and functional testing, and that’s not a risk to be taken lightly.
Jason Fullerton is the customer technical support engineer at the Assembly Division of MacDermid Alpha Electronics Solutions.