**12. Flux spattering from the soldering paste**

During the reflow process, the flux may spatter from the soldering paste. If the PCB is cleaned after soldering, flux spattering need not necessarily be considered a problem. If PCB cleaning is not included in the process, then flux residues can not only be the cause of future corrosion, but flux spatter can leave stains on the test surfaces intended for the subsequent electrical testing methods.

If a flux stain on the test surface gets into sites where the contact test tip is in contact with the test surface, then, due to the non-conducting nature of the flux, the electrical test evaluates the product as a reject, even if the product is fully functional. In addition, these stains are, in most cases, clear and therefore difficult to detect. An example of a photo of the test pads on a PCB is given in **Figure 22** along with an image from an electron microscope, where a dark stain is evident; the source of the stain was flux spattering.

There are various sources of non-conducting stains on the test surfaces, for example, the solder mask, organic coatings, inappropriate materials used when maintaining technological equipment, etc. Diagnosis of the cause can be very problematic.

**Figure 22.**

*Test surfaces for electrical testing methods: (a) photo from an optical microscope and (b) photo from an electron microscope—a dark stain is obvious, and the source of the stain was flux spattering.*
