Why is flux residue bad for your board?

Flux residues are one of the most common contaminants found on the PCB at the end of the soldering process. During soldering, flux is applied to aid the bond between the solder, the component, and the PCB. Failure to clean the remaining flux residues can result in problems with conformal coating and cause corrosion, electrical leakage, and short circuits. 

Types of flux residue

Rosin and Water-soluble flux

  • These flux leave behind ionic material, which forms dendrites that could lead to current leakage and eventually electrical shorts 
  • Flux residue should be removed as soon as possible to prevent damaging the board
  • Typically, this is not a concern as it is standard procedure to clean boards after using rosin or aqueous flux; however, the cleaning process has to be thorough to avoid any missed spots

No-Clean Flux

  • It is not always necessary to clean after using no-clean flux, but it is recommended
  • No-clean flux leaves behind negligible residue that may still build up over time
  • Any impurities left on the board may cause defects in conformal coating application
  • Industries that require technically-clean boards (e.g. aviation or semiconductor) will still require cleaning of boards after no-clean flux is used
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