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Flux residues

Appearance
Thermically decomposed residues of the flux are in principle unavoidable with soldering processes which use flux. This is normally not important as long as qualified flux is used. There is, however, the risk of local flux accumulation at insulated areas. And contact areas are also under constant risk.

Dark flux residues

Description:
Wrong flux application or removal results in dark residues on the board as well as on the solder joints in most of the cases. Boards which have not been cleaned directly after soldering can show dark (brown) residue. Strongly activated fluxes can also cause brown dicolouring and etching.

Causes/Remedy:
  • wrong / unsuitable flux
  • insufficient cleaning of PCB
  • cleaning of PCB too late
  • unsuitable soldering process parameter

Source: visual, optical inspection - general view
dark flux residues

Light (white) flux residues

Description:
Light (white) residues of organic nature can often be found after soldering and/or cleaning the PCB. Fluxes on rosin basis facilitate the removal of residues. . The white residues are mostly caused by layers still adherent before soldering. In fewer cases they are due to the flux only. Incompatibility between flux and solder resist can also lead to such residues. The ageing of the assembly (cracking of flux residues, water absorption) can also make residues visible (flux on resin basis).

Causes/Remedy:
  • wrong / unsuitable flux
  • insufficient cleaning of PCB
  • contamination (depositions) on the PCB before soldering
  • incompatibility of solder mask and flux

Source: SEM
light (white) flux residues
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