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When Should You Remove Flux Residues?

When Should You Remove Flux Residues?

Flux is an essential part of the soldering process.  Flux removes the oxide layer to help the solder bond to the surface as well as easing the flow.

When you solder, there is always a flux residue that is left behind.  Certain kind of flux residues can continue to corrode long after assembly if not removed.

However, the flux that you use will ultimately determine when you need to clean your parts.

Which Flux Should be removed from your assembly:

If you are using no-clean or rosin-fluxes, the flux residues are generally safe to leave on your device.  It is important, however, not to leave on water-soluble residues because they can ultimately damage the components and must be cleaned off.

For the most part, consumer products with a short life don’t have to be cleaned, but high-reliability critical products may require all residues to be removed.  

If your J-STD-001 Class I or II PCB (consumer or industrial) was assembled using a no-clean flux, it is generally safe to leave any remaining residues in place.  If your electrical component normally uses Rosin fluxes, it can be safe to leave these residues intact as well.

Below are 5 reasons why you should clean the flux residues after soldering:

1. Prevents Corrosion

If the flux residues aren’t removed, it will cause corrosion.  Flux residues are acidic and if they aren’t cleaned properly, the residues will lead to the corrosion of the PCB and components from moisture in the air.  Eventually the joints will weaken and fail.

2. Reliability Issues

If the flux residues are removed around the PCB, it can help improve the performance of the PCB and enhance its durability.  If the flux isn’t properly removed, it can damage the PCB and its components.

3. Conformal Coatings Could Be Affected

Flux resides that remain on the PCB before there is a conformal coating process can stop the coating from sticking.  Because there could be steam or outgassing, it can potentially affect the adhesion.

4. Conductivity Issues due to Ionic Contamination

If flux residue is left on a circuit board, it could cause a short circuit or even leakage. This could be a result of the ionic or polar particles that are left on the circuit board from the flux residue.  When exposed to moisture and a current is applied, they can attach themselves to a branch or even a chain which is known as a dendrite and ultimately cause a short-circuit.

5. Cosmetics

By removing the flux resides, it can help prevent the spots that the residue leaves behind which makes it look dirty.   Solder balls could be captured inside the flux and therefore should be cleaned off.  This not only could cause a possible defect but by cleaning the flux, it will look cleaner and better.  This is important when selling the PCBs to consumers.


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