Cold joint

Poor wetting

Dull grainy surface

Flux trapped against lead

Cold joint

Poor wetting

Pits and voids

Dull gravity surface

Solder setting very slowly

Results in dry brittle joint

Disturbs the chemical bonding

Joint becomes mechanically stronger

Flux will not dissolve

It will corrode the joint

It results in oxidation of solder

It disturbs the chemical bonding take place

To help the corrosive action

Cleaning before soldering the joint

To break down the acid within the joint

Remove residual flux and prevent corrosion