What is Water-Soluble Solder Paste?

Solder 

What is water soluble solder paste? How is it different from no-clean solder paste?

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Editorial Team - PCB Directory

Jan 17, 2021

Water-Soluble Solder Paste as the name suggests is solder paste whose flux residue left after the solder reflow process can be dissolved or removed with water.

The PCB assembly world now mostly uses no-clean solder, where flux residues can also be safely left on the board, without creating leakage current or corrosion issues. However, though the flux residue of no-clean solder paste is not conductive or corrosive, it does need to be cleaned off the board as it can result in parasitic leakage, dendrite growth, electrochemical migration, and shorting. Removal of this residue has become mandatory for applications, such as military, aerospace, high frequency, semiconductors that require perfect elimination of the residue after reflow.

Ironically, the flux residues left behind by no-clean pastes are way difficult to remove from PCBs than other fluxes. Hence, the use of a water-soluble solder paste is an easy way out. Water-soluble solder pastes have residues that can be cleaned by means of heated deionized water that is sprayed on to the board. The temperature and pressure of the deionized water vary with each solder paste formulation. Also, there are two different cleaning “mechanisms” that solder paste manufacturers design into their water cleanable solder pastes. The cleaning equipment and the angle at which the water strikes the assembly.

When using water soluble solder pastes reflow is done in an inert atmosphere (nitrogen, vapor phase or under vacuum ovens) and where cleaning is compulsory for the majority of the products. Typically, water-soluble solder pastes have a higher-level of wettability because of their strong activation. Hence, these sorts of solders form a better bond. Having said that, they are also known to have a shorter stencil life and to be more sensitive to conditions as temperature and humidity, compared to the no-clean pastes.

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