This week Eric Bastow offers a series of 3 entries to explain the basics of water soluble solder paste:
“All these product names generally refer to a type of solder paste that leaves behind a post reflow flux residue that is cleaned off or removed with water. Removal of the post reflow flux residue is typically mandatory; although, there are some manufacturers that claim that their water cleanable flux residues can also be safely left on the assembly, without creating leakage current or corrosion issues.
The majority of water cleanable solder pastes have their residues removed by means of heated (~120F - 135F/~53C – 57C) deionized water that is sprayed on to the assembly at ~60 psi. Temperature and pressure of the DI water varies with each solder paste formulation, the cleaning equipment and the angle at which the water strikes the assembly. There are two different cleaning “mechanisms” that solder paste manufactures design into their water cleanable solder pastes.
1) The flux residue is softened by the heated water and “knocked” off by the force of the sprayed water striking the residue.
2) The residue is truly soluble and readily dissolves into the heated DI water. A good water cleanable paste will be designed to be cleaned with just heated, sprayed DI water. However, water’s natural surface tension makes it difficult to clean in areas where spacing is very tight. In these cases, it may be beneficial to use a commercially available cleaning additive to reduce the surface tension of the water allowing it to more effectively clean in tight areas.“ –Eric Bastow