A customer called into the EMPF Helpline to ask if a DI water rinse cycle is necessary when cleaning misprinted circuit boards and stencils.

A customer asked whether a deionized (DI) water rinse is needed when washing stencils and misprinted circuit boards with Isopropyl Alcohol (IPA) and Vigon SC200.  The customer was seeking the correct chemistry and equipment for a process that cleans dirty screen printing stencils and misprinted circuit boards.  The customer had heard that a DI water rinse cycle was not needed when washing with IPA or chemistries such as Vigon SC200. 

Typical contaminates removed when cleaning solder paste from stencils, misprinted circuit boards, and circuit board assemblies are fluxes, flux residues, oils, mold release agents, metal oxides, and plating and etching salts.  Nonionic contaminants may cause electrical “opens” and can cause adhesion problems for conformal coatings.  Ionic contaminants, such as flux residues, plating, and etching salts cause leakage currents if left on circuit boards and corrodes products if left on metal surfaces (since flux is slightly acidic).  In the presence of a voltage potential and moisture, ionic contamination may align in an electromagnetic field and form dendrites that may grow and cause electrical shorts.

Cleaning processes consist of wash, rinse, and drying steps.  There are several types of cleaning compounds: 

• Soaps—use salts of organic acids.
  
  
• Solvents and Synthetic solvents —dissolve contaminants.
  
  
• Aqueous cleaners—use blends of detergent compounds with water to lift contaminants from a surface.  Detergents, sometimes referred to as synthetic soaps, perform better than soaps in hard-water (mineral-laden) applications because they contain water softeners to effectively treat dissolved magnesium and calcium ions.

Water is called the “universal solvent” and is an important component of aqueous cleaners because of its ability to dissolve inorganic and some organic contaminants.  Due to its strong dipole, water forms stable solutions of other dissolved polar molecules (or ions) by arranging them on the alternating positive and negative ends of the water molecules.  However, not all contaminants dissolve readily in water. For this reason, aqueous detergent cleaners (such as Vigon SC200) are complex mixtures specifically formulated to create greater chemical and mechanical cleaning action.

Deionized water is water from which all the ions have been removed.  As a result, DI water readily dissolves residues left behind from the wash cycles.  Regular tap water, especially hard water, contains dissolved minerals and is not as aggressive in dissolving residues.
 With aqueous cleaning, the last thing to come into contact with the cleaned surface is the rinse water. A thorough rinse will remove contaminants which have been cleaned from the surface and any remaining detergent residue. Whatever contaminants are present in the rinse water may become residue after rinsing. Therefore, more stringent cleaning requirements increase the need for greater rinse water purity.

DI water rinses are preferred rather than tap water rinses because DI water has less potential for metallic cation (positive ion) deposition which leaves conductive residues on electronic components.

If doing batch cleaning, DI water is recommended for cleaning circuit boards, especially for High Density and Class 2-3 applications.  In fact, DI water is the preferred method of cleaning electronic assemblies.  In regards to stencil cleaning, it is not necessary to use DI water to clean the stencils.  Make sure the stencils are adequately rinsed and cleaned by the solvent cleaner used to remove the paste.  A DI water alternative to assure cleanliness of the stencils is to use heated water to clean the stencils more quickly and with less water.  Some advantages of a DI water rinse for stencils are:

• Mineral residues are not left behind due to the hardness of the water.
  
  
• DI water cleans more aggressively and less water volume is needed during the rinse cycle.

Reference:
McLaughlin, Malcolm C. and Alan S. Zisman. Aqueous Cleaning Handbook. AI Technical Communications, 2002 (3rd Edition).