April, 2012
www.us-tech.com Getting PCBs Really Clean
Continued from previous page orinated solvents.
Detergents are good most of the time, but are very specific to the type of fluxes to be removed. The deter- gent’s concentration is very impor- tant in water and can vary between 3 to 50 percent in some cases. The tem- perature can vary from 20 to 60°C, and the agitation used, sprays, spray under immersion or ultrasonics should be considered. In general terms, this is called the aqueous cleaning process. The drawbacks of these detergents: the incomplete removal of all residues under compo- nents because of the poor/high sur- face tension between 40-50mN/m; the aggression of these formulations and compatibility with materials becomes another issue; and the rins- ing with tap or DI water (high sur- face tension 70-80mN/m), the drying operation, the water-proof compati- bility and the disposal of used (conta- minated) water and solvent. The total cost of all of these factors should also be considered.
The petroleum distillates such as alcohols and ketones are mainly used for the cold cleaning operation, even though they can be found being used at warm temperatures. These prod- ucts are very flammable at room temperature (20°C) and when used under warmer conditions, can become very risky. Costs are accept- able, but disposal and annual cost can be significant.
Formulated hydrocarbons were developed after the CFCs and HCFCs passed out of vogue. When perfectly formulated, they easily out- perform any other cleaner. They are able to remove flux residues, solid residues and salts under any type of components because of their very low surface tension (approx. 20mN/m). However, as with the aqueous process, the same detergent draw- backs exist, whereas, with the co-sol- vent process, the PCBs are very nice- ly rinsed and dried with the vapor phase fluorinated solvents. The rins- ing solvent can be recycled by distil- lation and the formulated hydrocar- bon is easily disposed of. The formu- lated hydrocarbons have a very
The combination
of ultra-miniaturization and lead-free soldering have conspired to make the cleaning proposition especially difficult.
extensive lifetime and because of this, the total costs are the lowest of all types of cleaning systems. The surface tension of both formulated hydrocarbons and fluorinated sol- vents is outstanding. This combina- tion is one of the most user- and envi- ronmentally-friendly processes.
Brominated solvent formulations are very simple to use in a vapor phase degreaser. Nevertheless, there are some non-solvency problems and compatibility issues. For this reason, compatibility tests must be per- formed with all materials in contact. Due to the very low surface tension (20-30mN/m), the ions might not be totally removed and prevent the results from matching ionic specifica- tions. The costs are reasonably low, but the hazardous aspects for end-
• • • • •
users and the environment are of great concern. These products are severely restricted in Europe.
Glycols or modified alcohol for- mulations are used the same way as the formulated hydrocarbons with some surface tension between 25 and 35 mN/m. Unless used in a formula- tion, glycols can not dissolve all con- taminants. In most cases, these for- mulations have a good solvency power, but the disadvantages of these products are the need for rins- ing with tap or DI water — high sur- face tension, the drying operation, the waterproof compatibility and the need for disposal of the used mixture. The total cost of these elements com-
bined is considered to be high.
Fluorinated solvents. When used pure, fluorinated solvents and formu- lations can not dissolve all contami- nants. Even with the lowest surface tension of all families, approx. 8-15 mN/m, their solvency power is weak. But when combined with formulated hydrocarbons, then the co-solvent process is excellent for handling the toughest cleaning jobs. These prod- ucts should be used in the latest sol- vent vapor degreasers. The solvency power is the simplest way to express the ability to dissolve the contami- nants. The approved method is to use the Hansen Solubility parameters which will define for any product,
parameters of molecular bonding. While this is true for theoretical cal- culations, when real-world products are blended, the Kauri-Butanol method establishes a direct rosin sol- vency value
Surface Tension Understanding surface tension
is key to understanding a good clean- ing performance. It becomes especial- ly important as miniaturization makes parts as small as the contam- inant particles themselves. The smaller the parts become, the lower the surface tension of the cleaners should be. When this law is under-
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