February, 2017
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Page 71 Good Chemistry: Studying PCB Cleaning Processes Continued from previous page A simple way to quantify a
chemical’s ability to dissolve contam- inants is to apply the Hansen param- eters, which offer a strong theoretical method for measuring polar, non- polar and hydrogen bonding. When products are blended, the Kauri- Butanol Index is used to establish a direct rosin solvency value.
Surface Tension Surface tension is critical to
cleaning performance, especially with miniaturized PCBs, where
PCBs, no quartz was affected accord- ing to IPC-TM-650.
Comparative Analysis Every end-user must establish
a particular specification of control, which varies based on the applica- tion. In one study, Inventec produced 600 PCBs in quantity to triple the cleaning results. Each trial con- tained 30 components. To achieve a successful result, all cleaning res - idues had to disappear, including the contaminants under the components leaving no fingerprints, particles, or dust remaining. In addition, the
the study, the top two cleaning processes made use of ultrasonic immersion. The first, at 25 kHz, and the second at 38. Both used formulat- ed hydrocarbons/HFE (hydrofluo- roether) along with bi-co-solvent/
vapor degreasers, and reached a 9 on the cleanliness scale. At 8, an immer- sion jet, co-solvent, separated process with formulated hydrocarbons/HFE and bi-co-solvent/vapor degreasers
Continued on next page
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A comparison of various chemistries’ surface tensions and their changes with temperature.
lower surface tension is preferred. Demineralized water has a surface tension of around 80 mN/m, which lowers by about 10 mN/m when the temperature is increased. Demin - eralized water cleaning systems are often warmed up in separate wash- ing units to reduce surface tension and to enhance cleaning in cavities and underneath components. Similar issues apply to tap water, which has surface tension of about 70 mN/m at room temperature. When detergents are added to
water, the surface tension can drop down to 35 or 45 mN/m depending on the process temperature. But the question remains, how can a “wet- ting product” be rinsed with water that has a higher surface tension? Because of this, many suppliers use additives or isopropanol to rinse, wet, and dry parts. To clean beneath components,
glycols or formulated hydrocarbons are commonly used. They have sur- face tensions that are lower than that of water and detergents, in the range of 25 to 35 mN/m for glycols, and about 20 mN/m for formulated hydrocarbons. These products share the same rinsing problems as water, but that can be solved by using a final rinsing solvent that has an even lower surface tension than either of these cleaners. The solvents then evaporate and condense on the free- boards of the vapor phase equip- ment, leaving no residue on PCB sur- faces or underneath components. In addition to chemicals, many
types of mechanical agitators are com- monly used to help penetrate, dissolve and unfasten contaminants. These include sprays, sprays under immer- sion, ultrasonics, agitators, and rotors. PCB assemblers often avoid the use of ultrasonics for fear of damaging com- ponents, however, according to trials performed using over 60 different
rosins, underfill, substrate, or com- ponents themselves could not be damaged by the cleaning operation, and at the end of the washing step, the parts had to be fully dried with- out degrading their ink markings. In Inventec’s study, the final
results of the various cleaning processes were marked on a scale of 1 to 10, with 10 being the most suc- cessful. The comparative results were made using an established value of ionic contamination remain- ing below 0.2 µg of Eq NaCl/cm2. In
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