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February, 2017
Good Chemistry: Studying PCB Cleaning Processes
By Patrick Duchi, Inventec Performance Chemicals
which offer outstanding solvency power and low surface tension to dis- solve and remove contaminants with- in minutes, leaving electronics clean and dry. Meanwhile, the exponential growth of miniaturized devices has challenged the marketplace to meet ever more stringent technical specifi- cations along with increasingly rigor- ous environmental regulations. These trends have been of partic-
T
ular importance for high-reliability applications, including the medical, military and aerospace, communica- tions, energy, and transportation markets. The phase-out of CFCs and
HCFCs has caused standard solder pastes and fluxes to evolve from RA and RMA fluxes to no-clean and low- residue no-clean, and now to very- low-residue no-clean formulas. The ongoing trends of miniaturization and lead-free soldering have further
he ability to effectively clean a PCB has been complicated by the ban of CFCs and HCFCs,
complicated the use of aqueous and semi-aqueous PCB cleaners.
PCB Cleaning Basics For the best results, the key
parameters and physical laws behind these cleaning chemistries must be understood. Cleaning performance is
solvency power as defined by the Kauri Butanol Index. The second key trait is low surface tension, needed to penetrate underneath components, which is expressed in mN/m (millinewtons per meter). Next, physical parameters also come into play, including tempera-
als. Furthermore, there is high sur- face tension (70 to 80 mN/m) associ- ated with rinsing PCBs with tap or DI water, along with the issues asso- ciated with waterproofing and the disposal of contaminated mixtures. The high cost of cleaning process equipment and water remediation, as well as disposal needs to be con- sidered. Petroleum distillates such as
PCB cleaning products include detergents, light petroleum distillates, formulated hydrocarbons, brominated solvents, glycols, and fluorinated solvents.
affected by four main criteria. First, there is the Hansen Parameter, which characterizes the ability to dissolve contaminants based on the
ture, mechanical activity and the duration of the processes required by customer specifications. Finally, var- ious general performance features such as chemicals, efficiency, quality, performance, and cost, all affected by requirements of RoHS, REACH, ISO and related specifications. Miniaturization is reducing the
size of solder pads, resulting in less flux residue, but also less space between legs, boards and compo- nents. The components themselves are also becoming smaller and more complex, making reliability a greater concern. The typical contaminants found on a PCB are mainly organics, such as natural and synthetic rosins, ions, acids, solder balls, fingerprints, and particulates of the PCB sub- strate material. Lead-free alloys require higher
soldering temperatures and reflow profiles than standard Sn/Pb formu- lations, resulting in the hardening of active fluxes. This poses added risk, as there is a temptation to want “shiny” solder pads, which places greater emphasis on the ionic clean- ing of the PCB, as the absence of ionic contamination is sometimes seen as a false indicator of greater long-term reliability.
Cleaning Product Classifications
PCB cleaning products can be
generally classified as detergents, light petroleum distillates, formulat- ed hydrocarbons, brominated sol- vents, glycols, and fluorinated sol- vents.
Detergents are effective but
their use is specific to the type of sol- der flux. Concentration vs. water can vary from 3 to 50 percent, while the temperature can vary from 20 to 60°C (68 to 140°F). The type of agita- tion used, along with the sprays, spray under immersion, or the possi- ble use of sonication also needs to be factored in. The drawbacks of deter- gents include a possible failure to remove all residues under compo- nents due to high surface tension (40 to 50 mN/m), the aggressiveness of these formulations, and their possi- ble incompatibility with PCB materi-
See at APEX, Booth 2339
alcohols and ketones are used mainly for cold-temperature cleaning opera- tions, but can also be used at warm temperatures. These products can be flammable at room temperature and extremely risky under warmer condi- tions. The costs are reasonable, but the annual cost of disposal can be sig- nificant. Formulated hydrocarbons easi-
ly outperform any other cleaners to remove flux residues, solid residues, and salts under components due to their very low surface tension (approximately 20 mN/m). They must be rinsed with either a semi- aqueous process or fluorinated co-sol- vent process. The aqueous rinsing process
involves virtually the same draw- backs as detergents. The vapor phase co-solvent process leaves a PCB nice- ly rinsed and dried due to outstand- ing surface tension. The rinsing sol- vent can also be recycled by distilla- tion, making it a user- and environ- ment-friendly solution. Formulated hydrocarbons also offer long storage life, with the lowest total cost of all cleaning systems. Brominated solvents are rela-
tively simple to use with a vapor phase degreasing process. However, certain non-solvency and compatibil- ity problems can arise, so compatibil- ity tests must be performed with all materials. In addition, despite offer- ing very low surface tension (20 to 30 mN/m), the ions might not be totally removed and thus prevent matching ionic specifications. The costs are also reasonably low, but brominated solvents are considered dangerous to health and to the environment, so their use is restricted or banned most countries, especially in Europe. Glycols or modified alcohols
have a surface tension between 25 and 35 mN/m, delivering excellent solvency power. However, unless used in a formulation, they cannot solubilize all contaminants. Other drawbacks include high surface ten- sion when rinsing with tap or DI water, water-proofing of components, and disposing of soiled mixtures. Also, the total cost of these chemicals is typically high. Fluorinated solvents cannot solu-
bilize all contaminants when used in a pure state, so even with a very low surface tension (8 to 15 mN/m) their solvency power is weak. However, once combined with formulated hydro- carbons in co-solvent process, these chemicals can achieve excellent clean- ing results. These products should be used in conjunction with the latest sol- vent vapor degreasers.
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