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Most people are not good at predicting the future, but here’s one forecast that’s certain: Electronics are going to get smaller, smarter, denser, and hotter. This puts more computing power and snappy features into the mobile devices we all love, which is a good thing. But it also puts pressure on PCB manufacturers as they deploy ever more complex miniature components with ever tighter tolerances.
Smaller, more densely populated circuit boards make the issue of managing faults, quality and product longevity highly challenging. This is why so many companies consider their PCB cleaning as a mission-critical process. If the cleaning is not effective the device simply will not function reliably for the required life of the product.
Manufacturers also need a cleaning process that’s flexible, because today’s components won’t be used in next year’s designs. Speed is important, but so is the ability to clean new shapes or unusual contamination. A large, rigid, inflexible cleaning process may be very cost-effective today, but how will it handle the parts and fluxes five years from now?
To future-proof any investment in cleaning, a company needs to understand the complexities of cleaning. But that’s truly difficult because the trade-offs are not always obvious; many diverse factors crowd the equation. So the question is, which critical cleaning process takes into account all the important requirements, whilst still cleaning components today and tomorrow? The answer, in my mind, is solvent cleaning.
A Time for Change
At the turn of the century, the trend in cleaning leaned towards the use of water-based products. Water was cheap, the boards were big and the stand-offs high. The cleaning systems grew to enormous size, able to spit out hundreds or even thousands of clean PCBs an hour.
But as the dinosaurs found, bigger may not always be better. Aqueous cleaning may no longer be optimal simply because the systems are so large and complex, which ossifies them. Companies with rigid cleaning processes have difficulty adapting to new component designs and new contaminates. They run the risk of being left behind, especially as smaller components are making cleaning processes increasingly more difficult to implement. Many manufacturers now are opting for solvent cleaning because they find it to be more effective, flexible, and less expensive.
Here’s the essence of vapor degreasing: It is a closed-loop system with few moving parts that inherently recycles the solvent. This keeps costs down and through-put up. In particular, vapor degreasing offers excellent performance when cleaning miniature components.
Figure 1: Vapor degreasing.
In the simplest configuration, a vapor degreaser consists of a top-loading cleaning machine composed of two chambers. The first chamber is called the boil sump and the second chamber is the rinse sump. Both chambers are filled with a special nonflammable cleaning fluid that boils near room temperature.
In the boil sump, the solvent is heated and immersed parts are cleaned in the roiling fluid. The heat also generates vapor (e.g., steam from the solvent). The vapors rise inside the machine until they are captured by refrigerated coils that encircle the perimeter of the system. The refrigeration condenses the vapors back to the liquid state. This pure, clean distillate liquid is channeled into the rinse chamber which eventually overflows back into the boil sump.
Without any air knives, blowers or dryers, the parts come out clean, dry, spot-free and immediately ready for further processing. Depending on the process requirements, vapor degreasing can handle the largest parts, the highest volumes and the most challenging shapes.
A few simple additional features can make this simple system even more flexible. An automated hoist can move the dirty parts through the cleaning system. Ultrasonics can enhance cleaning and ensure residue-free results. The sumps can be fitted with filtration systems to remove insoluble contamination (particulate). “Super heat” and external distillation are also money-saving, performance-enhancing choices. These systems are easily programmable and allow for excellent repeatability. Companies can pick and choose to tailor their equipment and processes to their PCBs and budgets.
Vapor Degreasing Advantages
In comparison to aqueous systems, the advantages of vapor degreasing are clear. The systems are very easy to use, highly efficient, smaller in footprint, and lower in overall capital and operating costs. Let’s look and see how these benefits are delivered.
First, solvent cleaning leverages the chemical properties of the cleaning fluid, rather than fighting against those characteristics. Low-boiling solvents have a much lower surface tension and viscosity than water, so they easily clean even under the smallest of parts. Most vapor degreasing fluids are also very heavy and dense, typically 20–40% heavier than water, which aids in dislodging particulate from components. Because the solvent is tailored for the application, delicate parts are easily cleaned and dried. It is possible to combine all these factors into a “wetting index” that permits apple-to-apple comparisons (Chart 1).
Chart 1: Wetting index comparison between different solvent cleaning solutions.
Another advantage is the cleaning fluid boils at a low temperature (usually slightly above room temperature), so heating the cleaning fluid uses very little electricity. In comparison to an aqueous cleaner, a vapor degreaser of comparable capacity will use about 1/6th the electrical power. Since burning fossil fuels to generate electricity is one of the primary sources of carbon in the atmosphere, an energy efficient-cleaning system is highly attractive.
An unexpected cost savings stems from the fact the solvents, unlike aqueous saponifiers and surfactants, can be re-used indefinitely. This is because a vapor degreaser is a recycling system. While aqueous cleaning dump the cleaning fluids after a single use, the re-usability of solvent cleaning makes the choice extremely cost-effective.
These machines, when properly designed, equipped and configured, outperform the cleaning efficiency of any other cleaning technology.
Water—Not as Green as You Think
Aqueous cleaning is the most common alternative to solvent cleaning. Experience has proven that it can work well in non-critical applications because water cleaning uses the kinetic energy of pumps and sprays to “power-wash” contamination from surfaces. But as parts become smaller and the stand-offs tighter, the performance of aqueous systems suffer.