There are four steps to successfully cleaning PCBs (printed circuit boards). Wet, scrub, rinse and dry. The wet, scrub and rinse operations are obvious. Wet the board with a pure cleaning fluid. Scrub it using a good quality scrubbing brush. Then rinse it off with more clean fluid. However, the dry step is often overlooked or in some instances, completely left out of the PCB cleaning plan.
For reliable performance, circuit boards must not only be clean, they must be dry in order to function properly. Leftover moisture on a PCB surface can lead to a host of problems including corrosion, electrochemical migration, delamination, dendrite growth, pin holes and blow holes and adhesion problems with conformal coatings later in production. Modern PCBs often have bottom termination components like land grid arrays and QFN (quad-flat no-leads) with very tight standoff heights. Moisture can get trapped under these components, making drying even more complicated.
There are different ways to dry to ensure printed circuit boards are not only clean, but also dry for optimum reliability and performance.
Manual Cleaning & Drying with Wipes and Dusters
One of the most commonly used methods technicians use to dry PCBs during benchtop cleaning is a simple lint-free wipe. Whether using it to capture the excess spray of cleaning fluid on a circuit board or gently pushing the wipe under low-surface mounted components, wipes are a quick and effective way to dry the PCB without leaving lint or fuzz behind. There are many factors to consider when selecting a lint-free drying wipe including absorbency, material and packaging.
The most important factor when choosing a drying wipe is the ability of the wipe to absorb the type of moisture encountered. For example, some wipes will absorb cleaning fluids, but they will not absorb water. The key is to match the type of wipe absorbency to the contamination to be removed.
The material makeup or the type of fiber used to make the wipe is also important. Nonwoven wipes, since they are made without glues or binders, don’t leave residue on the surface being cleaned, resulting in a cleaner PCB. Another way to limit re-contamination to the PCB is to choose pre-washed wipes or wipes with finished or heat-sealed edges. This eliminates loose fibers and dust, ultimately leaving the printed circuit board cleaner.
For some benchtop post reflow (rework) assembly a good drying method is to use a dust remover. A quick blast of from a duster can be effective at blowing trapped moisture from underneath larger components that a wipe alone might not reach. The dust remover can be targeted to a very small area on the PCB and works extremely well in getting into and under intricate and hard-to-reach nooks and crannies on the circuit board. Using a duster in combination with a wipe doubles the drying power and prevents the fluid from being pushed back onto on the board and spreading contaminant around.
When selecting a duster to use for a drying application, safety is key. Inexpensive canned air is readily available from a number of retail outlets. However, many of these formulations may pose a hazard due to their high flammability ratings. A safer alternative is to select a dust remover that is made with nonflammable gas. A 360° type of dust remover also adds drying convenience since the can operates at any angle, even upside down, allowing operators the flexibility to quickly dry very complex or intricate PCBs. Using the duster may take a little bit of practice, but once the operator understands the velocity of the high-purity gas, the moisture can be blown away quickly and efficiently with no residue left behind.