Feature: Medical electronics
Drying: An essential step to cleaner PCBs for medical electronics
By Elizabeth Norwood, Senior Chemist, MicroCare T
here are four steps to successfully cleaning printed circuit boards (PCBs): wet, scrub, rinse and dry. Wet the board with a pure cleaning fluid; scrub
it using a good quality scrubbing brush; rinse it with more clean fluid; then, dry it. Typically there is consistent focus on
the first three steps, but the critically- important drying step is oſten overlooked or even forgotten. For reliable performance and longevity, PCBs must be clean and dry. If moisture stays on PCB surfaces it can lead to problems including component corrosion, electrochemical migration, dendrite growth and board delamination. Leſtover moisture may also outgas during soldering, creating blowholes or preventing proper coating adhesion during conformal coating operations. Modern PCBs are oſten densely
populated with bottom termination components, like land grid arrays and QFN (quad-flat no-leads). Te very tiny standoff heights of these components can trap moisture under them, making PCB drying more complicated; see Figure 1. Te PCBs inside today’s wearable
medical devices are densely populated to keep them smaller, lighter, less invasive and more discreet for patients to wear. In addition, these devices have delicate parts oſten made with varied materials. Soſter plastics and lightweight metals are easily damaged, so cleaning and drying must be done carefully to prevent harm.
The right drying method Tere are different ways to dry PCBs; which one to use oſten depends on the cleaning method used. So, whether it is heat, air, vapour, wipes or compressed gas, there is a method to help ensure PCBs are dry and ready to provide optimum performance and reliability:
Batch drying for aqueous cleaning Batch or bulk drying typically occurs aſter PCBs undergo water cleaning inside aqueous cleaning machines. Boards cleaned with water typically require intensive drying methods, which include convection drying ovens, heated blowers and air knives to displace and evaporate the water. Temperature and air flow must be carefully monitored when using these
38 July/August 2022
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drying methods. For example, if the circulating air flow isn’t strong enough or if the temperature isn’t high enough in the convection oven, it might not reach the water under the low-mounted components, leaving moisture and residue behind. Or if the temperature is too high, or the air flow too powerful, it could cause surface damage to the PCBs by baking any leſtover, residual contaminants.
Batch drying with vapour degreaser Vapour degreaser cleaning and drying is an affordable and convenient alternative to aqueous cleaning machines and drying ovens; see Figure 1. Vapour degreasing uses cleaning fluid immersion, combined with vapour rinsing and vapour drying, to remove all types of PCB contaminants including fluxes, pastes and particulate, without the risks associated with aqueous cleaning and drying methods. Vapour degreasing uses non-flammable
cleaning fluids in specially-engineered vapour-degreasing cleaning machines. Te vapour degreaser boils the cleaning fluid at a low temperature, usually between 40˚C/105˚F and 65°C/165°F, to produce a
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