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Every decision we face in life has pros and cons. Some have more pros, others more cons, but no decision will be purely positive or purely negative. Even the decision to have a slice of cake should be unequivocally positive, but for some reason, it isn’t. As living, breathing, sentient beings, it is our responsibility to weigh the pros and cons of our decisions so that the outcome will best align with our goals.
Deionized water (DI water) has been used in the PCB cleaning process for decades. Many companies still use it, but why have they decided to continue?
Cost is always one of the first considerations, and PCB cleaning with DI water has relatively low startup and maintenance costs. Attaining DI water and the equipment needed to maintain the process often comes with a palatable price for management. Legacy is another consideration, and DI-water does a “good” job of removing specific flux material and other impurities left over on the board. Remember, DI-water has served the industry for many years and will continue to be a viable PCB cleaning tool for some applications.
If you're curious about the opposite side of the argument, here are some of the cons.
The first con to cleaning with DI water is its physical limitations in the face of evolved technology. Every year, printed circuit board design becomes more intricate and complex. Stacked components, low stand-off heights, and unusual geometries allow engineers to do previously unimaginable things with their PCBs. But water isn’t good at cleaning under low-standoff surface-mounted components or in hard-to-reach places. Because DI-water has a surface tension of 72 dynes/cm, it physically can’t get under these tight gaps or geometries. Chemistry-assisted cleaning processes have been shown to reduce surface tension to 30 dynes/cm and below.
Another negative consideration for DI water relates to the pastes used today. There’s an increased use of lead-free solder pastes, which require higher soldering temperatures. This often results in more burnt-in fluxes and could produce water-insoluble contamination. In addition, DI-water alone has a limited to no ability to solubilize non-ionic residues on the board’s surface, such as fingerprints, mold release compounds on components, etc. Recent studies suggest that water in PCB cleaning applications has reached its physical limitation, favoring a chemically assisted cleaning process.
You may also want to consider whether the “good enough” cleaning with DI water is good enough. Components and boards left uncleaned by water may lead to in-field board failures due to electrochemical migration and dendritic growth. How much does a board failure cost regarding recalls, reputation damage, or even loss of life?
So, is cleaning with DI water right for you? I suggest weighing the pros and cons related to your particular situation. Ask yourself: How much of a factor is the cost compared to the level of cleanliness I need? What are the reliability needs of my PCBs? Can I do a better job of PCB cleaning? How high is my risk tolerance?
Once you weigh your answers to these questions, you’ll be better aligned with your goals and know if DI-water is right for you. As always, if you’re unsure, don’t go it alone. The help you need is out there- all you have to do is ask.
