You think about surface tension all the time, don’t you? Talk of surface tension is all around us these days. Maybe not the phrase, but every day we’re told the importance of washing your hands with soap. It’s the soap part that’s surface tension related.
Maybe you remember surface tension from your chemistry courses – causing the edges of a liquid surface to climb the side of your graduated cylinders forming a meniscus. It’s the same force that makes a small drop of water hang from your fingertip rather than falling to the ground. Surfactants (a.k.a. soap) reduce surface tension making the surface more “wettable” (literally able to be made wet), which is part of why washing your hands with soap and water is so much more effective than washing with water alone.
Surface tension is measured in dynes/cm (a dyne is the measurement of “the force required to accelerate a mass of one gram at a rate of one centimeter per second per second”). Sounds complicated, but think about it as a measurement of force over a distance. As a point of comparison, the pressure measurement PSI (or Pounds per Square Inch) is a measurement of force over an area. Water has a surface tension of 72 dynes/cm which makes it good for cleaning dirt off fruit, but far less effective at cleaning hard to reach places like under low-standoff surface mounted components.
The technological trend in PCB manufacturing is for surface mount technology to get smaller, denser, and lower to the board. The accelerating miniaturization of components makes cleaning with de-ionized water alone difficult due to its high surface tension. Water cannot clean where it cannot reach, so if you have QFN or MLF components utilizing leadless technology (CircuitNet.com discussion) standing fewer than 4 mil off the board, the space is too narrow for water to clean.
The solution (ha! Get it?) could be a high precision cleaning agent that reduces surface tension. These cleaning agents help break the surface tension of your cleaning medium in addition to cleaning all types of organic and inorganic process residues. A lower surface tension means that more of the cleaning chemistry is reaching into narrower spaces allowing more solder paste and flux residue to be washed away. While the residue under components is not visible (like the undercarriage of your car) cleaning where you can’t see is still important for the long-term performance of your assemblies. Thorough cleaning will mitigate the risk of field failures like electro-chemical migration (dendritic growth) and leakage current that may occur due to residues left over from the soldering process.
Accounting for surface tension when cleaning PCBs may not be the first thing that comes to mind, but if your boards (specifically under low-standoff components) are still not clean after washing with DI-water alone, you may need to consider precision cleaning agents to help reduce the surface tension.