A Dip in Ionic Contamination Test Keeps Mil/Aero PCBs 99.9% Clean
Whether we like it or not, ions are part of electronics manufacturing. And, in simple language, they are bad for electronics manufacturing. When applying pressure or water-based film on electronics circuitry with alloys on top of component leads, in due time, ions can lead to corrosion and dendrite growth. That, in turn, results in certain types of defects such as a short circuit, dendrite growth, and/or a catalyst leading to tin whiskers.
Dendrite growth can best be described as a sprouting formation or pattern of crystals. Tin whiskers, on the other hand, are an electrically conductive, crystalline formation of tin that can grow from places where tin is used as a final finish. Both adversely affect electronics circuitry.
This worrisome hotspot of ions is a major cause of concern in mission-critical PCB applications like aerospace and military. Therefore, it is extremely important to clean these boards to eliminate ionic residues that can be a source of failure, short circuit, tin whiskers, dendrite growth, and other related failure causes.
Thankfully, the industry has specialized contamination testers to detect and measure these impurities. Those testers are known as Resistivity or Solvent Extracted or ROSE and Process Ionic Contamination Testing or PCT. These testers use specialized chemistry to get rid of ions hanging on to the electronics circuitry. An example is an ionograph tester shown in Fig. 1.
Today, detection and measurement of the impurities is a well-proven technique, widely used, and is part of IPC J Standard, which is a military standard. This J Standard uses special chemistry and is a batch cleaning process characterized by testing one board at a time as opposed to a batch of 10, 20, or 100 boards.
Each board is dipped into a narrow and deep tank containing the chemistry and special chemical agents react to get rid of those ions. After a two to three minute cleaning cycle, the board comes out; a volumetric measurement per square centimeter is taken to assure the board is extremely clean. Typically, after being subjected to this ionic contamination test, a board comes out 99.9% clean.