IoT devices going into military/aerospace, medical and/or highly specialized industrial applications demand ultra-high reliability so they operate properly according to their tight specifications. That means the small IoT printed circuit boards (PCBs) must be ultra-clean and virtually free of any contaminants or chemical residues.
But to understand how best to gain that IoT reliability, we first have to check out PCB surface finishes or coatings, because therein can lie tiny and minuscule residue that sabotage that reliability. Regardless of size, whether large or small, like IoT PCBs, all printed circuit boards have these surface finishes. The purpose of these finishes or coatings is not only to protect the board’s copper finish, but also to keep the copper from oxidation and deterioration.
There are a number of PCB surface finishes. However, the most prominent are hot air solder leveling or HASL, immersion tin, organic solderability preservative or OSP, electroless nickel immersion gold or ENIG, and immersion silver. Of those, immersion gold is the surface finish most popularly used for rigid-flex or flex circuit PCBs comprising an IoT device. This surface finish is heralded because it offers a number of key benefits, among them are surface flatness and the fact that it works well with fine pitch devices. Fine pitch is a measurement that means the very short distance between a device pin or lead and its neighboring pin or lead. Considering their small size and advanced electronics functionality, IoT devices demand highly advanced circuitry packaged in ultra-small packages like micro ball grid arrays or µBGAs with 0.2 to 0.4 mm pitch between leads. That’s about the diameter of a human hair.
If immersion silver is used as a surface finish, it has a tendency for oxidation and is sensitive to the tiniest flux or chemical residue that remains after the IoT PCB is assembled and cleaned using regular cleaning techniques. Oxidation corrodes surface mount or SMT pads that are in contact with the leads of electronics’ circuitry. And that oxidation leaves a white residue as well.
If an IoT device is designed to operate at high frequencies or high speeds, as a lot of medical, industrial and military/aerospace IoT deployments do, those white residues as well as flux residues or tiny splashes of solder can adversely affect signal transmission and will keep the IoT device from performing at its intended higher frequencies or higher frequencies.
This is where Ionographic cleaning comes in to play a big role. It uses a set of chemicals to perform the ultra-clean process for IoT rigid-flex and flex circuits. This chemical is a combination of isopropyl alcohol and de-ionized water called IPA solution, which is a conductive solution in nature that collects all the contaminations and leftover residues to make the IoT PCB extra clean.
This cleaning and testing involves submerging the IoT rigid-flex or flex circuit boards in the IPA solution for about five to15 minutes. It’s left underwater for a short period of time so the solution has the time to fully circulate around the circuitry.
Then, the Ionographic cleaning system automatically tests the IoT rigid-flex or flex circuit boards for ionic contamination. Hence, changes in water resistivity provide the level of relative board cleanliness or, if any, remaining residue with specific quantitative figures so that the data out of the Ionograph system is absolutely unquestionable.