Ionic contamination is the reason for an estimated 25% of printed circuit board (PCB) failures, and it starts in the fabrication and assembly process. Here’s what it is, how it affects PCBs, and how you can find and prevent it.
What is Ionic Contamination and how does it happen?
Ionic contamination happens when substances such as salt, acids, sulfates, perspiration, flux activators, plating chemistries, ionic surfactants, or ethanolamines get onto a circuit board component during the fabrication or assembly process.
These “ionic” contaminants are so-called because they either already have an ionic (positive or negative) charge, or they gain one after settling on the PCB early in the fabrication or assembly process and are then subjected to further fabrication or assembly.
When these contaminants come into contact with a component of the PCB before it is properly sealed, they may leave behind an ionic residue if the PCB is not specially cleaned. Because contaminants may gain a charge later in the creation process than they were applied, it’s also possible for testers to miss residues before the process is complete.
When does Ionic Contamination happen?
During fabrication, ionic residues most commonly either come into contact with the PCB during the creation of plated-through holes, etching or drilling procedures. During assembly, flux residue in soldering or other ionic surfactants from soldering preparation may contact with the PCB. Or, biocides on the pick and place machines used to assemble the parts may leave a residue on the parts they contact.
The effects of ionic contamination manifest as the ionic residue continuously redirects the PCB’s directed charge to areas of the device where it wasn’t meant to flow, or amplifies that charge higher than it should be, overloading and damaging components.
Therefore, while ionic contamination can be said to have “happened” as soon as an ionic contaminant contacted the PCB, the problems caused by that contamination may take some time to manifest. This is why 25% of all PCB failures are “caused” by ionic contamination: because the original contaminants create the strain or malfunction that leads to the final damage.
What does Ionic Contamination do?
An ionic residue contains positively or negatively charged atoms or molecules. If the directed current flowing through a PCB during operation collides with charged ions, those ions can alter the charge by either amplifying it or redirecting it to parts of the device that it wasn’t meant to go. This can lead to electrochemical migration within the PCB, which can, in turn, lead to dendritic growth, short circuiting, corrosion, malfunction, or even total system failure.
A PCB conducts an electric field between two copper pads: a positively-charged anode and a negatively-charged cathode. When the electrolyte (the solution that absorbed the ionic residue and therefore facilitated the growth of ions) enters this electric field, the contained ions can drift or “migrate” across the field. This is called electrochemical migration, or ECM.
Many of these ions are diffused toward the cathode, because the metal ion concentration is greater near the positive pad than the negative pad. As a result, tiny shavings of metal from the anode drift across the solution and are deposited on the cathode, forming what are called dendrites: tiny, crystalline metal slivers that are highly conductive.
Dendrites are a major problem on PCBs because they can direct the current flowing through the electric field between the anode and cathode during operation, causing it to branch in different directions than intended.
This misdirection can affect the performance of the PCB by preventing current from flowing where it should, but it can also have even more damaging effects. When current flows to the wrong parts of the PCB, it can create short circuits, overcharge and damage different parts of the PCB, facilitate corrosion, or even damage the PCB beyond repair.
Dendrites grow and accumulate over time, and may even eventually contact one another and converge. The longer ionic contamination affects the PCB, the higher the chance dendritic growth will affect the performance of the device.
How can I prevent Ionic Contamination?
Ionic contamination requires both the ionic contaminants themselves and the presence of moisture in order to form the solution the ionic residues combine with. Unfortunately, it tends to be very difficult to control for either of these contaminants over time.
Many ionic contaminants are endemic to the fabrication and assembly process of PCBs; in fact, most contaminants come from the same materials used to make the PCB itself. Meanwhile, moisture can accumulate between the layers of boards within a PCB during regular operation.
While ionic contamination cannot truly be totally prevented over a long enough time, however, it can be slowed down considerably through aggressive control of board-level cleanliness and the constant cleaning and testing of residues on manufacturing equipment.
The most effective way to slow ionic contamination is to actively hunt for any and all potential vulnerabilities in your manufacturing process and systematically control for them continuously. To do that, you need to conduct periodic ionic contamination testing in compliance with IPC TM 650-2.3.25.
How can I find Ionic Contamination?
Innovatech offers ionic contamination testing that can find ionic contamination within your PCBs, identify where it came from, and even help you locate the area of your operation you need to clean, update, or control as a result.
If you think your PCBs may be affected by ionic contamination, get in touch with Innovatech to schedule testing today.