Inner and Outer Layer Electrochemical Migration in a PCB
Environmental exposure can create insidious failures in a PCBA during operation. Those same failure mechanisms can arise if a bare PCB is not thoroughly cleaned or handled during fabrication. The relevant class of failures that occurs in both cases falls under two categories: corrosion and electrochemical migration.
Electrochemical migration is related to another failure mechanism: conductive anodic filamentation, or CAF. These two failure mechanisms are similar chemically in that they are forms of migration mediated by an electrochemical reaction. In this article, we’ll explore each migration pathway in a PCB, specifically where these occur and how they can be identified in a PCBA.
Electrochemical Migration in Inner and Outer Layers
In a PCB or PCBA, electrochemical migration refers to a bias-driven chemical reaction between copper in the PCB, as well as the organic and inorganic substances that make up the substrate. Processing substances can also be present in the PCB, and these can participate in electrochemical migration reactions. The various chemical reactions involved in electrochemical growth can occur in outer layers or inner layers.
When electrochemical reactions lead to growth of conductive salts on the outer layers, we generally call this electrochemical migration. This form of material breakdown is similar to corrosion, but it does not only result in growth of oxides along the surface layer between exposed conductors. Because this happens between exposed conductors, which happen to be component pads in most boards, the danger here is shorts between component pads.
The conductive salts that grow on external layers tend to have dendrite structure. An example of these dendritic structures is shown in the image below.
Dendrite growth observed between two traces spaced 6.35 mils apart. [Source]
The exact reaction pathway depends on the surface finish material and the surrounding contaminant environment. Studies have examined electrochemical migration and dendrite growth under various plating chemistries, such as the study cited with the image above (involves HASL). The chemical precursors leading to dendritic growth include:
- Copper and its plating chemistry
- Water (either adsorbed from humidity or from microdroplets)
- Uncleaned flux residues
- Etchant residues
- Formation of hydroxide through dissociation of oxygen in water
Under an applied bias, even at low voltages, salts can begin to grow on the anode and cathode sides of copper on the surface layer. The growth of salts occurs even at low bias voltages, so this will be a common occurrence in contaminated PCBs. The influence of flux residues should illustrate the need to thoroughly clean PCBAs to remove these and other residues.
When electrochemical migration occurs on the inner layers of a PCB, we generally refer to it as CAF. This is because the growth of copper salts occurs along glass fibers in the PCB substrate material, and the copper material appears like a long filament when viewed in the board cross section. Eventually, this causes a short circuit as the conductive filament bridges two conductors in the internal layers. This makes any phantom short circuit between two elements in the inner layers very difficult to identify.
The structure of CAF is interesting in that it follows a sinusoidal shape. It also always begins at the anode, likely due to oxygen starvation in the interior layers. This is because the filaments tend to grow along glass fibers in the substrate material, as shown in the example image below.
CAF growth direction along glass fibers in the PCB substrate. [Source]
If there is any type of damage material damage, possibly due to thermal cycling or delamination, there will be a new pathway created for CAF. Also, if the top resin coat above the glass fibers is too thin, and any copper is present, the CAF will be more likely to make contact and create a short.
An important step to preventing CAF from continuously growing and eventually producing a short circuit is to ensure low void content in the PCB substrate, prevent further moisture uptake, and select the right materials. Tighter weaves are more susceptible to CAF due to their lower resin content, so they would be preferable over a tighter weave when reliability is concerned. However, for a high-speed/high-frequency board, looser weaves are not preferred because they create more noticeable fiber weave effects.