Mike Konrad’s Post

Transcript

Hi again, everyone. Mike Conrad here, founder and CEO of Aqueous Technologies and host of the Reliability Matters podcast. Back with you once again for another episode of Tips and Tricks for Cleaning Circuit Assemblies. Conductive anodic filament, or calf, is a significant reliability concern in the electronics industry, specifically in printed circuit boards. Calf refers to the formation of conductive filaments between the anode and the cathode within a PCP's layers, lending to short circuits and potential device failure. This episode explores the causes of calf formation and presents effective prevention methods to mitigate its occurrence. I'll explain more when I return in just a moment. Calf is one of the manifestations of electrochemical migration, or ECM. Unlike surface level ECM where the resulting dendritic growth can be physically viewed, CAFE occurs between the layers of the board, remaining invisible until failure strikes. What type of failures result when calf occurs? Calf can Causeway electrical shorts between conductors as well as parasitic electrical leakage. If an electrical short occurs, one of two events happen. In the best case, the fine metallic filament that grows between the conductors instantly becomes an open circuit, much like a blown fuse. In this case, one may never know a momentary short occurred. Another common scenario is battery drain. If an assembly is powered by an onboard battery, Battery life. Would be significantly reduced. Other manifestations of best case scenarios is if the board requires calibration, it just won't calibrate. In the worst case, the short circuit can damage individual components on the assembly. What are the causes of calf formation? Well, there's a number of factors #1 moisture. Moisture ingress is one of the primary causes of calf formation. When PCBS are exposed to humid environments, moisture can penetrate the laminate material. The presence of moisture. Facilitates the migration of ions leading to the formation of conductive metallic paths between adjacent conductive elements. Overtime, these paths can develop into metallic conductive filaments. There also needs to be an electrical bias. Calf formation is driven by the presence of an electrical bias between the anode and the cathode. When an electric field is applied, it encourages the movement of ions, particularly metal ions such as copper, towards the cathode. This ionic movement contributes to the growth of conductive. Elements There also needs to be some material defects. Defects in the PCB materials such as voids, cracks, or delamination, sometimes referred to as dry weave, create pathways for moisture and ionic movement. These defects can arise from manufacturing processes or from mechanical stress during the PCP's life cycle. The presence of defects exacerbates the likelihood of calf formation. There also needs to be contaminants within the layers of the board. Contaminants such as flux residues, ionic impurities, and processing chemicals such as plating solutions can contribute to calf formation. These contaminants provide additional sources of ions that can migrate under the influence of an electric field, accelerating the growth of conductive filaments. Another contributing factor is inadequate laminate material. The choice of laminate plays a crucial role in calf resistance. Inadequate or low quality laminate materials. The poor moisture resistance and high ionic content are more susceptible to calf formation. The properties of the laminate material, such as its glass transition temperature or TG and resin content, influence its ability to withstand calf inducing conditions. What are some of the prevention methods for CAF well #1 moisture control? Implementing strict moisture control measures during PCB manufacturing and assembly is essential. This includes storing materials in controlled environments. Using desiccants and ensuring the PCBS are properly baked to remove absorb moisture before assembly. If baking is just not an option and the assemblies are cleaned, it's vital that the cleaning machine have an effective drying capability. Using heat while inline or conveyor rize format. Cleaning machines generally use air knives to blow water off the surface of the assembly, they really can't penetrate within the layers of the assembly, allowing moisture that is accumulated during the board, fabrication, shipping, storage and assembly. Processes to remain Many batch format cleaning machines utilize a heated drying system that may perform at least some of the functions of a bakeout in terms of reducing moisture content within the layers of the assembly. Additionally, conformal coatings can be applied to assembled PCBS to reduce moisture ingress. While most conformal coatings do not provide absolute protection for moisture ingress, they can significantly reduce the amount of moisture. Another factor is material selection. Using high quality calf resistant laminate materials is crucial. Laminates with low moisture absorption rates and high TG values are less prone to calf formation. Materials such as high GFR 4 polyamides and certain high performance epoxy resins offer better resistance to moisture and ion mitigation. And then there's design considerations. Proper PCB design can mitigate the risk of CAF formation. Increasing the spacing between conductive elements, for example, reduces the electric field. Strength making it less likely for filaments to form and as I mentioned earlier, protective coatings are always helpful. Applying protective coatings such as conformal coatings or encapsulants to the PCB surface can provide a barrier against moisture ingress and contaminants. These coatings create a physical barrier that reduces the volume of moisture reaching the conductive elements and reduces, not eliminates the ion migration if conformal coding is to be performed for the purposes of reducing moisture ingress. It's exceptionally important to conduct a bakeout prior to conformal coating. If prior to conformal coating, the board contains moisture within the layers, the application of conformal coding will effectively seal in that moisture. Screening and testing is important. Implementing rigorous testing and screening processes during manufacturing can help identify potential calf issues, early techniques such as accelerated life testing, humidity testing, and electrical bias testing such as Sir or Surface. Insulation resistance testing under heat and humidity conditions can simulate real-world conditions and identify boards susceptible to CAF formation. Early detection allows corrective actions before products reach the market. So, in conclusion, CAF formation is a critical reliability concern in PCB assemblies, driven by factors such as moisture ingress, electrical bias, materials defects, contaminants, and inadequate laminate materials. Preventing calf requires a multifaceted approach, including moisture control, careful material selection, thoughtful PCB design, protective coatings, and rigorous testing. By implementing these prevention methods, manufacturers can significantly reduce the risk of calf formation. Ensuring the long term reliability and performance of the electronic devices. I hope you found this information helpful. If you have any additional questions on this topic or a topic for another episode, please reach out to me. I'd love to hear from you. My e-mail address is right over here on the screen. And be sure and listen to or watch the Reliability Matters podcast. This podcast covers the reliability of circuit assemblies. We interview subject matter experts on a variety of topics, all related to reliability. We currently have more than about 150 episodes with more than 80 thousand of those. Episodes downloaded to date. Listen to the podcast on your favorite podcast a or watch the reliability podcast on the reliability matters YouTube channel. Either way, be sure to subscribe to the podcast to be notified when new episodes are released. We release new episodes on the second and fourth Tuesday of every month. To learn more about the podcast, visit reliabilitymatterspodcast.com. Thanks again for watching until I see you again stay safe, stay healthy, stay happy and perhaps. Most importantly, keep doing it right. I'll see you again soon.