Effective Printed Circuit Board (PCB) design requires careful consideration of various factors such as the orientation of components, routing of signal traces, and flux/paste mixture. However, one thing that PCB designers commonly overlook is the importance of selecting cleaning methods and fluids early in the design phase. They often delay cleaning choices until the later stages of production, after finalizing decisions on solder pastes and conformal coatings. However, this approach can lead to manufacturing contaminants, such as flux residue, dust and moisture being trapped on the board resulting in poor performance and reliability issues. These can include dendrite growth, oxidation, noise on the board, shorting, electrochemical migration, intermittent operation and even complete field failure. To avoid production delays, product recalls and costly returns, it is crucial for designers to plan ahead and proactively specify an appropriate flux remover that works with their chosen solder paste, flux, and PCB substrate during the initial design phase. This will help avoid cleaning complications during production and ensure their PCBs perform reliably over time.
Like Cleans Like
Several brands and formulations of PCB cleaners or flux removers are available in the market to select from. To get good, consistent circuit board cleaning, the right cleaning fluid should be chosen for the job. The old saying “like cleans like” applies here, which means matching the cleaner to the contaminant. PCB designers should identify the contaminant on their boards and select the cleaning fluid that corresponds to it. Inorganic, polar contaminants, such as no-clean flux residue, require a polar cleaning fluid with aggressive solvency. Organic, nonpolar contamination, such as rosin-based flux residue, requires a milder nonpolar cleaning fluid that is safer for the PCB materials.
Avoid Damage
Choosing the right cleaning fluid and cleaning method is crucial, as it ultimately affects the functionality and reliability of the board. Using a cleaning fluid with too high of a cleaning strength or an inappropriate cleaning method can damage PCB components and substrates. Components can be loosened, connections are broken, and substrates impacted if the wrong cleaning fluid or inappropriate cleaning method is used. Designers should consider material compatibility when choosing a cleaning fluid. To avoid damaging the PCB, the cleaning fluid must be compatible with all the materials on the board. The cleaning fluid should be strong enough to effectively remove the contaminant but should not dissolve delicate plastics, coatings or other soft materials. Spot-testing the cleaning fluid on the components first to ensure its safety is recommended. If in doubt, start with a milder cleaning fluid and gradually increase the strength or Kb value of the cleaner until optimalclea ning results are achieved without causing any damage.
Plan to Rinse and Dry
Some PCB fabricators may overlookthe I mportance of thorough rinsing after cleaning. However, rinsing is a critical step to ensure that all contaminants are removed from the board. If not rinsed properly, flux residue can remain on the board and dry in place, leading to a sticky residue. Drying may also seem like an unnecessary step, but any moisture left on the surface of the PCB can lead to problems, especially for those undergoing conformal coating. Leftover moisture on the PCB can outgas during the conformal coating application, resulting in defects in the coating like bubbles and fisheyes.
Pre-Testing Prevents Headaches
Fundamental to PCB design is pre-testing. Understanding how to clean all the PCB elements early in the design process is critical. By instigating test cleaning at the initial concept stage, designers understand how their choices of pastes, fluxes, and cleaners impact the board’s performance. Test-cleaning a limited run of PCBs before full-scale manufacturing can be beneficial to achieving a functioning part.
■ Leftover moisture on the PCB can outgas during the conformal coating application, resulting in
defects in the coating like bubbles and fisheyes
Once the cleaning fluid and process have been demonstrated to work on a smaller scale, production can be increased to the higher volumes required. Through smallbatch preemptive testing, any surprises are eliminated to ensure better board reliability. Pre-testing can take place in-house if the facility has the appropriate capabilities. However, outsourcing is a popular choice for PCB cleanliness assurance. Specialist companies can undertake comprehensive testing and cleaning trials adhering to the standards specified by the PCB designer. This can range from simple visual testing to strict IPC-610 requirement testing.
Types of PCB Testing
PCB testing involves a range of methods and techniques to detect and measure the amount of contamination onthe bo ard’s surface. The choice of testing method depends on the type of contamination and the level of precision required. Surface Insulation Resistance (SIR) testing is a highly precise testing method that measures the reliability of an electrical component based on the strength and quality of the signal that comes through the component after cleaning. SIR testing is typically performed off-site in a research lab. During the test, the difference in an electrical current over time and in differingtemperatures and humidity levels is measured after cleaning. SIR testing also measures the corrosiveness of a residue by looking at the formation of dendrites. High resistance is desirable, indicating that the board is clean and free of contamination. Another widely used testing method is ionograph or Resistivity of Solvent Extract (ROSE) testing. During the process, the PCB is immersed in a bath of deionized water for a few minutes, and the water is then tested for ionic contamination. Any change in resistivity of the water indicates the relative dirtiness or cleanliness of the board. While ROSE testing is less precise than SIR testing, it is faster and more user-friendly. Ion Chromatography (IC) testing is the most precise and reliable testing method for PCB cleanliness. It is the gold-standard for judging cleanliness, as it detects and quantifies weak organic acids and individual ions. IC testing is an excellent process to troubleshoot the source of contamination quickly and predict the effect of each ionic type. Elizabeth Norwood is a Senior Chemist at MicroCare, LLC, which offers precision cleaning solutions. She has been in the industry more than 25 years and holds a BS in Chemistry from the University of St. Joseph. Norwood researches, develops and tests cleaning-related products. She currently has one patent issued and two pending for her work. For more information, visit www.microcare.com.
The choice of testing method depends on the type of contamination and the level of
precision required.
