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Conformal Coating Optimization
December 31, 1969 |Estimated reading time: 9 minutes
An overview is provided of the protective process' most effective options providing "smooth" integration into existing production lines.
By Bill Donges
Over the past decade, conformal coating processes have moved from relatively limited niche applications to use across a widening range of products. In addition to the military and other specialized applications driving early requirements, electronics system proliferation in such demanding environments as automotive, industrial, communications and mobile devices has increased the need for this form of product protection.
Unfortunately (as many process engineers have discovered), the traditional methods used for applying conformal coatings are not well suited for today's high-volume, tightly controlled assembly environments. In many cases, such methods depend on cumbersome off-line processes, which generally require labor-intensive operations such as component masking.
In response, the evolution of new, automated dispensing systems for the deposition of precise, selective conformal coatings has enabled engineers to integrate the new processes directly into in-line production scenarios. Specifically, a combination of closed fluid systems, programmable dispensing patterns, board-handling options and curing now may be melded with existing high-volume or high-mix printed circuit board (PCB) production lines that also emphasize the minimization of operator and off-line process intervention.
Selection Rationale and ObjectivesThe primary reason for using conformal coatings is to shield specific areas of the assembly against environmental factors that can damage and shorten useful product life. Properly applied, conformal coatings offer protection from vibration, moisture, dendrite growth, oxidation, dirt, solvents and other corrosive elements.
Coating objectives vary for different products and depend on variables such as the operational environment. In some selected instances, a conformal coating can enhance performance and allow for greater component densities by improving dielectric properties between closely spaced conductors.
Critical ParametersCoating selection factors that must be considered include the maintenance after application of consistent process control application, the required quality levels and sustained throughput rates. With current, densely packaged product designs, tighter manufacturing tolerances and mixed-function assemblies, the ability to control dispensing accurately to achieve precise edges has become a critical quality parameter. Thus, from a throughput perspective, conformal coating processes must fit smoothly within highly automated PCB production lines, operate continuously and maintain line pace.
Another key consideration is total cost of ownership (TCO), which takes into account a comprehensive evaluation of capital investments, ongoing labor costs, scrap rates, cost of material usage/waste, ventilation and maintenance schedules/costs, etc. Other important TCO components that sometimes are overlooked include the adaptability of capital investments for future products vs. the retraining costs for adapting labor-intensive processes.
Finally, for both OEMs and contract manufacturers (CM), the ability to quickly adapt existing processes for new or revised product designs is critical to maintaining a competitive edge. The table provides a typical single-shift, high-volume production cost comparison of spray booth application vs. selective coating.
Evolution of Process AlternativesConformal coating application methods have graduated from the early dipping and air spraying processes to more precise selective dispensing techniques. Generally, these different approaches can be grouped into two processes: the "subtractive," in which the areas to remain uncoated must be masked out (Figure 1); and the "additive," in which the coating is applied precisely where required without masking.Figure 1. Hand masking is required prior to dip coating.
As the name implies, dip coating consists of physically submerging the assemblies into a vat of conformal coating material, a process that can be performed either manually or mechanically. However, in either case, components that cannot be exposed to the coating material must first be masked carefully by hand. Further, after dipping, the masking materials must be removed manually and discarded (or recycled). Because hand masking inherently is labor-intensive and uses secondary consumables, it invariably adds time and cost to the production process.
From a process-control standpoint, dipping is subject to the inherent variations associated with manual operations, with results directly depending on the consistency and precision of the masking process. Dipping also requires an ongoing investment in training and monitoring to maintain appropriate operator skill levels and attention to detail. And lastly, the thickness and consistency of the coating material can vary if the dip tank viscosity changes.
Process ConcernsDipping: The open atmosphere around the dip tank coupled with the practice of dipping unclean boards can cause contamination. Accordingly, dipping processes require periodic purging and replacing the coating material in the tank. Depending on the type of material and the frequency of the purge cycle, this can be a major cost item for high-volume production.
Traditional advantages of dip coating include relatively low capital investment costs on the front-end and potentially high raw throughput rates. However, from a TCO standpoint, it is important to remember that the training, labor and consumables cost for masking quickly can cancel any expected savings as well as degrade overall throughput levels.
Board tooling also can impact overall costs. In some instances, where assemblies can be pre-designed especially to minimize masking requirements, dip coating can provide a cost-effective, high-throughput alternative. Unfortunately, with today's dynamically changing product designs and quick time-to-market requirements, pre-designing for dipping rarely is a practical alternative in high-mix CM environments in which little or no control over the design process exists.
Brushing is another conventional method for conformal coating application. It consists of dipping a brush into a container and manually applying the coating material to the board. Although brush coating is simple, with little investment in equipment or tooling, it is an inherently crude process that has many of the same labor-intensive, variable quality and contamination issues cited above. While brush coating can be a viable alternative for low-volumes such as prototype runs, it simply cannot keep pace with the high-volume requirements of the more mainstream production environments.
Figure 2. Air spray methods must control over-spray.
Air Spraying is yet another application alternative. The method has evolved to encompass both manual and automated techniques, ranging from spraying the entire assembly to targeting specific areas. While spraying by hand has the advantage of relatively low equipment costs, it poses the same variability and labor-dependent issues associated with brushing. Also, there is added time and cost for masking to prevent over-spraying on surrounding areas (Figure 2). For a more up-front capital investment, however, automated air spraying systems can provide a higher level of consistency with more targeted application possibilities over manual methods (but still require masking to prevent spray contamination). Finally, air spray technologies typically have a transfer efficiency of only 50 to 70 percent, i.e., 30 to 50 percent of the material falls outside the target zone.
Needle-based Selective Dispensing systems offer much greater transfer efficiency as well as provide better precision and consistent edge definition by putting the conformal material exactly where it is required. Needle-based dispensing also offers the advantages of closed-loop control over the fluid, thereby eliminating contamination risks, minimizing concerns with viscosity variability and avoiding the costs of frequent purges and discards of unused fluid. However, even with high-speed robotics, the throughput of needle-based systems can be limited by needle diameters and flow-rate constraints. Needle dispensing methods can be most appropriate for off-line, touch-up work in which accuracy and consistency are important but throughput and material thickness are uncritical.
Figure 3. An example of an in-line automated selective coating dispensing system.
Automation's AdvantagesFor more demanding in-line production requirements, most OEMs and CMs are turning to automated coating operations (Figure 3). These dispensing systems can use unatomized dispensing techniques to apply conformal coatings at high speeds while maintaining consistent accuracy and sharp-edge definition. The no-atomization dispensing module moves at a fixed height over the assembly, thus overcoming the speed limitations of contact-based needle dispensing systems and eliminating the over-spray associated with atomization.By incorporating specialized air-assist mechanisms for micro-shaping the fluid stream, these advanced selective-coating systems can apply various patterns and thicknesses, thus ensuring an optimal balance of coverage and speed. For example, selective coating can deliver bead, monofilament or swirl patterns at speeds ranging from 5.5 to 18.0 ips, or the method can dispense individual spots as fast as 0.01 seconds each.
Figure 4. Three types of patterns that selective coating can deliver: bead (a); monofiliment (b); and swirl (c).
The "bead" mode emulates the precision of needle-based dispensing but with greater throughput and without dripping (Figure 4a). In "monofilament" mode, the air-assist mechanism spins a single strand of material as it exits the nozzle, stretching the coating material into a conical-shaped looping pattern (Figure 4b). Monofilament mode provides good uniformity of build and edge definition with achievable film builds ranging from 0.003 to 0.012". "Swirl" mode uses increased air pressure and reduced material flow to create a controlled atomization of the material that expands coverage while retaining good edge definition and minimizing over-spray (Figure 4c). The swirl mode results in film thickness of 0.0005 to 0.0030".
Programming Selective Coating SystemsComprehensive software programmability is another critical benefit of selective coating systems. The technology permits process designers quickly to adapt the various dispensing pattern "tools" to their specific application requirements, optimizing the dispensing sequences for maximum efficiency and throughput. Process engineers can focus on desired results by using user-friendly software instructions, such as defining an "area coat" and "mask regions" by simply teaching or entering two points and noting results displayed graphically on the computer monitor.
Additionally, because automated selective coating solutions incorporate closed fluid systems, they also minimize the impact of differences in fluid properties or changes in viscosity over time. Unlike dipping processes that can be viscosity-dependent, selective coating processes are equally suited for either thin or thick fluids. Selective systems generally can handle the entire range of conformal coatings, including acrylic (AR), polyurethane (UR), epoxy (ER), silicone (SR), Paralyne and other materials.
With selective conformal coating, the manufacturing engineer has wider latitude for adapting the materials to specific requirements of the product design rather than being restricted by the coating process's narrow parameters. For example, for applications requiring different material properties, dual-headed systems can selectively dispense two different types or thicknesses of fluid in different locations during a single operation.
Creating Complete Production CellsRaw speed and dispensing flexibility are not all that is needed to ensure smooth integration of the conformal coating process within overall production-line requirements. Whether the need is full in-line processing or a flexible stand-alone work cell, the dispensing operations must be integrated tightly with other process requirements to give manufacturers the advantages of a complete production unit for conformal coating.
Figure 5. In-line solutions manage process control, contain labor costs and ensure increased product performance.
For example, curing process optimization is key to achieving sustained throughput objectives. Different conformal coating materials also make use of various curing methods, including heat-cure materials (using infrared or convection ovens), ultraviolet (using UV ovens), moisture cure (RTV) or reactive curing. By integrating production grade curing ovens with the automated selective coating system, manufacturers can leverage the full benefits of today's high-speed dispensing capabilities. By leveraging the programmability and flexibility built directly into advanced production grade ovens, manufacturers can accommodate high-mix requirements and a range of coating materials efficiently while minimizing changeover and production downtime. Other key elements for supporting conformal coating direct in-line use include the integration of standards-based conveyorized loading/unloading functions together with options for automatic inverters (board flippers) to enable efficient coating of bottom-side components (Figure 5).
Ultimately, the overall goal of the new in-line solutions is to bring conformal coating processes out of the "back room" and onto the production floor in direct support of mainstream PCB production operations. As requirements for conformal coating processing become more widespread, the availability and adaptability of new, comprehensive, in-line solutions will be critical for managing process control, containing labor costs and ensuring increased product performance.
BILL DONGES is the conformal coating products manager at Asymtek, A Nordson Co., 2762 Loker Ave. West, Carlsbad, CA 92008; (760) 431-1919; Fax: (760) 930-7487; E-mail: bdonges@asymtek.com