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Conformal Coating for Microelectronics: A Primer
December 31, 1969 |Estimated reading time: 8 minutes
By Hector A. Pulido and Gareth De Sanctis, Asymtek
Conformal coating of an electronic assembly is not often considered by designers at the beginning of a project. Until recently, this operation was equated with massive manual masking prior to spraying or dip coating rather than a selective, precise process. Because conformal coating often is mistakenly thought of as an added cost rather than an added value, it is seldom considered during the board design phase. As a result, the decision to use conformal coating usually occurs when field testing reveals its need.
Many OEMs and EMS companies will have to adopt automated and precision conformal coating processes to improve consistency and quality of their products. The competitive marketplace also is a driver for automated conformal coating because it can result in faster process times, reduced rework, easier handling, and other such factors. There are a number of steps that can be taken to ease this transition. They will help the process owner avoid common mistakes, oversights, and unnecessary pain, be it in the pocketbook or the process itself.
Product ConsiderationsThe first thing to look at is the product to determine its field performance demands: environmental, product geometry, and coating metrics. The combination of elements the product will be exposed to vibration, temperature extremes, humidity, chemicals will determine the optimal kind of protection.
Product geometry is a factor because some substrates may be perfect from an electrical functionality point of view, but can present tough challenges for the selective coating operation. Ignoring this may result in a poorly suited equipment application, slow process, or high rework rate, resulting in unexpected associated costs.
Product geometry includes overall dimensions such as length, width, and height; edge clearance; required top and bottom clearance; weight; whether it's a one- or two-sided application; and whether the loading method is batch or conveyor. If using a conveyor, another consideration is whether the product is being loaded individually or on a pallet.
If the product needs to be repairable, will the coating and process selected allow repairability during the process and/or after use?
Material Considerations The coating material will be determined primarily by the product performance needs, then by factors such as ease of application, cost, and handling. Typical coating materials are acrylic, urethane, silicone, and epoxy. They are also classified by cure method as heat, UV, or moisture cure. As mentioned above, give consideration to whether the coated part can be reworked or repaired using that material.
How material is handled can be critical, because it might require special equipment to maintain a consistent temperature or viscosity to dispense correctly. Some materials will require mixing to have the desired content of solids prior to the application. A mixing station is necessary or, in some instances, the material supplier will provide premixed product. The material's pot life will determine the production level that can be achieved before changing the fluid reservoir; the amount of material to be prepared also depends on pot life.
The way a conformal coating material is packaged can make a difference because exposure to ambient conditions can affect certain materials. Mechanisms used to enhance the performance of materials also can make them sensitive to light sources or ambient moisture, causing accelerated cure rate or decreased pot life if not handled properly. Safety is another factor; materials may require particular care when handling, including use in a ventilated area, with gloves and respirators, or caution with ignition sources.
Process ConsiderationsProcess considerations include type of operation, production volume, application method, automation requirements, takt time, and ancillary equipment. Operation type determines if the required production can be achieved in a normal eight-hour day, five-day work week shift. If the projected needs require more than what can be accomplished in two shifts, it might pay to invest in equipment with a higher production rate. A rule of thumb often used is:
- Batch <50 parts/8 hr. shiftIn-line >50 parts/8 hr. shift
The required production volume is a good place to start in selecting equipment and determining the type of operation required, but other, less subtle factors for increasing production should be considered. One is application method. Equipment that selectively coats parts can save considerable amounts of time by eliminating the need for masking/unmasking operations. Time and money are saved by reducing multiple coating passes, being able to adjust the coating speed, and using equipment that can be easily programmed and that is flexible enough to adapt to program changes or modifications.
There are several types of coating applicators. Spray coating is an atomized process that can achieve a thinner film pass; however, if selectivity is a requirement, the applicator usually has to move more slowly. The finish is a feathered or fuzzy coating on the edges.
Curtain, or flow coating, can maintain high application speeds, produce sharp edges, and provide a film build of 25 to 75 µm (1 to 3 mils); therefore, if suitable for the application, it has the advantages of fast process time, selective coating, and good film thickness. It also provides consistent application, reduced masking and unmasking, and a one-pass application to reach appropriate film builds.
Using a needle to apply coatings always has been the default choice for small areas and semi-controlled flow. It works well for discrete areas where it is okay for the needle to touch or get very close to the substrate to initiate material flow through capillary effect. Because it can slow down the process, it is sometimes combined with a second, faster applicator that can also be appropriate for discrete, small-area dispensing.
The jet is a relative newcomer to conformal coating. It has many advantages for selective coating of small parts, especially when coating needs to be applied in hard-to-reach spaces. The jet can shoot small volumes of material without touching the substrate and without concern for substrate warpage. It is faster than a needle because the material is shot out, eliminating the up-and-down motion. The combination of speed, selectivity, and precision provides advantages for these new conformal coating applications that other tools aren't achieving.
Figure 2. The jetting mode is especially useful for coating small substrates or substrates with high component density, and with tight coated-to-uncoated area tolerances.
AutomationAutomation results in many process control advantages that just aren't possible with a manual or semi-automatic process. Automated systems provide an additional level of quality control. New ways to refine automation processes are becoming available with the added benefit of bringing controls and settings into the programmable coating application.
Flow monitoring measures and maintains volume flow within certain programmable upper and lower limit levels. An out-of-limits alarm can notify the user if a process moves beyond them.
Viscosity control is important in the conformal coating process to coat parts evenly, because constant viscosity results in uniform thickness deposition and a consistent application pattern. Viscosity and coating patterns can change as temperature changes. The ambient temperature on a manufacturing floor can fluctuate up to 10°C during a manufacturing shift, so a process control that maintains a constant temperature in the coating eliminates one more variable.
Pattern width control, which is also called laser fan width, automatically verifies and adjusts the fan, or coating pass, width as needed. It is also beneficial to achieve tight keepout areas.
Event logging is becoming more important for traceability. Multiple parameters are recorded for analysis using external SPC applications or for interaction with special protocols related to factory information systems, where, through an ID method like a barcode reader, a process executed on a product can be traced and attached, together with their application parameters.
Takt time = cycle rate = drop rate. The term takt time refers to the amount of time it takes to load, execute a coating operation, and unload. It's used as a measure of production capability, to determine the bottlenecks and the number of machines required to fulfill a determined production rate.
Ancillary equipment depends on what the process demands are, with need based on what the cycle rate is and how the process is set. Examples of ancillary equipment are material handling and cure equipment. Material handling equipment consists of conveyor lengths, in-line inverters, stackers, or product accumulators. These are typical of in-line systems and are necessary to maintain process speed and minimize operator intervention, thus driving up the production rate.
Cure equipment is chosen as a function of material and process speed. The most common are heat-cure and UV-cure ovens. A heat-cure oven consists of a conveyor inside a heated chamber of a given length, which is calculated according to the required cure time and the cycle rate. It's usually at the end of the line. A UV-cure oven uses a very short cure chamber because UV-cure processes occur in seconds. Because of this, they often are used for high production rates.
Inspection stations usually are considered in two instances: touch-up or quality assurance. They have different needs such as open access, extra ventilation, or small tool holders, or just an inspection light to verify coverage. There are multiple combinations of these, and customization is often possible.
Economic ConsiderationsThe cost of conformal coating must be weighed against the costs of producing unprotected electronic components. One thing to consider is the replacement cost of the components if they fail due to factors that could have been prevented by conformal coating. When the life of a product is increased, the cost of returns will decrease.
Another consideration is coating method. Dip coating will greatly increase material usage and cost. Spray booths may use less material than dip coating, but the total process cost is higher because the number of operators and operator time both are greatly increased due to masking. Automated selective coating may have a high initial cost, but the total cost of ownership will be low due to the decrease in material usage, the reduced cost of operations, and the reduced return rate of the product.
ConclusionConformal coating started as a simple process performed on electronic substrates in need of extra protection from external elements, with little attention paid to quality factors beyond adequate component coverage. The increased capability of semiconductor assemblies to perform complex tasks in automotive applications, traffic control, signage, outdoor surveillance, and mission-critical elements has increased the demand for conformal coating. Because device failure could have dire consequences, the quality of coating material application is critical. New equipment and processes are in place to accommodate the conformal coating requirements of these emerging technologies. A smooth transition to automated precision conformal coating can be achieved through an analysis of the product being coated and the desired result, the coating material used, the process selected, and of the economic costs over a period of time.
Hector A. Pulido and Gareth De Sanctis, Asymtek, may be contacted at 2762 Loker Ave. West, Carlsbad, Calif., 92008.