Reducing Field Failures: Is Conformal Coating and Potting Right for Your Product?
Anyone who has spilled a cup of coffee on a keyboard or dropped a cell phone in water understands the negative effects that moisture can have on the operation of electronic products. Products used in harsh environments with regular exposure to temperature extremes, water, humidity, sand or salt spray can also have performance issues. One way to mitigate that risk is to either coat or encapsulate all or part of the printed circuit board assembly (PCBA).
That said coatings add cost and there is no one best option. Determining which coating is best for an application requires careful analysis of multiple variables. Additionally, just as there are best practices in terms of design for manufacturability, there are also best practices in design for conformal coating or potting.
As a contract manufacturer, Burton Industries builds a wide range of products. Some of those products are used in harsh environments and have requirements for conformal coating or potting to help protect the components. This article looks at considerations in choosing the best coating option and then discusses ways to reduce cost by designing product to minimize labor and rework requirements. It also discusses key test considerations.
Coating Options and Tradeoffs
Conformal coating is the most widely used coating method and acrylics are among the most popular conformal coating material. Acrylics are easy to apply and easy to rework. Silicone is also popular. However, it is harder to remove for rework. Rework on silicone-coated PCBAs requires use of solvents unless only a small area of the product will undergo rework. The main advantage of silicone is that it is stable at higher temperatures up to 200 degrees C, which makes it appropriate for high heat applications. The team at Burton Industries uses a one-part silicone formula. Both acrylics and silicone provide good protection from moisture, fungus, dirt, dust and salt spray. Silicone provides better protection in environments that include chemicals or solvents and vibration.
Potting provides additional protection in harsh environments by encapsulating sensitive electronics. When used on a single component in a process known as “glob top,” it can protect ICs from damage or strain. Some companies use potting compounds as a means to prevent theft of proprietary data. Potting may be a better solution than conformal coating for protecting products in environments that have a lot of vibration because it provides total encapsulation. It can also help with heat dissipation, since encapsulation spreads heat more evenly. Other harsh environmental conditions it can protect against include chemical or gas exposure, shock and drops.
Both potting and conformal coating add cost to the product, although prevention of field failures can eliminate a much higher cost. Once a product is potted, it generally can’t be reworked. Products incorporating potting must either be designed with an enclosure that won’t allow seepage during cure or a mold must be made to hold the compound during cure. Curing time with potting can be longer than that of conformal coating and the curing process requires control, since heat and humidity can affect cure time.
Both coating and potting require a clean substrate for the coating or potting to adhere properly. When no clean flux is used, it must be tested to determine if an additional step is required to clean the substrate prior to coating or potting. Use of non-wettable components will add cost. At a minimum they need to be masked or protected with a fixture if dipping or spraying is used. Thickness of the coating must be controlled to stay within the design specification including the thickness tolerance.
Determining the Best Option
The analysis process starts by looking at the end application. For conformal coating, it is important to understand what temperature range the product will operate in. Another factor is the operating environment. For example, will there be exposure to solvents, salt spray or condensation?
Once the conformal coating option has been chosen, it is important to review the Material Safety Data Sheet (MSDS) for the material to ensure there are no potential issues. Chemical composition and any identified use hazards should be reviewed to determine if any of these are incompatible with the product or its market. Cure time, physical properties and application techniques should also be reviewed, since these all factor into the cost equation. There should be a test of the coating in the end use environment prior to volume production to verify that it achieves the desired performance levels.
For potting, the same temperature and operating environment questions asked for conformal coating also apply. However, the MSDS needs a more thorough review. In addition to chemical composition, hazards, cure and physical properties, it is also important to look at ways the compound may change in use.
Will encapsulate stress components? How flexible is the compound? Does it shrink or expand with changes in temperature?
In selecting an appropriate potting compound, it is important to consider environmental factors and potential component stress issues. For example, a softer compound will put less stress on components, particularly when there are temperature extremes.
Designing for the desired coating application eliminates the potential for defect opportunities and reduces labor. There are five basic types of conformal coating application: dip, spray, brush, selective coat and deposition. Spraying is the most frequently used option at Burton Industries. The spraying method is a manual method using a spray gun in a properly vented spray booth that will contain overspray and remove fumes from the area. The selective coating method uses an automatic spray machine. Selective coating machines apply conformal coating to only selected areas as directed in the machine program with less masking required than for spray applications. While automated conformal coating offers consistency, the tradeoff is that clean-up requirements during product changeover are much more time-consuming then the clean-up for a manual spray gun. This may make a manual method more cost effective for lower volume product.
The application method should be chosen before the product design is finalized to ensure the printed circuit board (PCB) layout supports an efficient application process. The better defined “keep out” areas are from coated areas, the less masking is required in manual application methods. Sufficient space should be defined between keep out areas and coated areas. For example, 2.5mm is generally a sufficient space between the areas for selective coating machine applications. Spray applications and dip applications will require some type of masking to keep conformal coating from getting into unwanted areas.
Connectors require special attention, as conformal coating can wick up their leads. One option for preventing this is use of conformal coating gels to build “dams” around the connector.
The desired thickness of the conformal coating should also be defined based on the level of protection required. From a PCB layout standpoint, coated components should be grouped together and those that are not coated should be similarly grouped. Try to avoid creating areas on the PCB where the conformal coating can pool. PCB edges should also be kept clear, as conformal coating that goes completely to the edge of a PCB can cause fit interference if the end unit is installed in a housing.
Finally, it is important to develop a test to determine if the coating is providing the protection needed.
In potting, it is important to carefully evaluate the design of the enclosure for the completed unit. The housing should have no areas where potting compound will leak out. However, the housing will need to have a fill hole, vent hole or other means to dispense the potting compound into the housing. If the product is not potted in the housing, it is necessary to design a mold to contain the unit and potting during curing of the potting compound.
If components such as LEDs need to be visible, clear potting or clear RTV will be needed in these areas at a minimum. If more than one material is used in potting, they must be compatible.
Potting compound composition must be analyzed carefully, as well. For example, some potting compounds contain platinum catalysts that react to other components and chemicals in the manufacturing process. If other chemicals in the factory used in the process react, this reaction can prevent curing of the potting compound.
The shelf life of the compound and packaging must be considered in the overall cost. Custom compounds may have a large minimum volume purchase requirement. The compound must be used or discarded within the shelf life of the product. On high volume product, this isn’t an issue. However, with lower volume products, this “minimum buy” liability cost must be factored into unit cost, since the material may need to be discarded rather than consumed.
Dispensing methods are also a factor in potting. Large volume quantities will likely need to be dispensed with automated dispensing machines dedicated to one or a few different compounds. Changeover time and cost to purge compounds from a machine and switch to different compounds can be high. The designer should be aware of the dispensing methods available at potting contractor’s facility, to ensure that the dispensing method is appropriate for the likely product volumes.
The designer must also consider how to achieve complete coverage of the PCBA in the area or enclosure desired. Thick viscous compounds are very difficult to get to flow around and under small components and tight spaces. Very thin low viscosity compounds can leak around openings or holes in housings before they set or cure. Analyzing the tradeoffs is critical to selecting the right compound.
A final issue that must be considered by the product management team as a whole is the total cost when encapsulating products. Most potting compounds will not allow assembly level rework of completed units. Therefore failed units become scrap. If repair depot for field failures was never a desired option, this isn’t an issue. However, if the unit contains higher cost components and there was a desire to repair rather than replace failed units, potting may not be the best option.
Conformal Coating Test Considerations
As mentioned earlier, testing is an important part of validating a conformal coating choice. When applicable, test strategy should take into account any conformal coating standards that apply to the product.
The two most common standards are UL746E, which relates to electrical safety and flammability and IPC-CC-830, which provides general acceptance criteria for conformal coating. The IPC standard is similar to the old MIL-I-46058. MIL-I-46058 is now inactive for new design, but the Qualified Products List (QPL) is still maintained.
If desired, third party testing facilities can test conformal coating for compliance to UL746E or performance specifications. Additionally, other acceptance criteria may be added or substituted by the designer based on desired conformal coating properties for the project. Testing can be done with coupons or representative PCBAs, or even destructive tests with actual assemblies, if not cost prohibitive. Typical qualification tests include:
• Surface insulation resistance (SIR) testing
• Environmental cycling and thermal shock
• Humidity, salt spray and corrosive gasses
• Visual inspection for issues such as de-wetting, cracking, peeling, loss of adhesion, flaking, contamination, discoloration.
Visual inspection and coating thickness measurement with an eddy current meter are the most common methods of continuing inspection at Burton Industries once production begins. IPC-A-610, which deals with PCBA workmanship standards, and IPC-CM-770, which deals with component level workmanship standards, both address conformal coating.
Conformal coating and potting offer a viable solution for protecting products from harsh environmental conditions. Carefully considering the choices and tradeoffs during the design phase can save time and money. Manual application methods may have shorter changeover time for high mix product. However, automated methods may require less manual labor for masking and unmasking areas of the PCB that are not sprayed. Picking a manufacturing partner capable of outlining the tradeoffs and design considerations associated with any choice can help reduce cost and learning curve.
Jeff Brattrud is Burton Industries’ Engineering Manager and can be reached at firstname.lastname@example.org