Conformal coating and board cleanliness: ensuring product cleanliness and durability in any application.

Technology advances and the use of smaller components and denser layouts, combined with diverse environmental conditions, renew the importance of circuit board cleanliness and conformal coating.

Most circuit boards requiring conformal coating are manufactured for products used outdoors. SMC manufactures circuit boards with conformal coatings for the following applications:

* Commercial and industrial gas and water metering instruments located outdoors in harsh environments.

* Realtor lockboxes, which need to be highly heat- and moisture-resistant.

* Medical appliances, which are directly exposed to saline during operation.

* Commercial kitchen equipment, in which circuit boards need to survive washdowns.

* High-speed data processing systems.

Here we discuss some contamination and environmental impacts, and highlight processes and improvements that ensure product cleanliness and durability exceeds customer expectations in any application.

In general, ionic contamination, potentially the most harmful type of contamination, is defined as any electrically conductive residue remaining on a PCB. Ions are deposited on the PCB's insulated surfaces and are derived from metal cleaners, etchants, plating chemicals, fluxes, processing equipment, human contact or components placed on the PCB.

When ionic contaminants remain on the PCB after production, and the boards are subsequently introduced into higher humidity environments, salt residue begins to absorb moisture and corrode the anodes and cathodes. (1) Over time, these deposits could grow large enough to bridge conductors and create a virtual short circuit, possibly destroying the circuit.

Although the board wash process may remove most of the contamination, processes after washing also can place contaminants on the board. Once these contaminants are on the board, high current or current and voltage switching applications can speed the growth of the unwanted bridges.

To mitigate risks of product failure in the field from contaminated circuit boards, the assembler should work with OEMs to establish cleaning procedures and select conformal coatings that provide the protection needed to keep the product functioning in its operating environment.

Since contact with human skin is a big contributor to contamination (deposition of sodium, chloride and potassium ions), assemblies with many hand-inserted components, or hand-installed hardware are more susceptible to ionic deposits. PVA machines can apply coating evenly to boards while reducing human contact that can increase contamination issues.

When managing board cleanliness, educating workers comes first. Workers are trained to handle boards by the edges and to wash their hands before touching product.

At SMC, during the cleaning process, boards enter the aqueous wash machine on a mesh conveyor (Figure 1), first traveling through a pre-wash phase with deionized (DI) water that has a level of resistance of 17 to 18 M[ohm]. Boards then move to the wash phase, during which a saponifier (soap) in the wash tank is combined with DI water to wash the boards. The final rinse is completed with DI water; then boards are dried with hot air.

Based on customer requirements, SMC uses dynamic extraction to test random samples of boards every day to ensure constant monitoring of contamination levels. Early in the customer relationship, SMC determines the percentage of boards that will be tested, and a decision is made about the method of selecting a random sampling of boards throughout each manufacturing run. Technicians send board samples through an Ionograph SMDII 500 machine that pumps a heated 75% alcohol solution over the boards and collects contaminants in a container for testing. Technicians begin each test by entering the job number, board dimensions, fail point based on customer requirement, and test time. Contaminants are measured in micrograms per square inch as a ratio between the amount of contaminant collected in the container to the surface area of the boards in the machine.

[FIGURE 1 OMITTED]

With new projects, SMC often sends samples to outside labs for ion chromatography tests that provide more detailed feedback about the nature of contamination present. Advantages of this type of test include the ability to examine isolated spots on the board and the ability to determine the chemical breakdown of contaminants, so the data can be used to track the sources of contamination.

Following cleaning, conformal coating or potting is applied to the boards to protect them from environments that may cause contamination or condensation. Technicians apply a thin layer of material that "conforms" to components and solder joints, to protect the board from environments that may expose it to standing moisture, heat, salt spray and vibration.

Application methods and materials available for conformal coating and board cleanliness are numerous. SMC takes a specific approach concerning the processes and methods required to ensure PCB cleanliness and protection. Engineers start by examining the product application and helping select a coating or potting chemistry and application method. The coating choice is based on the final product's intended operating environment, compatibility of substrates and ease of use. Many board assemblers favor one type of coating in their manufacturing operations. SMC is set up to run five different types of coatings in two different machines.

Some points of differentiation between conformal coatings:

* Acrylics provide a thin, tough surface, but substrate adhesion can be a problem. Acrylics are best used in conditions where environmental exposure is minimal. Another advantage is that acrylics are easily removed if the PCB requires repair.

* Urethanes create a thick, durable coating with excellent adhesion, but full cure of the material may require a free-air environment for many days. Urethane coating can also be comprised of a dual-component cure of a resin and hardener combination to speed curing.

* Silicones, while easy to apply, create a softer finish, which may be a problem for boards that undergo manual assembly operations after coating. Silicone can fully encapsulate a PCB for extreme durability, tamper prevention and water immersion.

Many acrylics and urethanes include a solvent carrier such as xylene or toluene. Some coatings have trace amounts of lead or cadmium. These chemicals can create concerns regarding production material handling, waste disposal and product end-of-life disposal.

During the past 10 years or so, newer chemistries have been introduced. SMC currently applies "ecologically friendly" urethanes that outperform the coatings of the past. Solvent-free, 100% solid silicones are used when it is necessary to have thicker film build for increased moisture resistance.

SMC uses five PVA robotic dispensing systems for various types of coating applications. These systems offer several significant advantages to the production process:

* Automation helps reduce component-masking time and improves repeatability of the application.

* As environmental concerns grow, automated processes help reduce waste and increase throughput compared to dip or bulk spray methods.

* The use of automation reduces human contact that can increase contamination issues.

Circuit board cleanliness and conformal coating are important requirements. By continually monitoring processes and researching new innovations, assemblers can provide the highest quality products while ensuring reliable performance in the most extreme environments.

References

1. S.M. Dalessandro and T.D. Cabelka, "Identifying Causes and Effects of Ionic Contamination in PWB Assemblies," Electri.onics, vol. 31, no. 10, 1985, pp. 43-45.

Bryan Gillespie is a customer engineer at SMC (smcems.com); [e-mail]. Matt DeBenedetto is a manufacturing engineering technician at SMC; [e-mail].