Conformal Coating Application Methods

The most appropriate method of conformal coating application reflects the targeted substrate’s susceptibility to the coating material. Liquid coatings like acrylics, epoxies, silicones and urethanes possess specific performance properties. Their optimal protection and operational efficiency depends on matching these properties with the appropriate application process as well as supporting good interaction between the coating and the printed circuit board (PCB), leading to reliable assembly performance. This process limits the development of coating defects before initiating the coating process.

Choosing the appropriate application method is further dependent on material considerations of both the PCB’s purpose and performance expectations throughout its lifespan as well as the resin’s properties. This selection criteria include coating compatibility in relation to PCB function, operational temperature range, sources of potential contamination (chemical, moisture, salt/sand, etc.) and specialized, project-specific conditions (coefficient of expansion requirements, etc.).

Regardless of the method used, ensuring robust, stable and uniform coverage requires a coating thickness of 1-2 millimeters, another factor determining the choice of application method relative to resin selection and assembly function.

This optimal coordination between PCB, conformal coating and application achieves the coating quality specified by the project. Matching application equipment and procedure to project requirements assure technical replication, making the coating process more efficient and less costly while generating enhanced film uniformity during application. Throughput time – coating application speed, curing time, material cost and rework also figure in the overall selection of coating application methods.

Spray Application

Manual or automated spray techniques can be used for all liquid conformal coatings. Cost-effective spray applications offer better coating surface quality than other liquid application processes. Automated spray procedures further enhance project accuracy for high-volume spray assignments, which reduces masking requirements. In addition to high-volume advantages, spray booth applications are effective for medium-level production while manual benchtop spray coating is recommended for smaller-scale rework and repair assignments.

Dipping Methods

Acrylics, epoxies, silicones and urethanes employ manual or automated dip immersion processes for producing conformal films. Large product batches of epoxy respond to machine dipping procedures. With dipping, electrical assemblies are completely immersed in the selected liquid solution so that coating coalesces around the PCB during immersion. 

Unlike spray methods, dipping offers dependable under-component penetration. Extensive manual masking can be necessary, but dip processes are otherwise quick to finish. If an assembly’s components are situated too closely on the circuit board, coating problems may develop. Also, despite thorough immersion in the dip tank, inconsistent edge or tip coverage is a potential disadvantage as are cases of irregular coating thickness. 

Brush Application

Simple and cost-effective, brushing is best suited to small-batch production; it is too time-consuming to be feasible for larger-scale assignments. Other brush application disadvantages include:

• Only one side of an assembly can be coated at a time, slowing production
• Complicated/irregular PCB surfaces interfere with the application of uniform film coating
• Overly thick films readily crack during thermal cycling, leaving the component vulnerable to infiltration by moisture, oil, salt/sand and external agents
• Overly thin coats similarly produce inadequate protection

Despite procedural drawbacks that slow production, liquid brush application is still recommended for acrylics and urethanes. For epoxies, prototype assemblies or those requiring a lot of masking benefit from brush application. On the other hand, brush coatings for silicone are used primarily during coating touch-up operations and reworking imperfect surfaces.

Ultimately, a conformal coating’s operational effectiveness depends on the selection of an application method that reflects the PCB’s functional environment and expected longevity.