Pot life and Curing of Coating
Pot Life
The chemical composition is also very important as often materials have finite work times, commonly referenced as pot life; the hardware must be cleaned to avoid curing inside the equipment. The presence of hazardous solvents is common, so consideration needs to be given regarding process controls and the environmental impact of these materials. This specifically applies to the health of the employees, potential for cross contamination to other processes, and ventilation of the vapors.
The cure mechanism is determined by the materials as well, be they acrylics, urethanes, silicones, or epoxies. The time to cure is related to the material itself, and automated equipment selection depends on the cure mechanisms of the specific material. To minimize the time to cure and the associated work in process as well as line length, designers need to look at the specific coatings and not the equipment.
Considering the need for conformal coating, applying the required process tolerances for the material, and evaluating conformal coating equipment and characterization at the design phase greatly aid in manufacturability. It makes the manufacturing process less difficult, less time-consuming, and less costly, and avoids having to use specialized equipment, applying additional conformal coating, or using masks after the fact due to unplanned-for parts or areas that have zero tolerances.
Curing the Coating
The equipment used to cure the coating material also has input variables depending on the cure mechanism. For thermal cure materials these are typically the temperature of the oven, belt speed, temperature ramps, and for some materials, the humidity inside the oven. For UV cure materials variables are the height of the UV lamp and the speed the board passes through the oven. The output of these variable settings, in conjunction with the material thickness, provides the throughput of the cured product as well as defects, such as incomplete cure or over cure defects.
lso has input variables depending on the cure mechanism. For thermal cure materials these are typically the temperature of the oven, belt speed, temperature ramps, and for some materials, the humidity inside the oven. For UV cure materials variables are the height of the UV lamp and the speed the board passes through the oven. The output of these variable settings, in conjunction with the material thickness, provides the throughput of the cured product as well as defects, such as incomplete cure or over cure defects.
Through first characterizing the equipment capabilities, the manufacturer is able to best develop a high yield, maximum throughput process for the end product. There are variables associated with coating a specific board due to board topography, keep-out areas, surface wetting, cleanliness, moisture retention, and component placement. Decoupling the equipment characterization from the final product enables higher reliability coating processes in a shorter and less costly development timeframe. The timeframe is shorter because it is a simpler process without confounding variables, and less costly because of engineering time-savings as well as reduced potential scrap product generated in process development. Each coating material has slightly different rheological characteristics.
Understanding how those interact with the equipment input variables subsequently provides the best manufacturing processes. The majority of manufacturers who have successful coating operations manufacture more than one product. With multiple products, there is a need to develop a process that works with many different selectivity requirements. By initially developing a coating process that is end-product independent, it allows companies to introduce new products more quickly because each product is not a unique project.
Finally, the conformal coating process is a very dynamic process because there are many tolerances that are inherent in it. Some of these are associated with the acceptable limits of material viscosity variations, substrate surface energy affecting wetability, surface cleanliness, fixture tolerances, and the coating equipment’s own tolerances on volumetric dispensing. All of these processes can be characterized for high yield; however, it is advantageous to have a basic characterization process that can be referenced so as to provide a standard to work from. The equipment can be selected to have closed-loop heaters to control the viscosity, software-controlled pressure regulators to dynamically adjust the spray patterns based on closed-loop monitoring, fiducial recognition for increased placement accuracy, bar code scanners for automated program selection, and many additional options as defined by the process requirements.
Characterizing the equipment before introducing the final product also helps to ensure that the initial process is
defined in the center of the process window, allowing for the widest range of variation, which is much easier to do when it is defined outside of a specific product. It also decouples trouble-shooting issues from the incoming product (upstream issues) or issues with the coating process itself.
This understanding minimizes the need for process support and increases the number of good parts being produced per shift. Automated selective coating equipment is very reliable and designed to run in 24/7 environments; the challenges are with the process itself. It’s important to consider the need for and type of coating at the design phase, and properly characterize the equipment. This will help ensure that the product can be manufactured utilizing automated equipment and at the lowest cost.
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