This is the first in a short series on material viscosity, in which we will review what it is and how to measure it. We will start with the definition of viscosity. Viscosity is the internal friction of moving fluid and a measurement of the resistance of a fluid deformed by shear stress. It is measured in centiPoise. The lower the number, the less viscous the material, as can be seen by this comparison chart: starting with water at 1 cPs and ending with honey at anywhere between 2,000 – 10,000 cPs.
UV curable conformal coatings have attained widespread use in applications requiring fast through-put times, such as automotive, white goods, and industrial control manufacturing. In addition to speed of cure, UV curable conformal coatings provide the advantage of high physical protection and chemical resistance for electronic systems. Traditionally the UV curing of coatings is an energy intensive process. One that uses high-power mercury discharge lamps, which as a result of use produce considerable amounts of heat and ozone. The recent introduction of new, high-output LED UVA light units allows for the successful cure of conformal coatings by using between 365nm and 395nm wavelength of UVA light, effectively curing with minimal heat and no ozone generation.
The PCB industry continues to evolve with ever increasing demands for efficiency and manufacturing cost reductions. From the design phase onward, suppliers and contract manufacturers are being asked by their OEM overlords for increased performance at lower and lower costs. An obvious potential source for cost reduction is the use of alternative cheaper raw materials or their elimination where possible. At some point in the discussion, it is sometimes asked, “Why don’t we just eliminate the conformal coating?” or "Are conformal coatings necessary".
The electronics industry is exploding with the introduction of innumerable consumer devices designed with ever more complex functions and controls. These range from the relatively mundane such as controlling the temperature in your home to potentially life-saving including deploying airbags in your car or sensing hazardous road conditions. The demand for smarter and more complex electronics is becoming more important in everyday life.
Xylene and Toluene. Two words that can evoke concern and add to paperwork for environmental health and safety departments and manufacturing professionals. Aromatic solvents have been safely used for decades by employing basic safety equipment including gloves, masks, and proper workplace ventilation. Nevertheless, concerns for maximizing employee safety and minimizing effects on the environment have led electronics manufacturers to seek out alternatives. For over 20 years, HumiSeal® has supplied a wide-ranging group of aromatic-free products across all traditional chemistries to manufacturers for whom these concerns are critical.
Picking a Conformal Coating
You and your organization have spent weeks or months researching conformal coatings to meet the demanding specifications of your customers. These can include factors such as dielectric strength, thermal and mechanical shock properties, processing considerations, and so on. At some point, the critical consideration of price and cost must be added to the equation. The amalgamation of cost and performance could be considered a measure of the true value of any specific coating.
Conformal coating thickness is one of the most important characteristics to ensure long-term reliability of your electronics. A minimum coating thickness is essential to provide the required function of the conformal coating, but if a conformal coating application is too thick, it can actually have negative effects on your level of protection.
Which conformal coating thickness should be applied to reach optimal protection?
In a previous blog “How to Save Time and Money by Measuring the Right Amount of Viscosity,” viscosity was described as critical to the conformal coating process. This post will cover some of the technical details related to coating flow rate and the significant potential for savings of both time and money for PCB manufacturers.
UV curable conformal coating materials such as HumiSeal UV40 are frequently used in military, aerospace and automotive applications due to superior resistance to chemicals and mechanical stress. These properties make them difficult to remove when rework under the coating is required.
There are products available for chemical removal of conformal coatings, but many are restricted by government regulation and health and safety concerns.
An effective method for localised removal of conformal coatings, allowing the rework of the components, is the powder or micro-abrasion equipment.
The conformal coating process, like any other processes, has different variables, inputs, and outputs. In order to be able to control the results of your application (outputs), we need to fully understand how each variable affects the process.
The importance of the variables
To get the right amount of liquid, thickness, right coverage, no contamination on keep out areas, etc., you need to identify and understand the variables of the process.
The most important variables:
- Machine parameters (speed and Height)
- Curing method
In this blog, we will concentrate on reviewing the first variable: Viscosity