Conformal Coating Selection: Conventional vs. Two-part

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With electronics increasingly playing a part in daily life, the applications for electronic systems continue to develop at a rapid pace. Miniaturisation is ever-increasing with systems expected to perform more operations more quickly and more reliably whilst becoming smaller and lighter with every design iteration. As assemblies become more densely populated, and housing/casing designs become more permeable to save weight, the use of conformal coatings is essential to protect the assembly from its operating environment and ensure acceptable reliability for the application intended, especially when operating in hostile environments.

Kinner-Fig1-Aug2019.jpgFigure 1: Tin-whisker formation.

Conventionally, conformal coatings are thin polymeric materials intended to protect assemblies from harsh conditions experienced in their operating environment, including moisture, condensation, salt spray, corrosive gases, conductive particles, etc. Coatings can also provide further dielectric insulation, allowing components to be placed in closer proximity than would be possible in the air, and mitigation against the growth of tin whiskers (Figure 1).

To fully protect against corrosion and provide an effective barrier to mitigate against tin whiskers, which can potentially cause failure in electronics by shorting the circuit, it is crucial that the coating material covers all metal surfaces at an appropriate thickness to provide these barrier properties. To understand coating coverage with regards to potential tin-whisker mitigation, the IPC TR-57 “Conformal Coatings: The State of the Industry” white paper investigates hundreds of boards coated with a variety of common coating chemistries and application methods, which were meticulously cross-sectioned for thickness. The results were surprising with numerous areas of extremely low coverage being found along with wide ranges of thickness on different component types. A typical result for a gull-wing device, such as a QFP lead, is shown in Figure 2.

Kinner-Fig2-Aug2019.jpgFigure 2: Typical cross-section showing difficulty achieving coverage (blue layer) with traditional coating materials.

The 2K series of coatings is significantly different in its protective capabilities due to its capacity for greater coating thickness and enhanced edge coverage. 2K coatings are VOCfree, fast-curing, high-performance, two-part conformal coatings that provide a solvent-free alternative to both UV and silicone materials, require less capital investment than UV materials, and improve on the performance of most silicones in harsh environments. The majority of coatings in the 2K range are hydrophobic, giving excellent protection against water immersion, salt mist, and humidity, making them ideally suited for automotive electronics.

Why Was the 2K Range Developed?

The main reasons why Electrolube decided to develop a new breed of two-part (2K) conformal coating systems simply results from the increasing expectations for conformal coatings to perform effectively in higher temperature extremes and withstand more thermal shock cycles and to address the coverage concerns addressed by the white paper. Many conventional materials are becoming obsolete because they are failing to meet new maximum operating temperatures or provide insufficient coverage required for successful condensation tests, which are needed by global automotive manufacturers. Electrolube also wanted to dispel any myth that applying a coating material more thickly achieves better coverage. Overapplying a coating only increases the amount of stress on the coating and the assembly, which can lead to disaster during thermal shock testing, especially with UV-curable materials, which crack and then provide no protection at all.

Addressing these challenges was the driving force behind the development of the 2K range. Electrolube’s 2K coatings have been designed to withstand high-temperature extremes and have the application ease of a coating with the performance properties of an encapsulation resin. 2K coatings can be applied more thickly without placing unnecessary stress on the assembly or the coating, which eliminates any concerns about cracking. 2K coatings also accomplish the coverage requirements for successful resistance against condensation, tin-whisker mitigation, and reinforcing dielectric functions. The 2K range is solvent-free, low-odour, and low-hazard, which enables users to reduce their VOC emissions and improve the safety of their process.

To read the full article, which appeared in the August 2019 issue of SMT007 Magazine, click here.



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