Selective Coating for PCBs

Printed circuit boards (PCBs) often need protection from damaging external environments, for which, Rush PCB coats them with a thin layer of protective finish such as casting resin or similar during the process of conformal coating. Although sealing the entire circuit board with the conformal coating is the norm, Rush PCB can also cover only sections or individual components on the substrate. For this, we have developed several methods such as flip-chip under-fill, dam and fill, and glob top for the purpose.

A protective finish is necessary with all industries now using PCBs as the most frequent carrier and connecting component for all types of electronic components. These industries include medical, security, communications, household, airspace, transportation, and computers. The protective finish helps in permanently protecting the intricate electronics on a PCB against ingress of chemicals, dirt, moisture, and other damaging influences such as impact. Although potting does help to achieve most of the protection necessary, Rush PCB UK has developed methods depending on the specific electronic components (sensors and processors, etc.) on the board, or specific potting functions the application demands.

Generic Conformal Coating

Typically, a conformal coating is the application of potting compounds or special coatings on the PCB to protect the onboard components. Depending on the application, PCB assemblers can apply the coatings manually using brush painting or by spraying then on. With PCBs growing more sophisticated, it is necessary to apply conformal coatings with high precision and reproducibility. Therefore, Rush PCB frequently opts for robot-controlled or automated applications using suitable metering heads.

Flip-Chip Under-fill

Rush PCB has developed the flip-chip under-fill process specifically for mechanically stabilizing flip chips. After mounting, a thin gap is normally present between the flip-chip and the substrate, which can lead to stress and even deformation. Filling this gap with a low-viscosity material, an under-fill, reduces the stress on the chip. Once applied, capillary action forces drawing the under-fill within the narrow gap, until it fills completely with the casting resin.

Dam and Fill

Rush PCB has developed this selective process for enabling potting of individual areas on the PCB, and without affecting the surrounding components and surfaces. This process is also known commonly as frame and fill. The process involves making a dam or frame with a high viscosity material by dispensing it around the area of the board that needs the protection. This creates a cavity, which the assembler proceeds to fill with a liquid casting resin until they have completely covered the particular structure.

We use this dam and fill procedure for optical bonding as well. For this, we first dispense a dam on the substrate, thereby forming a gap between the display or touchscreen and the cover glass. We then fill the dam with an optically clear adhesive. We have improved the process for achieving better heat dissipation, greater stability, and superb display readability.

Glob Top

Rush PCB has an additional option for protecting selective and sensitive areas on the PCB. This is the glob top process, with the potting compound being the only difference between this and the dam and fill process. In the glob top process, we dispense a viscous casting resin on the semiconductor chip, thereby fully encapsulating it and its wire bonding contacts. The viscous casting resin we use for this process does not flow so easily, and it does not contaminate adjacent components or coat areas of the PCB that need to remain uncovered. These are the main considerations for choosing the type of casting resin and for determining its quantity as a potting compound.

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Dispensers for Protective Finish

Rush PCB generally uses two types of dispensers based on the application. These are gear pump dispensers and volumetric piston dispensers. While gear pump dispensers are ideally suitable for under-fill applications, volumetric pump dispensers are great for dam and fill and glob top applications. Gear pump dispensers have an optional swivel needle for providing precise and fast material dispensing even when the PCB has highly complex component geometries.

Volumetric piston dispensers are a cost-effective alternative, depending on the requirement and material. We precisely tailor the dispensing cylinders in these dispensers to the volume necessary, or to the specific mixing ratio, thereby ensuring maximum process reliability.

Thermal Management

As applying a conformal coating affects the natural cooling process on the PCB, it is necessary to consider effective thermal management aspects as well while potting. Rush PCB is increasingly selecting materials with thermal interface properties. For such applications of liquid thermal interface materials, the volumetric piston dispenser is most suitable.

Rush PCB also uses a compact and complete solution for dispensing liquid thermal interface materials. We pre-configure and parameterize the system for thermal management applications. This ensures short delivery times and fast start of production, while offering an attractive price-performance ratio.

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Potting SMD Boards

It is important to select potting materials with low coefficients of thermal expansion when potting circuit boards with SMD. Rush PCB recommends this to prevent risking shrinkage of material at low temperatures, as this can damage soldered connections. Another consideration should be the glass transition temperature of the potting material must be above the temperature at which the board operates.