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Design007-May2020

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MAY 2020 I DESIGN007 MAGAZINE 67 tion with water, causing corrosion beneath the coating and compromising the adhesion of the coating or a combination of all of these factors. Variations in surface energy of the incom- ing bare boards will be exacerbated by the as- sembly process (reflow/wave thermal cycles). The surface energy of the board will determine the way that the liquid coating interacts with the substrate and the degree to which the sol- id coating will adhere to the board. The higher the value of incoming bare board surface en- ergy, the more repeatable and consistent the application of the liquid coating will be, and the greater degree of adhesion. The process will achieve a decent degree of repeatability and reproducibility (R&R), enabling a high de- gree of process control and consistent perfor- mance in the end-use environment. Converse- ly, the lower the incoming surface energy, the less repeatable and consistent the process will be, and the likelihood of field failures will be increased. The actual design of the board often deter- mines the success, or otherwise, of the coating process. Boards that are well designed for con- formal coating will have good separation be- tween areas that must and areas that must not be coated. Taller components will be grouped together away from low-profile components, and low-profile components will not be clus- tered together close to vias or SOIC, QFP, QFN, or BGA components, which will tend to pull material beneath them. This can lead not only to potential reliability issues with these com- ponents themselves but also leave low-profile SMT discretes short of coating or with bridg- ing bubbles. Conformal coating is a process. Ensuring a repeatable and consistent conformal coating result depends on the way the process is de- signed, just as much as the material selected. Manual processes, such as spray or brushing, will inherently have more variation in the end result due to the human operator, whereas ma- chines will consistently perform the same rou- tine over and over. However, machines cur- rently have no feedback loop, so if the material behaviour changes, the end result will change, whereas a human operator may compensate for this (either deliberately or possibly subcon- sciously). As previously discussed, the surface energy of incoming boards, board design, and pre-existing contamination can all affect the way the liquid material behaves. In addition, the temperature of the material will affect the viscosity of the material. The wider the change in ambient factory temperatures throughout the day (or even us- ing material that has been stored somewhere else, such as outside), the greater the effect of these changes in material properties and the wider the variation in material application. Controlling the temperature of the material or compensating by machine measurements and process control equipment can be important in ensuring a repeatable machine-based process. Airflow can affect the rate at which solvents evaporate and the material viscosity will build. Inconsistent airflow can lead to differential flow rates and unexpected issues, such as or- ange peel. If an oven curing process is used, the application of temperature can initially lead to a decrease in viscosity and increase the spread of the coating before the viscosity starts to build, possibly leading to coating areas in- tended to be left free from coating or a high- er degree of bubbling than expected. Similar- ly, the rate of change of temperature can lead to excessive bubbling, so controlling the ramp rate or providing a solvent "flash-off" period can be necessary to ensure a uniform and de- fect-free finish. 5. What is wicking, and how do I avoid it? Wicking is often used interchangeably with capillary flow, especially in a conformal coat- ing context, to describe the ability of liquids to flow into narrow gaps without the assistance of any external forces. The narrow gaps are most often unsealed connectors or the standoff be- tween component bodies and board substrate. In the case of unsealed connectors, switches, etc., the main issue is the material "wicking up" and coating the connector mating surfac- es, therefore reducing the contact or potential- ly insulating the mating surface and preventing either the form, fit, or function of the connec- tor or switch.

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