Conformal coatings are designed to protect printed circuit boards (PCBs), assuring they work under all operational circumstances. However, cases emerge where boards fail to function despite conformal coating protection. Such non-performance can be a consequence of:
- problematic conditions already existing on the PCB prior to coating,
- using the wrong coating materials for the particular project, or
- inappropriate coating methods.
PCB manufacture is a complex process, open to numerous sources of board failure. In some cases, failure mechanisms develop slowly enough their origins can be detected, and compensated for, maintaining the unit’s function. More rapid and unexpected PCB failures can lead to immediate PCB failure, often negating the prospect of repair.
This is unfortunate. Due to their properties, conformal coatings should protect your board unless the wrong coating material is used, or it is applied inappropriately. However, this PCB security is of little value if:
- the coating is applied to unclean components and/or surfaces, or
- the unit itself is poorly constructed or installed.
Inspection Before Coating
To safeguard against these potential threats to optimal performance, board inspection prior to coating is strongly recommended. Inspection is essential because it verifies the PCB meets workmanship standards and customer specifications. Testing during product certification processes helps identify conditions that generate board failure after conformal coating is applied. These include:
- Component failure, the inability of one or more PCB components to function independently or in union with other parts of the board assembly. Included are elements like diodes, driver chips, IGBT modules, microprocessors, rectifiers, resistors, transformers, and transistors. Inadequate board thickness can cause the PCB to bend or break.
- Contaminants like dirt, oil, or even the presence of insects under conformal coatings can gradually degrade PCBs. These debris corrode assemblies, reducing their overall lifespan and destabilizing performance prior to breakdown. Thorough cleaning of all substrates is essential prior to coating application, to assure contaminants do not nullify PCB function.
- Divergent thermal expansion rates among such basic PCB materials as component pins, PCB trace/trace coating and solder represent a further source of assembly failure despite coating. All materials within the PCB experience temperature changes (heating and cooling) during operation. Each PCB component has a specified range of heat it can absorb, a quantity dependent on its size and structure. Overheating can generate considerable mechanical stress which may disrupt physical solder connections, damage the effected component, or delaminate the PCB trace. If insufficient space exists around the component, in relation to others in the assembly, high operating temperatures can cause component overheating and burning, underneath coatings.
- Solder improperly heated during board construction can generate cold solder joints and bad surface-mount connections that may burn components and cause power issues. Left over soldering-flux may also produces PCB-corrosion. Electromigration of the elements in the solder occurs when the wrong type of solder is used within a PCB. Brittle, intermetallic layers form, generating broken solder joints that often are undetected, even with inspection.
- Loose connections may lead to poor connectivity between board layers causing inefficient performance.
- Tin whiskers’ growth from lead-free solder joints can bridge contacts or break-off, causing short circuits. Conformal coatings can combat the formation of tin whiskers during operation, but are less effective when whiskers exist within the assembly prior to coating application.
- Traces are the conductive pathways, tracks or signals etched from copper/silver coated sheets and laminated onto the non-conductive substrate. If accidentally placed too near each other, they can short circuit during operation. Use of inappropriate acid core solder is one source of trace shorts, and an example of a pre-existing PCB condition that good inspection can uncover and correct. Conformal coating cannot protect a board from a problem already extant when coating is applied.
Coating CrisesIt is also possible that none of these pre-coating issues are a problem, and your board still malfunctions, despite being treated with a conformal film. In some cases, it may be that the coating was inappropriately applied, leaving pinholes or gaps in the covering; larger-scale delamination can develop, rendering the conformal film ineffective. Not all coating materials – acrylic, epoxy, parylene, silicone or urethane – work equally well for all projects or purposes. If the wrong coating material was selected, PCB breakdown can occur even though the assembly is conformally coated. Under these circumstances, conformal protection alone is unequal to the task of maintaining PCB functionality.
PCB failures occurring despite the ostensible protection of conformal coating can compel a multitude of testing, probing and preparation processes, enacted to determine both:· the precise failure mechanisms effecting board performance, and· appropriate repair/rework procedures. Poor manufacture or stresses to the assembly emerging during operation that may cause failure can be minimized by:·
- thorough board inspection prior to coating, and·
- ensuring the selected coating methods/materials are germane to project specifications.
The objective in all cases is to maintain board function and avoid coating repair, removal or reapplication.
To learn more about conformal coating removal and rework, download our whitepaper: