Printed circuit boards (PCBs) and related electrical assemblies benefit from the protection of conformal coatings. However, because the films are insulative when dry, they can disrupt operation of the assemblies’ electrical components, items like capacitors, connector contacts, diodes, operational amplifiers, resistors, or transistors. Conformal coating masking protects specified regions of PCBs and related assemblies from being conformally coated during film application. These components must remain uncoated to function as designed. Consisting of masking appliances constructed with appropriate materials, masking systems prevent migration of conformal coatings into designated keep-out areas. Masking processes enacted prior to coating application assure the conformal materials DO NOT invade designated keep-out areas.Read More
Parylene Coating Blog by Diamond-MT
Parylene deposition takes place at the molecular level. Applied at room temperature through CVD processing, the typical thickness of parylene conformal film is in the microns-range.Read More
Conformal coatings are surface treatments applied to a wide range of products and devices used for aerospace, automotive, biomedical, consumer, military and numerous other purposes. Their primary objective is providing a protective film that supports a selected device’s ease of use, operating function, and service life, through an exceptional variety of working environments. Liquid Teflon (PTFE) and parylene are two of the more widely used hydrophobic conformal coatings.Read More
Selection of the material used to coat a medical device is very influenced by the operational environment it will encounter when implanted in the body. Pertinent operational/performance factors typically include:Read More
Biocompatible parylene conformal coatings provide superior protection for medical stents. They represent an enabling technology consistently applied to medical devices of all types for 35 years, to diminish problems stemming from surface microporosity and consequent biofluid corrosion after implant. Providing a reliable barrier to chemicals and moisture, parylene’s static and dynamic coefficients of friction are comparable to those of Teflon.Read More
A metal alloy of nickel (Ni) and titanium (Ti), nitinol (NiTi) exhibits the properties of shape memory and superelasticity, which make it very useful for adaptation to conformal coatings. However, like parylene, nitinol is often difficult and expensive to produce; the extreme reactivity of the alloy’s titanium component requires exceptionally tight compositional control during combination and manufacture.Read More
It is possible to remove unwanted conformal coatings from PCBs in-shop. The process can often be accomplished by either the assembly’s original equipment manufacturer (OEM) or an end-user, but the capacity to do so doesn’t always exist. For these parties, conditions affecting the poor coating may:Read More
Sometimes problems with conformal coating are too complicated or difficult to repair. This can occur when bubbles develop in the coating during the application process; bubbles cause voids in the coating that defeat its protective, insulating purpose, suggesting the need for removal. Other situations that lead to inadequate coverage, and may favor coating removal, rather than repair include:
- Coating application that’s either too thick or thin for the project’s purposes.
- Component surface finishes that adapt poorly to the conformal material chosen for coverage.
- Disparities in surface tension/surface energy.
- Gravity issues that negatively impact application of liquid coating.
- Improper mixture of two-part materials.
- Inadequate fixturing or placement of assembly components in the coating area.
- Inadequate masking implementation.
- Incorrect interpretation of coating requirements.
- Residue on the coating surface during coating application.
- Poor, uneven coating application.
Overly thick film application or use of coating equipment/materials unsuited to the assignment are major causes of coating problems. In these cases, complete or partial removal of the conformal film from the PCB may be the best solution.
Thus, it is important before beginning any conformal coating assignment for designers and users to recognize the various types of conformal coatings and their interactions with the parts/materials they cover, to protect the products in their respective end-use environments, for the expected design-life of each component.
When removal is the best option for your coating problem, it is advisable to consult prevailing industry standards for appropriate process guidelines. For instance, IPC-7711/7721 delineates recommended procedures for conformal coating removal from, and replacement onto, PCBs. IPC-A-610 is a widely-held standard for electronic assemblies, offering users limited but valuable criteria for conformal coating applications. Designed and constructed with the intent of obtaining maximum confidence in the materials with minimum test redundancy, IPC-CC-830B qualifies the definition, use and conformance of all conformal coatings types for PCBs. In most cases, coating removal is required when assemblies don’t meet the requirements of IPC-CC-830, concerning overall quality conformance of each
The Logistics of Coating Removal
The logistics of coating removal are largely dependent on the type of coating material, its position on the PCB, and the board’s components. Proper identification of the coating material, and the methods used for its original application, are essential to correct determination of the removal method. Once these have been identified, determination of the appropriate removal method can be achieved.
In many cases, chemical strippers can dissolve conformal coatings from PCBs. Acrylic films are typically removed easily by soaking in a solution of stripping fluid, followed by mild mechanical abrasion if necessary. These two processes also work for coatings such as epoxy, silicone and urethane; however, since these substances have higher levels of chemical resistance than acrylic, complete coating removal is more difficult and time-consuming. In all cases, the stripping solution’s compatibility with the PCB’s components needs to be verified to minimize potential damage during the removal process.
Chemical removal does the least damage to PCBs; it is effective for the liquid coatings -- acrylic, epoxy, silicon and urethane. Chemical methods work less well for parylene films, since the substance is chemical inert. Abrasion, laser, mechanical, plasmatic and thermal removal methods are more successful for parylene films; they also work for liquid coatings in many cases.
Recently applied coating is more easily detached from substrate surfaces than older coatings, regardless of the material, unless the coating itself has begun to decay with age. Larger areas of the board respond best to complete submersion in a tank of stripping fluid. Gentle abrasion using a soft bristle brush will also eradicate coatings.
Please remember that the removal of conformal coating generally requires use of exceptionally caustic and potentially dangerous chemicals; the safety of process operators, the product being treated and the immediate environment can be jeopardized by use of inappropriate removal materials and methods. Consultation with a certified conformal coating specialist is highly recommended prior to removing conformal coating. To this end, the professionals at Diamond MT are eminently qualified, and would be glad to be of assistance.
To discover more about conformal coating rework and removal, download our whitepaper now:Read More
Defects to either the PCB assembly or its conformal coating can be sufficient to cause coating removal. Whether repair technologies address the circuit board’s components or the conformal film, subsequent post-repair coating (recoating) processes need to address:Read More
- Conformal coating applied incorrectly can cause PCB malfunction.
- Selecting the wrong coating material from among acrylic, epoxy, parylene, silicone or urethane can be a source of board failure, if it does not support the PCB’s operating environment.
Removing the coating may be necessary if these conditions prevail.Read More