Repairing Parylene Delamination

Applied as a conformal coating through a unique chemical vapor deposition (CVD) process, Parylene conformal coatings provide an ultra-thin, pinhole-free barrier protection for a wide range of components.

With appropriate pre-treatment, CVD penetrates deep within substrate surfaces, rather than merely attaching to substrate exteriors. Ultra-thin protection negates operational dysfunction caused by excessive coating mass or viscosity, conditions that can impede assemblies’ performance. Parylene is chemically and biologically inert and stable, generating excellent barrier material to abrasive chemicals, bodily fluids, solvents, liquid water and water vapor. It is also readily adaptable for MEMs and nanotechnology.

Excellent but Not Flawless

PCBs and similar electronics can feature angular surfaces, crevices, exposed internal surfaces, pointed and sharp edges or similar specialized assembly configurations. Due to Parylene’s vapor deposition process, all areas and crevices will receive protection when coated.

Durable, heat and liquid resistant, the Parylene coatings are often difficult to rework, and occasionally require repair. Delamination, a lack of adhesion occurring when a conformal coating lifts away from the substrate, exposing the assembly’s workings, is a major Parylene failure mechanism, necessitating repair.

Delamination and Parylene   

Delamination results from faulty adhesion between the coating and substrate, producing a torn, unattached and non-conformal coating. Even partial lifting of the Parylene coating is sufficient to qualify as delamination. Since appropriate adhesion of conformal coatings is critical to their success, exposing even some fragment of the component negates their purpose and utility.

Delamination is frequently the result of:

• Surface contaminants on the substrate as Parylene is applied
• Poor Parylene and substrate compatibility
• Excess moisture on the substrate surface
• Overly thick coating
• Inadequate masking or de-masking techniques

Lifting the Parylene film from the substrate, delamination leaves an unprotected area that may be subject to further coating peel-back and additional assembly exposure. Care must be taken both prior to and during CVD application procedures to assure subsequent delamination episodes do not occur.

Repairing Delaminated Parylene  

Chemically resistant, Parylene seldom responds to solvent removal. Parylene films are difficult to:

• Mechanically compromise by chipping and related methods
• Detach by thermal treatment due to melting and burning temperatures of 350°C+, which can exceed those of assembly components, rendering them inoperable

Specialized techniques, such as micro-blast abrasion, laser ablation or plasma etching, successfully remove delaminated Parylene coating. Another effective removal method is mechanical incision, typically a costly, labor-intensive and time-consuming method. Manually enacted, the treatment surface is cut by incisive tools, further lifting already separated film from substrate. The damaged Parylene is then removed from the assembly with tweezers or similar implement. Because workers’ unprotected skin can contact the substrate surface during removal procedures, leaving bodily oils, sweat or residue on the substrate, it is suggested operators wear protective gloves. Otherwise, additional cleansing processes will be required, complicating the repair process while adding to its expense.

In most instances, localized, spot removal is all that’s necessary. Stripping the entire board is seldom required. Use of a soldering iron to carefully burn through the Parylene is recommended in these cases, followed by touch-up with urethane or other liquid coating to tack down the Parylene.

Regardless of the repair method, the goal is successfully stripping damaged Parylene from the affected surface and restoring it, so assembly function can be maintained. Reworked and removed Parylene can frequently be recoated to original specifications.

Prevention of Delamination

Prevention is the best cure. To lower the risks of delamination and the need for subsequent repair, ensure the following prior to CVD:

• Surface cleanliness removes all sources of substrate contamination, such as dirt, bodily substances, mold release agents and process residue.
• Compatibility is confirmed between Parylene type and substrate material. Positive interaction of surface energies between coating and substrate materials supports reliable adhesion, minimizing delamination. Applying a primer material that bonds well with the substrate can help.
• Reduce coating thickness. Thicker Parylene films tend to become brittle, leading to instability and subsequent delamination.
• Use reliable masking methods to assure coverage of only designated PCB areas.

Post-production and inspection procedures must target the possibility of delamination, in order to limit its incidence.