How to Test Parylene Adhesion

The Need for Adhesion Testing

Applied mechanical processes stimulate the binding force between surface molecules required for Parylene adhesion to substrates, which is essential to both good Parylene performance and assembly/component functionality. The emergence of conditions characterized by non-adherence and delamination squander the coating’s typically exceptional protection against chemical attack, corrosion and moisture, as well as its superior dielectric insulation.

Parylene adheres poorly to low surface energy substrates, under any circumstances, leading to delamination, separating from the surface its meant to protect. A variety of adhesion promotion materials and methods can significantly improve interface adhesion between the substrate and Parylene.

Suitable Methods of Parylene Adhesion Force Testing

Improving Parylene adhesion requires a close review of all of the current processes affecting its bond with the selected substrate. As with all aspects of the adhesion process, care needs to be taken in this regard. At present, there is no single widely-accepted test for conformal coating adhesion. Determination of adhesion priorities needs to be made on a case-by-case basis. It is very important to match the test methodology to the type of Parylene being used, and the expected conditions under which the component will function when operative.

ASTM D3359 – Tape Testing for Adhesion

The tape test consists of two variations:

• X-Cut Tape Test: A technician applies a utility knife and straightedge to mark the Parylene coating with two intersecting, small-scale cuts situated at 30°-45° to the surface. The cuts need to penetrate to the substrate surface, forming an “X” shape. Tape is placed at the X’s vertex (where the lines intersect) and then is briskly pulled from the cut surface. The center of the “X” is then inspected to see if the tape caused any of the Parylene to pull away from the component’s surface, indicating poor adhesion.  
• Cross Hatch Tape Test: Typically applied in a clinical setting, cross-hatched parallel lines are cut into the coating, using either a customized cross-hatch cutter or a utility knife/straightedge. The result is a series of slightly raised squares across the coating surface where the cross-hatched lines intersect. The tape affixed to the surface is rapidly removed. If any of the squares pull away from the surface, the coating adhesion is faulty, requiring repair.

ASTM D3359 is valuable because it offers a high standard for adhesion testing; passage verifies exceptional levels of conformal performance for Parylene bonding to the selected substrate. Tape testing is very similar to the “Knife Test” described below.

Knife, Pull-off and Scrape Tests

Additional testing methods of value for verifying Parylene adhesion include:

• Knife Tests: Very similar to the X-cut and Cross-hatch methods, a utility knife and straightedge mark the Parylene coating with two intersecting cuts that penetrate to the substrate surface, forming an “X.” Rather than applying tape to attempt lifting the Parylene film from the substrate, the knife’s tip is inserted into the X’s vertex in an effort to separate the Parylene from the substrate. Described in ASTM D6677, the knife test accurately determines the quality of Parylene adhesion, as a variable of two factors – the level of exertion required to remove the Parylene from the substrate, if removal does in fact occur, and the size of the removed coating. Any removal indicates coating issues.
• Pull-off Tests: In comparison to the shear stress forces applied by other adhesion testing methods, pull-off testing maximizes tensile stress. A loading-fixture known alternatively as either a “dolly” or “stub” is bonded to the Parylene film. Then, a portable pull-off adhesion device applies increasing pressure to the surface until the dolly either is detached from the surface or withstands a predetermined level of force, measured as tensile strength in pounds per square inch (psi) or mega Pascals (MPa). Failure of the Parylene film results in a fractured surface at the weakest plane along the coating.  Pull-off adhesion tester devices of different sizes are adaptable for use with specified substrate substance materials. Pull-off testing application and performance standards are available in ASTM D4541 and ISO 4624.
• Scrape Tests: As described in ASTM D2197, scrape tests are recommended for assessing smooth, flat panel surfaces only. The coated panels are placed underneath a balanced-beam scrape-adhesion tester, a stylus or loop that exerts increasing quantities of abrasive pressure until the coating is worn through either in selected areas or across the entire substrate surface. Adhesion is determined by the amount of pressure required to penetrate the coating, and how long it takes to do so. Ideally, removal is minimal and time-consuming, suggesting appropriate adhesion.

Conclusion

Inadequate adherence of Parylene conformal coatings significantly lowers the operational life of components and parts, causing them to malfunction during use. Systematic cleansing of substrate surfaces stimulates better interface adhesion. In comparison to using Parylene alone, adhesion promotion methods used on PCBs and related assemblies improves the overall adhesion and performance stability of active, wired devices, particularly in cases where superior adhesion is essential to persistent functionality.