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 parylene’s typically exceptional substrate protection against chemical attack, corrosion and moisture, as well as its superior dielectric insulation (er = 3.1).
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 preparatory substances -- including hexane, propylene carbonate (PC.), A-174 silane, tetrafluoromethane (CF4) plasmarface, and toluene -- can significantly improve interface adhesion between the substrate and parylene, when applied to the substrate surface prior to CVD.
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
- 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 angles of 30-45 degrees 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 rather than an X 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. Here again, tape affixed to the surface is rapidly removed. If any of the squares pull away from the surface, 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 – (1) the level of exertion required to remove the parylene from the substrate, if removal does in fact occur, and (2) the size of the removed coating. Any removal indicates coating problems; greater quantities of removed film suggest a poor bond between parylene and the surface.
- 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 (1) is detached from the surface or (2) withstands a predetermined level of force, measured as tensile strength as (1) pounds per square inch (psi) or (2) 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: (1) the amount of pressure required to penetrate the coating, and (2) how long it takes to do so. Ideally, removal is minimal and time-consuming, suggesting appropriate adhesion.
Whatever method is used, it is a mistake to opt for too-rigorous an adhesion standard for parylene films. Parylene has repeatedly demonstrated its capacity to provide superior conformal coating under exceptionally ruggedized conditions. While its adhesion capacities should always be subjected to appropriate testing, there is no value in creating test standards so demanding that a new collection of coating materials must be invented to meet them.
Inadequate adherence of parylene conformal coatings significantly lowers the operational life of components and parts, causing them to malfunction during use. This can be a problem because the chemical structures of the parylenes may actually reduce good interface adhesion, sometimes significantly. Systematic cleansing of substrate surfaces stimulates better interface adhesion.
Heat-treated parylene (140°C, 3 hours) can also respond beneficially, engendering dependable adhesion and film insulation, minimizing delamination and other coating erosion. Heat testing can determine whether a high, consistent adhesion strength is the outcome, as it generally is with heated parylene. Appropriate heat-treatment can generate parylene adhesion-improvement as much 800% greater for conditions of prolonged operational duration.
In comparison to using parylene alone, bilayer encapsulation of 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.
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