Effects of Adhesion Promotion on Sensors

Sensors measure specific aspects of data-driven technology, including performance properties such as acceleration, fluidity, humidity, temperature, position, pressure or vibration. Sensors collect data and respond with feedback for a multitude of electronic devices utilizing printed circuit boards (PCBs) and related sensitive electronics. Sensors have been successfully adapted for use across a wide range of applications, including aerospace/military, appliance, automotive, communications, consumer, industrial, medical and transportation uses.

PCBs and the larger devices they power often need to function in harsh operating environments. Conformal coatings, including liquid acrylic, epoxy, silicone and urethane resins and chemical vapor-deposited (CVD) Parylene, provide PCBs and similar electronics excellent barrier, dielectric and insulative protection through most performance conditions, sustaining their expected utility. Substrate adhesion is necessary to ensure conformal film reliability; in other words, coatings do not work if they delaminate or otherwise disengage from the components they are applied to protect.

Cases emerge where typical application methods cannot guarantee appropriate film adhesion. Surface adhesion promoters – carbonyl or hydroxyl for liquid coatings, silane for Parylenes – improve the bond between coating materials and substrates. In this respect, evidence shows that the application of A-174 silane via manual-spray, soaking or CVD, following substrate masking, significantly improves film adhesion for a wide range of substrate materials. Examples of substrate materials include elastomer, glass, metal, paper and plastic, among many other substances.

Adhesion of conformal films is also affected by pH sensitivity, and coating failure frequently causes sensor dysfunction. Evidence suggests siloxane bonds between the polymer and the substrate reformed, offering the possibility of sensor regeneration through vacuum oven curing. Additionally, higher-level bonded-energy keeps siloxane bonds from breaking at temperatures of 200°C, while providing additional chemical stability and weatherability. Maintenance of the siloxane bond can promote ongoing film adhesion and prevent sensor failure.

Microelectromechanical (MEM) surface acoustic wave (SAW) sensors use modulations of surface acoustic waves to sense multiple physical phenomena within a device’s operating environment. These include force, humidity, mass, pressure, strain and temperature. Water barrier studies of SAW sensors show that introduction of promotion materials improves film adhesion through formation of a chemical- or acid-base type bond for conformal films frequently adapted for MEMS uses.

Solid adhesion of conformal coatings lessens the impact of sensor failure mechanisms. Silane adhesion promoters form unique chemical bond with substrate surface, sufficient to improve conformal coating’s mechanical adhesion. Yet, silane is not effective for all materials or sensor uses; additional adhesion promotion options are also available for consideration, including proprietary formulations that are available from coating service providers. As always, much depends upon the sensor’s specific application, its material structure and operating environment.

References
EL Gowini, Mohamed & Moussa, Walied. (2015). Evaluating the adhesion of SU-8 thin films using an AlN/Si surface acoustic wave sensor. Journal of Micromechanics and Microengineering. 25. 10.1088/0960-1317/25/3/035031.