Effects of Adhesion Promotion on Sensors

Sensors measure specific aspects of data-driven technology.  Included are such performance properties 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.  They 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 — 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 conformal film reliability; 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 parylene – improve the bond between coating materials and substrates.  In this respect, evidence shows:

• application of A174 silane via manual-spray, soaking, or CVD,
• following substrate masking,
• significantly improves film adhesion for a wide range of substrate materials,
• including elastomer, glass, metal, paper and plastic, among many other substances.

For instance, γ-MPS (γ-methacryloxypropyl trimethoxysilane), a silane coupling agent, promotes adhesion of photolithographically patterned polymer hydrogels to conductometric sensors.  Situated on oxidized silicon wafers, sensors subjected to peel-tests of adhesion after immersion in solutions of buffered potassium chloride, with differing pH, as well temperature-changes analysis, showed varied results:

• Poorer adhesion resulted for hydrogels tested in a high-pH (9.0) medium, compared to those tested in lower-pH media (6.0/7.4).
• Elevated temperatures of 40°C exhibited comparable adhesion to room temperature (23°C).
• Hydrogels cycled between pH 9.0 and a dry environment adhered better than items cycled between pH 9.0 and 6.0.

Conformal film’s adhesion is affected by pH sensitivity.  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.

Other evidence exists.  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.  Conformal film adhesion promotion 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.  Adhesion promotion options to consider include:

• Plasma-surface treatment methods limit conformal film delamination problems attendant to implanted medical devices.
• Silicon substrates roughened with xenon difluoride gas demonstrate enhanced conformal coating adhesion.
• Another study of SAW sensors focused on adhesion of viscous SU-8 epoxy thin films. The Omnicoat adhesion promoter increased the adhesion of the SU-8 coating, lowering the value of center frequency shifts within the SAW device, a condition diminishing wave concentration and velocity, while enhancing film adhesion.

Adhesion failure between conformal film and the substrate is a primary cause of sensor malfunction.  As always, much depends upon the sensor’s specific application, its material structure and operating environment.  Establishing conformal film adherence solutions for sensors should be an important topic of research in years to come.

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