About this document
Bubbles cling to the walls of blind vias when they are filled with solution and when the solution is exchanged by nanoflushing. This Technical Note explains the problem through the hydrophilicity of PDA-coated surfaces and the way bubbles detach from them.
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Summary
Published work reports that a PDA coating lowers the water contact angle from 66° to 23° and suppresses bubble build-up under ultrasonic agitation and long immersion. On that basis the document sets out a continuous liquid film, low pinning force and an open exit path as conditions for bubble-free via walls, and presents how they apply to nanoflushing.
Conclusion
In blind vias, bubbles cause trouble both when the via is filled and when its solution is exchanged. A bubble stuck to the wall blocks solution contact at that spot, and if a grown bubble cannot escape, the solution inside is not renewed. This document assembles the theoretical basis for carrying over a published observation, that a PDA coating changes surface wetting so bubbles cannot stay on the wall, to vias. The cited study, Ye et al. (2019), reports that PDA coating on HDPE lowers the in-air water droplet contact angle from 66° to 23°, and that under ultrasonic agitation at 60 °C bubbles accumulated on the uncoated surface while none were seen on the observed area of the coated one. When abrasion reduced hydrophilicity, bubbles reappeared near a contact angle of about 45°, and suppression held through 24-hour immersion, −40 to 70 °C thermal cycling and a cumulative 500 hours at 60 °C and 80 °C. The document notes that 45° is an empirical threshold for that experimental system, not a universal critical angle for via drainage, and that the long-term tests used diesel exhaust fluid, so lifetime in an actual plating bath is a separate question. Applied to vias, three conditions keep bubbles from sticking: water occupies the wall first so a continuous liquid film prevents direct gas–solid contact; low contact-angle hysteresis and few defects, contaminants or exposed areas keep contact-line pinning weak; and an upward-facing entrance with liquid flowing in along the wall keeps an exit path open. As nanobubbles grow in the liquid, buoyancy rises with the cube of the radius while the PDA liquid film reduces wall pinning. Once the driving force exceeds wall pinning plus entrance resistance, the bubble leaves through the top opening and fresh solution takes its place. The document presents this sequence of bubble supply, growth, exit and solution inflow as the application concept for Cools nanoflushing.
References
- 1. Ye, Y., Klimchuk, S., Shang, M., McDonald, K., & Niu, J. (2019). Acoustic Bubble Suppression by Constructing a Hydrophilic Coating on HDPE Surface. ACS Applied Materials & Interfaces, 11(18), 16944–16950. https://doi.org/10.1021/acsami.9b04038
Topics
- Plating
- Glass substrates
