Issue link: https://iconnect007.uberflip.com/i/1269815
JUNE 2020 I PCB007 MAGAZINE 73 • Has the breakpoint of the resist changed, meaning the resist appears to be breaking or is being cleaned off later in the cham- ber? Perhaps additional time is required. Additional time in the developer is needed to improve the breakpoint (Note: A late breakpoint may cause incomplete develop- ment in areas with poor mass transfer or poor spray impact, typically at the bottom of narrow resist channels. Excess copper will be seen here especially between narrowly spaced traces). • Low developer concentration: Check the pH. Effective development is seen between a pH of 10.4–10.8 for most fully aqueous resists. Regardless, if the developing operation is controlled with an automated feed and bleed system tied to pH, this should not be an issue. However, it does not hurt to spot check the pH and concentrations of the carbonate. Finally, be cognizant of resist locking onto the surface. This would make developing more difficult. More on this when lamination and imaging are presented in a future column. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. The last bullet point requires a deeper dive, depending on what final etching process is in use (cupric chloride or alkaline ammoniacal). The specific features of these two processes will be presented in a future column. After reviewing and making any adjustments in the etching process, one should revert to the developing, surface prep, and imaging process- es. Clearly, there are spots of copper remaining on the inner layer. One is then able to discern extraneous copper near the edge of the trac- es. It would be prudent to investigate lam-pre- cleaning, resist lamination, hold times, expo- sure, and development. For now, the focus will be on development. Underdevelopment of Resist Underdevelopment of resist will leave resid- ual resist on the surface of the copper where it acts as an etch resist. Certainly, resist film thickness will play a role. It is reasonable to conclude that the remaining residues are easily able to inhibit the etching reaction. There are several possible conditions that are the poten- tial root cause of the unetched copper: • Operating temperature of the developer is too low: verify the temperature gauge reading periodically with a hand thermometer. • Time in the developer is too short: while this is rare in conveyorized modules, it is possible. Use a stopwatch or timer to gauge time in the developer.