Quality Killers in PCB Manufacturing – Photoresist Semi-Polymer

In the PCB manufacturing process, many tasks require the use of “photoresist materials,” including inner layer wet film, inner layer dry film, outer layer dry film, and green solder mask. It is a chemical material that, after exposure to light, produces different solubility in the developing solution between exposed and unexposed areas, thus generating patterns. The patterns formed by the exposure machine UV light are the DES etching layer or the surface green solder mask of the PCB product, while the photoresist that has not been exposed (the black area of the film) retains the characteristics of the photoresist monomer. The “unreacted photoresist monomer” will react as long as it is given some strange “energy.” The UV light from the exposure machine is energy, any white light outside the yellow light room is energy, the heat from the liquid temperature in the tank is also energy, and high acid or high alkaline substances can also catalyze reactions. It is incredibly easy to cause the unexposed photoresist monomer to undergo polymerization or semi-polymerization reactions, generating viscous, water-insoluble scum pollutants. The incomplete polymerization of the photoresist monomer is a terrifying quality killer in PCBs, causing anti-etching short circuits from dry film re-adhesion during DES development, copper dross, open circuits, and gaps from re-adhesion during outer layer development, as well as hole breakage in secondary copper dry film types and the infamous gold plating failures on gold fingers… There are numerous serious quality issues caused by dry film contamination. The industry’s standard treatment for the dissolved dry film monomer in the developing main tank is to quantitatively add developing liquid overflow replacements during production and to replace the entire tank with fresh developing solution during daily shift changes, maintaining a certain load of dry film monomer in the developing tank. When the PCB board exits the developing main tank, it is rinsed with water to wash away the residual dry film monomer. The monthly maintenance plan also involves cleaning the tank and tank walls to remove the accumulated brownish-black viscous sludge (sludge) using alkaline and acidic cleaning agents. When you finish cleaning the tank on the first day of the month and prepare the new developing tank, the first production board of the night shift begins production, and the photoresist monomer dissolves into the developing tank, starting a cyclical process of accumulating contaminants again over the month. The photoresist monomer undergoes long-term cycling in the developing equipment, where light and heat catalyze it into semi-polymerized, insoluble gel-like molecules, which is a significant factor affecting PCB quality and even reliability. The accumulation of contaminants, tank replacement, and cleaning operations create a persistent presence of this re-adhesion factor in the PCB production process, which is why the walls of the developing tank and the first two roller sections of the washing tank always appear contaminated and deep blue. Is there a better solution to the phenomenon of contamination and re-adhesion in continuous PCB products? The answer is yes. After years of research and development by material suppliers, the industry has special chemical additives that can overcome this problem, known as “photoresist neutralizers” or “photoresist inhibitors,” which can be added in the washing tank or even in the main developing tank, effectively removing photoresist monomers and keeping the equipment tank cleaner with increased production use.

Personal opinion, for reference only , November 28, 2024 12:13 PM , ,