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46 PCB007 MAGAZINE I DECEMBER 2022 each other, leading to PCB failure and reduced yields. More effective photoresist stripping is needed to avoid these manufacturing pitfalls. One way to remove these entrapped particles is to employ a phase-transfer catalyst (PTC). Quaternary ammonium salts (quats) are the compounds most commonly used as PTCs. ese compounds transport free hydrox- ide ions (OH - ) into the photoresist polymeric structure, network, or interphase boundary with the liquid medium of the solvated cata- lyst. e hydroxide ions then serve to chem- ically break the photoresist network apart. Concentrations are critical, as are the selection of the additives to break up the resist into small particles. us, optimizing the resist stripper formulation is essential to ensure a smaller par- ticle size. Regardless, overplated circuit traces will present a significant challenge in clean removal of entrapped resist. More on this later. Perhaps high spray pressures in the resist stripping chamber or the final rinse will improve one's chances of entrapped resist par- ticle removal. Other remedies (not necessarily recommended by the author) include a dou- ble pass through the stripper solution or hav- ing a separate offline tank with a resist strip- ping formulation capable of solvating small entrapped particles. Employing a plasma etch to successfully remove these particles is also an option. But due to the additional process times required to deploy these methods, there is a risk of attacking the tin etch resist, as pre- sented in Part 2 of this series on resist stripping. As for resist stripping effective- ness, effective stripping of a photore- sist implies the chemical destruction of the photoresist's polymeric network. rough this chemical dislodging of chemical structures, removal of small photoresist particulates becomes fea- sible. Most of today's photoresist strippers are water-based and contain multiple components, such as amines, solvents, and quaternary compounds. Many of these off-the-shelf products were developed prior to the need for these higher density designs. Most will deliver large particulates or lead to a sheeting of the resist. Because of this charac- teristic, they fail to strip photoresist from diffi- cult geometries and cause an overall decrease of yields in PCB production. We have found that it is possible to achieve such a dislodgment down to very small partic- ulates with a water-extended concentrate. is involves the following: A specific ratio of quat and solvent is nec- essary to achieve outstanding results. At the same time, the presence of amine in a spe- cific range will facilitate very good results as well. ereby, a PTC situation is achieved in the attacked polymeric structures of the pho- toresist, giving cause to an especially effective attack on the polymer network. is action allows for the exposed resist film to break up into finer particles. e spray action within the resist chamber and subsequent rinses help to further dislodge particles, effecting a clean removal. PCB007 Michael Carano is VP of quality at Averatek. To read past columns, click here. Figure 2: Example of overplated circuit features.