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72 PCB007 MAGAZINE I NOVEMBER 2018 desired circuit pattern atop the laminate. Bare (uncovered) copper (foil) remains in the ar- eas where the unexposed photoresist was re- moved. Thus, a later step is the not-so-trivial job of removing the exposed resist. This is far from simple and requires a thoughtful evalua- tion of process parameters and resist stripping formulations. Cleanliness: Removing All the Exposed Resist What exactly does that mean? For most, this means an efficient and rapid removal of the exposed resist from the panels. One may con- clude that cleanliness and resist stripper par- ticle size are related. This includes dislodg- ing and removing the exposed resist from tight spaces (more critical today for HDI designs) as well as any resist that may be trapped under an over-plated trace or pad. One performance attribute of resist stripping technology is the ability of the chemistry to break down the exposed resist in small par- ticles. As the resist stripper attacks the back- bone, the resist will release from the copper surface and break into particles of various siz- es. The particles must be small enough to be dislodged from recessed resist channels be- tween pattern-plated features. This is especial- ly important if the resist pattern is "over-plat- ed" (i.e., the plating has mushroomed over the resist and makes it even more difficult for the resist particle to escape). There is also the no- tion that the resist should have some degree of solubility in the stripper, so that a thin res- idue of resist left on the copper after the ini- tial breakup of the resist matrix into flakes will dissolve later in the stripping cycle to leave a clean surface. It is noteworthy that various resists will react differently when they come into contact with the particular resist stripping formulation. Size of Stripped Particles To enable a smooth operating resist stripping operation, remove the resist as small particles or chips. As one may surmise from Figure 1, different photoresist composition strips in var- ious particle sizes at different speeds in numer- ous resist stripping formulations will lead to varying results. Further, the initial size of the stripped particles reduces under the impact of sprays. However, not all resists have the same propensity to break up under a given spray im- pact. Fortunately, one can manipulate the par- ticle size (within limits) by selecting the strip- per chemistry, temperature, and concentration (Figure 2). Figure 1: Commercially available resist (L) versus a second (R). Under the same resist stripping conditions, the right resist strips much faster and more completely in the given time.

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