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PCB007-June2019

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66 PCB007 MAGAZINE I JUNE 2019 Article by Nikolaus Shubkegel When you compare direct imaging of solder mask with contact exposure of solder mask, the positive aspects and the advantages are clear. Without a doubt, direct imaging short- ens the throughput time and eliminates art- work production. It also eliminates the costly measurement of the panels and manufacturing of artwork with different scaling factors. In this article, I present seven of these de- fects. These defects occur rarely on contact ex- posing and only when the process is not well balanced. The occurrence of these defects can be explained by the fundamental differences between contact exposure and direct exposure. But first, let's review the direct imaging pro- cesses. The biggest advantage of direct imaging is the ability to scale and change the dimension of each individual exposure for the best fit. With direct imaging, it is possible to expose different cards on the same panel with differ- ent spreads. The scrap rate due to misregistra- tion tends toward zero. This is because, with artwork, you need to produce photo tools with different scaling factors, making nonlinear scaling impossible. Direct exposure solder mask, however, does have its own set of unique defects that occur. Thankfully, these defects are well known. Due to the increased use of direct imaging of solder resists, these defects occur more often. These defects depend on the solder mask chemical composition and the photoinitiator package (a compound that reacts to light, initiating or cat- alyzing a chemical reaction), so they can be a bit different from solder mask to solder mask. How the defects appear depends on the direct imaging unit, the wavelengths of the system, the general process parameters, the drying conditions, and the developing parameters. Even the pre-cleaning process can sometimes have an influence. These direct exposure defects occur early on during implementation of a direct exposure unit. Once the process is optimized, the de- fects no longer occur. In general, there are also possibilities to prevent these defects; in other words, they are manageable. Conventional Exposing: Direct Imaging Solder Mask

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