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68 PCB007 MAGAZINE I JUNE 2019 Four Fundamental Differences Between Direct Exposure and Contact Exposure 1. Oxygen Inhibition of Surface Crosslinking First, a fundamental difference is the oxygen inhibition of surface crosslinking. Contact ex- posure occurs under vacuum. A side effect of this is that oxygen adsorbed on the solder mask surface is desorbed. This low oxygen concen- tration does not interfere with the crosslinking of the solder mask. With direct imaging, rath- er, the exposure happens in the air and under normal pressure. The oxygen inhibition effect strongly depends on the photoinitiator pack- age. Some solder resists are very sensitive to oxygen inhibition while other systems are in- sensitive to the presence of oxygen. Exposure transforms certain photoinitiators from the solder resist into free radicals. These free radicals start the crosslinking of the sol- der resist. A portion of the free radicals stabi- lizes by a reaction with atmospheric oxygen. These stabilized radicals are no longer avail- able for crosslinking. As a result, the crosslink- ing on the solder mask surface is disturbed by the loss. When crosslinking is incomplete, the solder mask surface will be sticky, dull, and porous. Some photoinitiators start the polym- erization via a cationic photopolymerization. In this case, oxygen inhibition generally does not occur, but there are exceptions. 2. Narrow-band Light Frequencies for Direct Imaging The second problem with direct imaging hap- pens because light in the 200–350 nm range is missing. Contact exposure relies on mer- cury vapor lamps, short arc lamps, ultra-high- pressure lamps, etc., for its radiation sources. These lamps radiate throughout the UV range (200–400 nm), in the visible range (400–760 nm), and far into the infrared range. In contrast, direct-exposure light sources emit a few discrete wavelengths in the 355–420 nm range. This radiation penetrates deep into the solder resist and starts the crosslinking in the entire mask. But these light sources do not penetrate into the solder mask because the sol- der mask resin absorbs strongly in this area. The solder mask starts a massive crosslinking on the solder mask surface, creating a protec- tive skin. The surface is sealed, the chemical resistance is improved, and the moisture ab- sorption is reduced. 3. Quantum Yield Mismatches Due to Wavelength and Solder Mask Not Being Compatible A third difference is the quantum yield in the formation of radicals at different wavelengths. In contact exposure, the difference in the quantum yield between the individual wave- lengths does not play a major role. The whole UV range is available. Conversely, in direct exposure, one or more wavelengths are used, which form free radicals with a certain yield. Some wavelengths are very efficient with some solder resists while other wavelengths are very ineffective, which explains the different results when exposing a solder resist on different di- rect imagers. 4. Longer Exposure Times in Direct Imaging The fourth problem with direct imaging is the need for longer exposing times compared to contact exposure. This is not a defect, but it does make implementation more difficult. Pres- ently, more and more companies are switch- ing entirely to the direct imaging of the solder mask. Some companies have maintained con- tact exposing and partly switched to direct ex- posure for reasons of capacity. However, con- tact exposure is still the first choice for other companies. Since the exposing times are long, solder mask manufacturers have responded to the re- quirements of PCB shops by optimizing coat- ings for direct exposure. Existing solder masks have been modified so that all qualifications remain. It should be noted that these are still more traditional coatings and are not devel- oped purely for direct exposure. There are also solder masks available that have been developed specifically for direct im- aging. These products require an unbelievably low exposing energy. In most cases, this means the introduction of a new solder mask with all of the necessary qualifications. In an extreme case, a coating for direct exposure might mean

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