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46 PCB007 MAGAZINE I OCTOBER 2020 have introduced a photoimageable coverlay to the industry with great acceptance. Solder masks and consumables are grow- ing, so that's a market we were counting on, and it's coming to fruition. One of my favor- ite subjects is LED solder mask, or white sol- der masks, for different industries because of reflectivity and color stability. The automotive industry uses a lot of white solder masks that a lot of people are not even aware of. Starkey: Is the white mask that you just men- tioned a crack-resistant version? Monn: You nailed it. We were very fortunate when we started looking at this. We went to some OEMs and said, "What do you need that you don't have?" In the automotive in- dustry, crack resistance is very important. But what we weren't aware of, and they educat- ed us on, is not only do they need white sol- der masks that have crack resistance, but they still require color stability and some high- temperature applications. Simply making it crack-resistant and losing other characteristics wouldn't be acceptable. We had to keep those other good capabilities and make it crack-re- sistant as well. Starkey: I can visualize the sort of harsh service conditions that such a product would have to endure. Tell us a little more about what drove the development and what specific problems it overcomes. Monn: When you talk about crack resistance, there are a lot of different ways to view cracks. Typically, in circuit board manufacturing, when we think about a crack, we think about a surface crack based on temperature, reflow temperatures, and contaminants on the sur- face, but there are many kinds of cracks. When you're talking about LED boards, a lot of these boards are single-sided with copper or alumi- num backs. They don't typically always get routed; some of them get punched. The punching process, which is especially dependent on the size of the parts, can cause a tremendous strain on the board, the panel, and the solder mask. That was something we never considered initially, but once we learned more about what our customers were doing and how they were doing it, the little light bulb went off in our head, and it was another prob- lem to solve. Starkey: You're looking at the mechanical stresses involved in fabrication, as well as the stresses induced by a CTE mismatch. Monn: Right. You phrased it a lot better than I did. Starkey: As I said before, I know the automo- tive market is getting more and more demand- ing, and the service conditions of these ma- terials become more and more and more se- vere. We've talked about overcoming the me- chanical stresses and the thermal expansion mismatches between the coating and the sub- strate. Also, because it's in an LED application, it's going to get hot, so it has to have high-tem- perature resistance, as well as to get alternate- ly hot and cold, depending on whether the sys- tem is switched on or not. It's also white and has to stay that way. There's not a big advan- tage if it yellows upon long-term exposure to intense white and UV light. Don Monn

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