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48 PCB007 MAGAZINE I JULY 2018 holes. A panel such as an OLED requires mi- crovia holes to be plugged, which is a very dif- ficult thing to do. Many companies, as you all know, want via holes cleared or plugged. Now there are microvia holes, blind via holes, and they need to be plugged with solder mask. That has been a challenge for us. Goldman: And that's because you're basically screen coating with the vertical system, right? Kojima: Yes, correct. Well, not just limited to our products, but in general, whether it is spray coating or dry film or whatever. It is not easy to plug microvia holes. Goldman: Why is it so difficult to fill them? Is it because there is extra mask right there? Kojima: Well, you can set up the process to meet some require- ments. For example, there are parameters we can set so that ink can go into the hole easier. The challenge is that there are a lot of different sized via holes out there. Plugging is very diam- eter- and volume-specific; you can plug big holes, but then the small holes won't be plugged, and vice versa, that type of thing. Also, with microvia holes, obviously there's air inside the hole, and the air has to go someplace. Therefore, we print and capture some, but we can safely say we can plug probably up to 99% of the via holes. But there's always the 1% or 0.1% of the holes that we cannot plug. So that's a big challenge. Goldman: You've mainly seen that challenge in Asia at this time? Kojima: That's correct. Mainly where they do OLED panels and such products. Lindland: Yuki's right. One of the challeng- es that we have faced in production is apply- ing some of the things like getting a very even coating, minimal mask in the holes, and do- ing it on a daily consistent basis. For 20 years, we've been keeping ink out of the holes, and then we hear, "Oh, by the way, now will you please fill the holes?" And we try to be right up front and say 100% is very difficult to do. As Yuki explained, the holes are different sizes and will fill at different rates. We struggle with that, not being able to give them 100%. At the same time, we've worked very hard to provide extremely even mask thickness, which enhanc- es the next step of imaging. We used to mea- sure solder masks in mils, but now we're mea- suring in microns, and that's been a big advan- tage as we do some of the higher-end boards. Goldman: You are referring to thickness, right? Lindland: Right, the finished thickness. Sol- der mask usually is about 25% solvent af- ter you tack dry to get your fin- ished thickness or when you lay it down wet, and we're measur- ing that in microns now. Tom Meeker: One thing, if I could add, is that there's a wide vari- ety of difference in the ink for- mulations. In setting up for one customer's ink, you get totally different results if they switch from a matte to a gloss, or some- thing. There's a huge variation in performances that we've seen based on just the specific ink choice that the customer uses. Goldman: I would think most customers use more than one ink. As you said, matte versus glossy, and then there are the different manu- facturers and the different colors, also. Lindland: That's true. Customers are using many different colors now. Meeker: There are the colors, formulations and rheology. I'm working with a current custom- er who is routinely doing four different ver- sions or colors, switching back and forth ev- ery 10 panels. They've got all these screens set up and different setups for going back between Yuki Kojima

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