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In a recent conference call, our I-Connect007 editorial team was joined by Circuit Automation’s Yuki Kojima, VP of engineering; Larry Lindland, sales and applications manager; and Tom Meeker, CEO, for a lively discussion about solder mask. Spoiler: It’s not all about the equipment.
Patty Goldman: Gentlemen, let’s start with some background on Circuit Automation.
Yuki Kojima: Circuit Automation has been manufacturing double-sided screen printing systems for the circuit board industry since around 1980. Mainly it’s been for the rigid circuit board industry, but now we’re getting into the flexible circuit board industry, too.
Larry Lindland: I’ve been with Circuit Automation since 1995. As Yuki said, we’ve been working with solder mask manufacturers all over the world trying to provide the best application system out there.
Goldman: Yuki, if you would, please give us an overview of what you see happening in the solder mask world. What are the current problems and where do you see things are going?
Kojima: The requirements and specifications for solder mask have been diversifying. It’s been just computer boards and consumer products. But now, with HDI, smartphones, flexible circuit boards, communication panels, there is a wide spectrum of different types of circuit boards out there. Each has such unique requirements, and we have to meet different demands. For example, I think the biggest challenges right now are the exposure, or I would say imaging, after photo masking. Now, many people are getting more and more into direct imaging—laser direct imaging, UV direct imaging—and we need to meet the requirements, which are basically the mask thickness, evenness, and distribution and that sort of thing. I also do sales and marketing in Asia where the biggest challenge I see right now is via holes. A panel such as an OLED requires microvia holes to be plugged, which is a very difficult 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 requirements. 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 diameter- 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 challenges that we have faced in production is applying 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 enhances the next step of imaging. We used to measure solder masks in mils, but now we’re measuring in microns, and that’s been a big advantage as we do some of the higher-end boards.
Goldman: You are referring to thickness, right?
Lindland: Right, the finished thickness. Solder mask usually is about 25% solvent after you tack dry to get your finished thickness or when you lay it down wet, and we’re measuring that in microns now.
Tom Meeker: One thing, if I could add, is that there’s a wide variety of difference in the ink formulations. In setting up for one customer’s ink, you get totally different results if they switch from a matte to a gloss, or something. 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 manufacturers 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 customer who is routinely doing four different versions or colors, switching back and forth every 10 panels. They’ve got all these screens set up and different setups for going back between gloss and matte. There’s a huge difference in performance between them.
To read the full version of this article which originally appeared in the July 2018 issue of PCB007 Magazine, click here.