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Does inkjet solder resist have the potential for volume production? Mariana Van Dam, Global Sales Manager PCB Imaging Solutions, and Dr. Frank Louwet, Business Unit Manager Advanced Coatings & Chemicals, discuss Agfa's latest developments, plus some novel applications for inkjet etch and plating resists.
Pete Starkey: I’m at electronica 2018 with Agfa, and am delighted to have the opportunity to sit down with Mariana Van Dam and Frank Louwet to talk about developments in inkjet resists. It's lovely to meet you both again. We seem to catch up with you at most of the major events, and we're very interested to hear about the progress of your product developments, particularly in inkjet solder mask.
In my previous experience, it appeared that ink-jettable solder mask was an unachievable objective, but you seem to have made it into a reality. Can you bring us up to date with recent developments in the formulation, qualification, and customer acceptance?
Frank Louwet: Yes. We have focused on the green-coloured solder mask for rigid PCBs. The inkjet solder mask development in Agfa started about two or three years ago. During our interview last year at productronica, this ink was almost ready for field testing.
And that's what we have been doing for the last 12 months—getting this ink validated at a selective number of customer sites. This time was required because it's not only about the ink; the complete system needs to perform. Just as important is the printer hardware and software, including the printing and curing strategy for the different layers that build the solder mask. A lot of hard work and optimization was going on, not the least on the side of our customer. Basically we did not change too much on the ink itself.
Starkey: You've chosen some end users to cooperate with. Have you chosen to run with one particular supplier of equipment, or have you kept to an open policy that you'll cooperate with anybody that's interested? When you're doing customer trials, have these been with specific printing machines—what the end user already has—or have you provided ink and a machine to print with?
Louwet: We have an open policy with respect to printer integration. Of course, the above-mentioned optimization between ink, printer and software requires a lot of capacity. Cooperating at a specific customer site builds a lot of know-how and the “ink and system” are tuned to one another. Doing this effort in parallel for all printers would be tough to organize. But we are open to cooperate with any and all worldwide.
Mariana Van Dam: We have invested in an application lab at Agfa with several inkjet printer brands and we have the different printhead choices. All regions in the world are represented in our lab with state-of-the-art printers.
Louwet: So, we try to get all the different printer heads and technologies together in one room so we can print for customers and can optimize together with the customers.
Starkey: Do you think that you will be able to provide a small number of standard formulations that will be of universal acceptance and will run with any machine and customer, or do you have to optimize the formulation, printing strategy, etc., specifically for each machine and each end user?
Louwet: Having one ink serving all printers would be the optimum. But in reality that will not be the case. A first selection depends on the printhead technology. A Konica Minolta printhead or a Fuji Samba printhead have requirements that are a little different. And in that sense, you have to play a bit with the formulation. The next difference will be the type of curing: bulb versus LED and the wavelength. So some adaptation for this is also required.
Agfa continues to develop solder resist inkjet inks “in-depth and in-width.” In-depth new concepts are tested for next generations and in-width new type of inks (white inks, black inks and flex inks) are being worked on.
Starkey: That was a big obstacle in previous generations of solder mask development. When ink vendors had developed a formulation that would jet and cure well with a successful print strategy, then it would struggle or fail to pass international qualification tests or to win customer acceptance.
Louwet: In the process of the optimization and bringing it to a customer production environment, problems and issues occur. Up to now, these could be solved by workarounds known in this industry. Working together with companies that supply pre-treatment chemistry, like the antitarnish or the copper pre-etch, opened up possibilities before laying down the solder mask. Also post treatments like final UV and/or thermal cure determine the properties of the mask layer. But of course the basic fundamentals need to be good.
Van Dam: You cannot separate just one step from the rest; you really have to optimize the full process for inkjet, because it is very different from the traditional technologies.
Starkey: I understand. I think you have taken a systematic and professional approach to it. The evidence is there in the product. Are you in a position to place this material on general order, or is it still a development?
Van Dam: It's available.
Louwet: Here at electronica Fela and Würth presented what they call the 'S-Mask concept.' It is a smarter way of putting down the mask. Because you can do it digitally, you print the solder mask only where you need it as you can see here (Figure 1).
Starkey: What I'm looking at is a PCB that's not covered completely in green mask. The green solder resist is placed only where it is necessary; at least 50% of the surface area of this example is just the bare laminate. So, it's probably not the best way to go commercially for an ink supplier.
Louwet: Depends what side you're coming from [laughs]. For some competitors that would probably not be a good way to go. But yes, ink consumption will indeed go down. It is not only because you print selectively but it's also a 100% UV curable ink so there are no solvents involved. We calculated that ink consumption of solder mask would decrease by at least 50%.
Van Dam: Yes, that's right. And depending on the functional requirements, digital inkjet printing can also vary the solder mask thickness. For example, if you have a high break-through voltage requirement in a certain area on your board, you can put a thicker coating there. That's what's demonstrated in this example (Figure 2). It offers so many possibilities.
Starkey: Again, such a logical solution. Why hasn't anybody done it before?
Van Dam: The way Fela and Würth presented it, “it is not just an evolution, in fact it's a revolution!” There are so many advantages with this new way of applying solder mask. For instance, you don't put ink in the holes because you can control this with your inkjet; you don't have undercut because there is no development step, etc.
Starkey: What sort of edge profile can you achieve in terms of squareness and waviness? We'll always have some sort of meniscus at the edge, but how sharply can you define the edge? And is that edge free of the residual geometry of the print strategy?
Louwet: The printing is done in different stages. It's not one-pass printing, but multi-pass: printing dams for definition followed by filling. So 125-micron line/space with dams in between are demonstrated on production tools. The ultimate L/S and edge definition depends on the accuracy of the printer and the printhead. Samba heads have a two-picoliter drop size.
Starkey: That is very impressive.