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14 PCB007 MAGAZINE I JULY 2018 area of solder masks? What are the challenges facing circuit board manufacturers? Tibbals: I would say the two main challenges are 1) smaller features, and 2) final properties of the boards and therefore solder mask. Circuit boards are getting smaller, and the features are getting tighter. So, what does that mean for a solder mask? Well, it really means two main things. Via holes are getting smaller and more densely packed and overall registration is getting tighter. So, from a solder mask per- spective, you need to be able to resolve small features, but you also need to be able to re- move the solder mask from very small holes. Those two things really pull in opposite directions, because the more you try to clear small holes, the more potential problems you face with undercut of small features on the board surface due to the prolonged developing stage. Barry Matties: How do you overcome that chal- lenge? Tibbals: Washing smaller holes clean is just a fact of life when dealing with photoimageable solder masks. Now, there are new solder mask technologies coming along which eliminate ink in the holes, and we will come to these shortly, but with current application methods you've got two choices: You either have to plug them or you've got to wash them completely clean. The issue for the circuit board manufacturer is being in no man's land. Being halfway in between is not desirable; the concern for PCB manufacturers and end users is trapped chem- istry in those partially filled holes and the sub- sequent impact on reliability. If we decide to completely plug them, then there are offline processes well established for that. For via processing, there is machinery to plug those holes, but that's generally done with a dedicated hole plug material. Alternatively, a hole plugging stage could be used as part of the solder mask application process. However, to wash them clean is a perennial challenge. You will get variations in the amount of ink de- posited in holes depending on the application method. The screen printing process would tend to put quite a large deposit in the holes and a spray application process would gener- ally put less ink in the holes. But, once you get down to very small hole siz- es, then irrespective of your application method, it starts to become a challenge. And that is something which our customers are aware of and a problem they have to deal with using harsher devel- oping conditions—and that starts to impact the ability to resolve smaller and smaller features. In terms of registration, tight registration and resolv- ing smaller features on the circuit board are being dealt with by moving towards direct imaging, which takes away the issue associated with registration on the circuit board. Up to now, customers could just man- ufacture smaller size panels, in which case registration becomes slightly easier. In reality, customers need to be able to produce circuit boards on large panel sizes to make the whole thing economical and efficient. And if they're going to achieve that, trying to do it by expos- ing with an indirect, thermally unstable pho- totool becomes a challenge. Some phototool registration capability can be clawed back by making solder dams smaller but that does not help mask defined pads or via registration. So customers have reached a point where tight registration just can't be done without yields dropping off, which is clearly a problem. The good thing is that direct imaging equip- ment has come along to solve this problem. Direct imaging has obviously been around for quite some time now, but in more recent years, the machinery has moved on in the sense that it's now multi wavelength, multi-head, and what that does is give a broader UV spectrum; Shaun Tibbals

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