Issue link: https://iconnect007.uberflip.com/i/1002143
22 PCB007 MAGAZINE I JULY 2018 ing nowadays. The technology itself seems to have reached its limitations, really, with track heights and feature sizes. There are advantages and disadvantages. As we said before, no actu - al application method is the be all and end all. Tibbals: The other area we have to consider is the future demands of solder mask regardless of application method. It doesn't really matter whether its screen, spray, curtain-coat, conven - tional image, direct image, or inkjet, there are increased demands being put on a solder mask's final properties. If we look at the requirements of a solder mask, we see those becoming more demanding in two or three key areas. Thermal storage and thermal cycling re- quirements of a solder mask are increasing. Traditionally, thermal storage may have been done at 140°C or 150°C, but we're now see- ing demands up to 170°C or even higher. And, we have traditionally seen cycling being done from -40° to +140°C; more recently this went to +150°C, and we are now seeing those tem- peratures again increase to 160−170°C. This is all down to reliability. One of the driv- ers for it is automotive, because of the increase in hybrid and electric vehicles. These are put- ting more demands on the circuit boards due to the high level of electronics within the vehi- cle and a consideration that a device may be in a prolonged "on" state, unlike an internal com- bustion engine where the vehicle is turned off. What we're seeing is the automotive manufac- turers and automotive component manufactur- ers put in increasing reliability demands on cir- cuit boards and in turn the solder mask which is on those circuits. When we test materials now, we're not just testing them to the established test procedures. We're looking to test them to beyond where we believe those tests are going in the future. If we want to bring an inkjet solder mask to the mar- ket, there's no point in us testing it and formu- lating it so it passes thermal storage conditions 'A', when in two years' time it's going to need to meet thermal storage conditions 'B'. So there are the thermal conditions and de- mands, and there is the dielectric properties of the material. With the implementation of 5G technology and the Internet of Things, connec- tivity is going to be the major factor going for- ward. To facilitate that, we need to have higher and higher frequency boards and therefore the dielectric properties of the board itself needs to meet those requirements. And the solder mask starts to become an important factor in that. When we're formulating materials, we have to be aware that we're meeting these future dielec - tric demands, whether that be dielectric con- stants, dissipation factors, these are things we have to take into account when we're formulat- ing materials. That's our job as a solder mask manufacturer, to make sure that we're deliver- ing materials that meet these future demands, not just next year, but years in to the future as circuit board technology starts to move on. Happy Holden: Getting back to Patty's question, over the years, solder masks have taken on a lot of roles other than resisting solder because of rugged durability and epoxies and things. I wonder if in order to get things like inkjet print- ing, people have to go back to saying, "This is only suitable to resist solder. It doesn't do all these other characteristics of solder masks you've been getting for free." Tibbals: I think the problem is that Pandora's Box is open and no one ever wants to go back- wards in terms of capability, so I think from our perspective it would be a very hard sell to get somebody to accept that a solder mask is only a mask that withstands solder. Resist- ing solder is not the challenge, in many ways that's the easy part. They key is formulating a universally acceptable product that withstands electroless nickel gold or immersion tin, gives the required moisture and insulation resistance values, has good dielectric strength properties, and all of these other things that don't nec- essarily form part of one particular published standard but are all day-to-day things which the solder mask has to do. Solder, whether leaded or lead-free is actually quite a small part of the solder mask's job now, it's quite possible that solder mask goes through its whole life and never sees any solder, unless the solder paste spreads on top of it a little bit. It