Issue link: https://iconnect007.uberflip.com/i/1243344
52 SMT007 MAGAZINE I MAY 2020 that are highly reliable. You can do a lot of these things with ACFs and ACPs, but they're not as reliable. Solder delivers interconnects that cannot be matched with anything else. To our thinking, we are here to provide a solution that is equal or better. Feinberg: Is the heat generated only on the con- ductor, not on the dielectric? Ghosh: Heat is generated in anything that is absorbing. You can make a dielectric that is dark in color that will absorb and get hot. But, in most cases, your dielectric layer is clear and doesn't get hot. Feinberg: If you have a photocurable coating like a solder mask, does this add to the cure of the solder mask? In other words, would there be any thermal change to the solder mask except the immediate solder mask that is right next to the conductor? Ghosh: It shouldn't because, first of all, if the solder mask is of a specific color—such as green—if you think about the visible spectrum, yellowish-green is pretty much the predominant color in the visual spectrum. It's mostly going to reflect that light back directly. The only light that it's going to absorb is the UV part, which was what is used to run the photoactivation. Those photoactivation steps are usually self- limiting. Once it's done, it does not continue further. Again, if you make that solder mask black, then it becomes a problem, and then we would have to most likely put a mask on top of that. But if it's in those areas where it normally has the yellow screen, it's great because it's going to reflect most of that spectrum anyway. Holden: When do you sinter instead of solder? Ghosh: The first step would be to sinter after you put the conductive tracks. You sinter that, make them dry, and make them conductive compared to bulk metal. Then, you stencil the solder, you pick and place the components, and then do the reflow. They are two differ- ent steps with different timescales and energy requirements. If I bring in the same amount of energy on a wet ink as I would for the solder, it is going to completely destroy the metal. The amount of solvent that's in a conductive ink would cause it to explode violently, and your tracks would not exist anymore. But the ability to take it the liquidus temper- ature and then ramp it up much higher than that temperature over less than a second and drop it quickly allows us to have these inter- metallic formations that are thin and continu- ous. Also, the cooling down process is in the seconds. When you turn the light off, there's no energy left behind. That allows us to have this nice, granular microstructure that makes the solder joints very strong. Holden: We have an enormous problem with conventional lead-free soldering for very expensive military and high-reliability products in that holding a temperature at 230–260°C for the amount of time necessary for reflow has been breaking the microvia joints. Photonic soldering doesn't heat the substrate to anywhere near those temperatures. We're talking about some of the most expensive electronics in the world, and if there's an alternative method of soldering that does the solder of lead-free but doesn't heat the substrate, that's an enormous advantage that nobody else has. Ghosh: Absolutely. The big advantage is that we are bringing enough energy for the solder reflow, but not enough energy to heat up bulk material. Some of our tools heat up thick metal, which is mind-blowing, but that's a different regime. That is a very specific application that could see the advantage of this. This process is self-limiting: once it becomes this nice solder joint, now it has a good thermal pathway, and it does not get much hotter. It does not give out heat to everything else that is not metal around it. It's trying to find as many metal tracks to go around and get that to even temperature. Matties: With respect to what Happy is talk- ing about regarding quality improvement, have you done any laboratory testing to talk about the quality and structural difference?