Issue link: https://iconnect007.uberflip.com/i/1243344
50 SMT007 MAGAZINE I MAY 2020 Ghosh: All of our work so far has been done with conventional off-the-shelf solders. We have done SAC305—anything from Type 3 to Type 7. We have done single solder balls and tin-bismuth in a wide variety of sizes. We want this to be as widely available as possible to customers. It takes years for companies to qualify a sol- der type and the supplier. We have gone to our initial customers and told them, "Give us the solder that you have, and we will use that." We are working with technology partners devel- oping specific solders for a variety of appli- cations. Our technology partners have been very impressed with how well this technology works with conventional solders, plus with solders that are being specifically designed for laser soldering or some other technologies as well. Feinberg: What if the light were only able to hit the solder at the very top? Let's say that the solder joint was down in a cavity created by solder mask. Can the eutectic point still be reached throughout the solder joint? Ghosh: We are using a white light source, so it depends on the size of the cavity. Nothing's beating physics. There's going to be shadow- ing, etc. However, if a few microns or larger, then yes, it will go into that cavity, and it will heat up the solder. Again, this is a light ini- tiated process, but this is a thermally driven process. Feinberg: It's thermal; it's not a photoreaction. Ghosh: Correct. The light is only to bring in the energy. That light gets absorbed, scattered, etc., and then turns to heat, and that is driv- ing the whole process. That's why we can use completely conventional solders. The solders are still using the same energy. Holden: When you consider how much energy goes into an infrared reflow oven, the pho- tonic method can't be requiring the same total energy. You're not heating the substrate. This is more a surface phenomenon. Ghosh: Exactly. IR heaters are designed specifi- cally to be absorbed by polymers, solvents, and organics. That's how they are designed. That's why they work so well for drying applications. You cannot keep a PET substrate under most IR heaters because the PET is again designed to absorb in the IR area. Ours is white light. We do have some IR component, but it's signifi- cantly less compared to the visual part because it's a broadband light source, we also can filter out different spectrums of light if needed. You can cut out the UV, the visible, and the IR. That allows us a level of selectivity. Holden: Would it be akin to the energy of con- densation soldering where the heat is only applied to the surface of the solder—not the entire one? Ghosh: The amount of energy that is absorbed and converted for the reflow process. The amount of energy that the whole thing is exposed to is not because the other parts of the substrate are still seeing this energy. It's letting it pass through. In our design, we use something called a "beam dump." The energy that we're delivering needs to go somewhere. A lot of that energy that is being delivered gets dissipated. Keep in mind that this is pulsed white light. The pulsing is very important here. If you're going to put in the same amount of heat with- out the heat being lost, you are cultivating too much energy, and you are going to destroy everything. Even with PET, there are two things: first, the light has been converted to heat, and second, that heat is now trying to find the best thermal pathways. One of the pathways it sees is to go through the solder and make it reflow. We are bringing in light and it is converted to heat, but then it's all about the substrate, com- ponent, solder, and conductive pathways. The substrate can be paper, fabric, or TPU. Holden: I'm a big believer in the future of printed electronics, especially for disposable medical electronics. Ghosh: That has been one of the main drivers for this technology: making disposable systems