Issue link: https://iconnect007.uberflip.com/i/899995
18 The PCB Design Magazine • November 2017 concerned about the adhesion of the layers, of the copper to the substrate, and the internal ad- hesion of the substrate layers. What I've seen in the past with boards that aren't HDI is that you'll get issues with moisture in the boards and then you'll get issues with delamination as we run through the reflow process at the surface metal level, and at the wave solder and through-hole level. And that's not even mentioning the re- work. Unfortunately, the way that the systems work is that they go through the reflow or the SMT process, and parts don't always go where you want them to go, and so you end up with rework. When we rework these things, now we're applying a 700°F solder- ing iron to a pad and you're more likely to have that glue that's holding the boards together come apart on you, so a big concern that we have are our thermal profiles and how these things are going to stick together. Another concern is that the smaller these parts get, the more dif- ficult it gets for us to put solder paste on the board and have things that we can see, like the bottom side underneath the BGA. When things flow in there, we must look at it with X-ray. If we're talking about micro BGAs, we don't have a whole lot of clearance for the solder to reflow and hopefully not bridge. Matties: How are you facing those challenges? Burns: We're starting to use a lot more nano- coating on our stencils; we're focused in on dif- ferent designs. Instead of typical round BGA pads, we're doing the square with the rounded corners, and we have to have carriers. A lot of the times, we're running boards through the re- flow oven or an exterior that gives it more rigid- ity. It goes through reflow so you don't have any warping of the board, which can also damage it. Matties: This of course adds cost and cycle time. Mike Smyth: Right, and we also have an inline SPI which we are definitely checking. Matties: For the solder paste, is jetting the solu- tion for the solder paste in this application? Paul Petty: It's something that we can look at. We do not have that capability at this time. We have looked at different equipment that could do that, but right now what we do is we rely on our SPI a lot. That monitors our height, volume, size. It keeps all the pads in check so that we can ensure that everything is meeting spec. I was also going to say we're some- times constrained by our custom- ers on which solders we can use. We have a variety of tin-lead sol- ders and we have the RoHS-com- pliant solders, so we'll have a va- riety of the SAC solders and some others. For one customer, we use a low-temperature solder that has a high bismuth content, so we've got a huge variety of solders. Some of our customers don't allow high- or even medium-activity fluxes. We have to use ROL0 fluxes, which again creates other additional is- sues. Matties: How often do designers come to you prior to designing the boards to discuss the best layout for assembly? Burns: In about 12 1/2 years that I've been in this industry, I've had one company come to us and talk to us actively about the design of the board prior to them bringing it to us for manu- facturing, and that was an interesting journey. It ended up working really well. They went through a lot of iterations of the board design before we finally got one that would work, but the vast majority come to us with a product and say, "Here, we're ready for you to start building this." Matties: Obviously having the designer come talk to you first would be your preference, I would think. How do we get more designers to do that? Burns: We have to convince them that there's value to be added. It's not just designing the THE HDI ROUNDTABLE EXPERTS DISCUSSION Mike Smyth