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November 2017 • SMT Magazine 15 COMMUNICATION STILL THE BEST TOOL of the things that we're all discussing here is the temperature cycles. Just starting out, every- thing that we build here has to go through a re- flow oven. It has to go through a reflow profile. We have to melt the solder so that we can get the parts to stick to the board, so the thinner the layers and material get, the more we're con- cerned about the adhesion of the layers, of the copper to the substrate, and the internal adhe- sion of the substrate layers. "What I've seen in the past with boards that aren't HDI is you'll get issues with moisture in the boards and then you'll get issues with de- lamination as we run through the reflow pro- cess at the surface metal level and also at the wave solder and through-hole level. And that' s not even mentioning the rework. Unfortu- nately, the way that the systems work is 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 re - work these things, now we're applying a 700°F soldering iron to a pad and you're more like- ly 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. Anoth - er concern is that the smaller these parts get, the more difficult it gets for us to put sol- der paste on the board and have things that we can see, like the bottom side underneath the BGA. When things flow in there, we have to 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." To address those issues, Petty says they are starting to use a lot more nanocoating on their sten- cils, and they are focusing on differ- ent 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 reflow oven or an exterior that gives it more rigidity. It goes through reflow so you don't have any warping of the board, which can also damage it." "We also have an inline SPI, which we are definitely checking," says Smyth. Is jetting the solution for the solder paste in this application? "It's something that we can definitely look at," says Smyth. "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, vol- ume, 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 sometimes con- strained by our customers on which solders we can use. We have a variety of tin-lead sol- ders and we have the RoHS-compliant solders, so we'll have a variety 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 addi - tional issues. Testing One of the problems HDI has is limited ac- cess for in-circuit tests. Are there solutions to help people that use HDI in terms of being able to use it for assemblies? "That's a big concern of ours," says Jervey. "As a contract manufactur- er, our in-circuit test traditional- ly has been a big part of our test solution. Over the last couple of years, I see that dying out very quickly. Our response to that in the immediate mode was to tran- sition a lot of our work into fly- ing probe with very small-pitch flying probe leads down to 3 mils on the point of the probe. We're doing a lot of flying probe work. The obvious extension to a physi- cal test is to utilize boundary scans where we're capable of using vector- less type tests or physical access isn't there to test things. It's going to be a challenge. As things get smaller and smaller, ultimately you have to touch it somewhere to test it in some fashion and it's a challenge every day." Mike Smyth