Issue link: https://iconnect007.uberflip.com/i/1307491
NOVEMBER 2020 I DESIGN007 MAGAZINE 19 than Class 1 because Class 3 military allows a minimal annular ring. Data shows that a minimal annular ring basically guarantees you barrel cracking and corner cracking. In many cases, everything built to military Class 3 can be less reliable than Class 2 and Class 1. Gary Ferrari wanted to test it because I showed him the data that proves that all of this Class 3 stuff isn't really good data. Our Japanese partner introduced landless vias to us, and we said, "That won't work. It doesn't have any lands," and they said, "Test it." We built an elaborate test vehicle with different sizes of annular rings all the way down the landless, and the landless vias were 10 times more reliable than the vias with the land. The smaller the annular ring, the less reliable the via. We put a bunch of Ph.D. metallurgists on this to explain this to us, and then they showed us the Coffin-Manson models and the data. We said, "We understand now." Chavez: I heard about this many years ago at one of the IPC APEX EXPO Design Forums. But to my knowledge, landless vias just haven't taken off in the aerospace industry. Within aerospace, it's extremely important to be well within the safety zone of design and manu- facturing margins, so we're not pushing the envelope, as you would see in the commercial industry. Aerospace is usually several years behind in the industry, and it's purposefully that way because we always want to be in a safety sweet spot for reliability. Holden: Aerospace companies are still using tin-lead because that solder is highly ductile. Chavez: Yes, we're still using the industry-stan- dard tin-lead, but some designs are produced lead-free. It all depends on the customers' requirements. With lead-free, tin whiskers are a serious issue that comes into play and must be addressed. Parts, such as some BGAs, only come in lead-free. And today's BGAs are get- ting smaller and smaller ball pitch that requires HDI and micro technology vias. Often, we don't have a choice regarding which via technology to implement. Because of these fine pitch components, we have to use microvias. These parts are getting smaller, smaller, and smaller, and we don't have a choice. The minute an EE selects a part with these 0.65-mm pitch BGAs or smaller, they've already boxed us in a corner to HDI and micro- vias. We have no choice. Holden: I'm judging a board right now that's 782 I/O at 0.4-millimeter pitch. The board that this thing is mounted on is an eight-layer ELIC, in which there are no through-holes. Every single layer is "microvia-ed" to the next layer. Now, they don't talk about what material it's made out of, but I sure hope the fabricator and the material are well tested because I took a look at that BGA and said, "My gosh." Chavez: In that case, it is what it is, and you have to adapt to it. As engineers, we have to do what we can that's in our repertoire not just for design but also for manufacturing. This is where you had better be talking to your suppli- ers. They need to be all in with the engineering aspect as well. Shaughnessy: Does it matter to a designer whether the vias are going to be laser drilled or mechanically drilled? Chavez: You should know what you're get- ting into. You should know those details up front when you're dialing in your stackup and adjusting your stackup with your supplier. You should collaborate with them to achieve your results because you want to minimize your expense and maximize your yields. That way, you're getting the best bang for your buck at As engineers, we have to do what we can that's in our repertoire not just for design but also for manufacturing.