Microvias Can Be Stacked in Certain Package Densities

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Summit Interconnect’s Gerry Partida recently spoke with the I-Connect007 Editorial Team about his research into root causes of weak microvias. Rather than a single manufacturing process cause, Gerry suggests that microvia reliability is the culmination of several material interactions and that contrary to popular belief, microvias can still be stacked in small, tight packaging densities. He highlights the need for simulation, as well as some of his findings that he plans to publish in a paper at IPC APEX EXPO 2023.

Nolan Johnson: Gerry, I understand your team has been doing some research into microvia stacking and will have a paper at the upcoming IPC APEX EXPO on this topic. What have you been learning?

Gerry Partida: Remember back in the early days of HDI, we would stack microvias as deep and plentiful as we wanted to? Then people started experiencing intermittent failures. Boards got hot, the components would fail, and it went back and forth. Manufacturing did something wrong, the assembler overbaked the boards, and it would go back and forth again. A lot of designs started to suffer, especially certain military products that would stack microvias. We would ask, “Why isn’t it working? Why does it work when it does work?” Most of the microvias that were stacked originally were small BGA packages. They were 0.4 mm or 0.5 mm, and those densities drove you to stack. These designs often were for the commercial OEMs, but if something failed, the commercial guys didn’t come back to discuss the issues.

But for the military guys who have ASICS that cost hundreds of thousands of dollars each, the stakes are much higher. If it is for space, then it can only be assembled once for flight; it cannot be taken off and reused. The military packaging then was a much wider pitch than the commercial guys who were stacking microvias initially.

When we looked at where the failures were happening, they were still happening with the commercial guys who were going three or four deep stacking microvias. They weren’t trying to make short, squatty, wide-diameter microvias because they were using thicker dielectrics to get wider lines for impedance. Consequently, we went for a time where there really didn’t seem to be a problem. Then it became, “We see a fracture at the target pad on the stack of the microvias,” and everybody thought there was a weakness in the electroless copper.

We all came up with these rules of thumb: Don’t stack more than two. A lot of DOEs were done, and they almost always concluded, “Do two stacks, then stagger off.” That seemed to work. Even fabricators we would work with had rules like, “Keep your aspect ratio for a single microvia at 0.75 to one. If you’re stacking them, keep them at 0.6 to one.” That seemed to work; we got good results.

Now, during this time we employed reflow resistance testing to monitor the strength of connections in the finished product. We started learning more about what works, and what doesn’t work. Some designs would slip through, where they do a three-stack on tight pitch, and they were passing. We were asking that if our rule of thumb was only two, then why is it working at three? When you look at the design, it’s a 0.4 mm pitch.

To read this entire conversation, which appeared in the September 2022 issue of PCB007 Magazine, click here.


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