Via Reliability and Robustness in Today’s Environment

Estimated reading time: 8 minutes

By the time you get to failure analysis, your microvia is blown apart, you have this big wide chasm in most cases, whereas with a single via, you could stop it before catastrophic failure. If it reached 10% resistance change in your single via, you could stop the test, and you would be able to find the beginnings where the failure is starting, where 10% resistance change would equate to a 10% crack or separation in your microvia.

Holden: This will be great because as we focus on 5G and new materials, and the new materials are going to have to go through this kind of reliability component testing.

Neves: Exactly. One of the things we have, and it’s a little bit confusing to people, is that we clump everything into the word “via reliability,” and via reliability is one path to go through that relates to reliability in the environment that your product is in, but we don’t want to wait that long.

We moved toward what I like to call “via robustness,” where we take the coupons and test them above the T_g of the material, take them to solder temperatures multiple times, and run them as hard and as fast as we can until they break to try and see differences between these new materials and differences between processes. People tend to confuse the results from via robustness with via reliability.

Holden: Since we’re on the topic of reliability, why don’t you give us an idea of the European Space Agency where they have a full program including the leakage testing and CAF.

Neves: They have CAF and via reliability, and they want to keep the materials below the T_g point of the laminate system because at or near the T_g point, you start getting some extreme expansion in the material that causes degradation to your vias that doesn’t exist in real life. You never see that stress in your product’s life cycle. Even in a satellite, you don’t see that material going over the T_g of the material during operation. They also want to get a very high-temperature gradient in their testing. They want a 200°C delta gradient, so we go from -55 to 145°C to give them that 200°C gradient. We cycle through that, which allows them to then take that information and relate it to real-life reliability and see how long it’s going to last in their environment in the field. Meanwhile, a lot of the testing that’s out there right now is, “Let’s take it to 220°C, 230°C, 250°C, or 260°C and cycle it until it breaks.”

This via robustness approach may tell you which material is better than the other, but it’s not necessarily going to relate to what’s happening in your end life use situation. You could have two materials with one that is significantly worse in a via robustness test that may be as reliable in the intended end-use environment, and there’s going to be cost differences between materials of different via robustness as well. People are going to have to decide, “How much am I willing to pay for this additional via robustness that may or may not have any influence on the via reliability in the end product environment that I’m using it in?” 

Holden: Reliability is getting more and more important. If you don’t understand a lot about that, I’d recommend the seventh edition of The Printed Circuits Handbook, where Chapter 53, "The Acceptability and Quality of Fabricated Printed Boards," is written by Bob Neves. It’s a great place to start, but there are six chapters on bare board and assembly reliability written by Dr. Reza Ghaffarian, which are outstanding and provide you with the fundamental knowledge about modeling, reliability, and testing. If you want to know about these topics, get the handbook, read it, and then they can contact your organization for further information.

Neves: Thanks, Happy. It’s always a pleasure, and congratulations on your “night of Happy-ness” at IPCA APEX EXPO 2020.

Holden: Thank you very much.

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