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In this interview with the I-Connect007 Editorial Team, TTM’s Julie Ellis and Richard Dang drill down into stackup design, detailing some of the common stackup challenges that their customers face when designing for both prototype and volume levels, and offering advice to designers or engineers who are struggling with stackup issues. They also discuss why having too many different prepregs in a stackup can be asking for trouble, and how proper stackup design can optimize both the fabrication and assembly processes.
Andy Shaughnessy: Julie, why don’t you start by explaining why proper stackup design is so critical.
Julie Ellis: A stackup not only has to meet all of the customer requirements and industry standards, such as IPC-6012, but it also should be designed for best cost for fabrication using the least number of processes and available material that we can buy in time to meet the delivery, and be planned for special requirements, such as laser microvias or thick copper for high current. For products destined for volume manufacturing, stackups and minimum design guidelines should be verified with the final fabrication site capabilities in mind. We also take into account long-term reliability issues when we create our stackups for very high voltage devices (>500V) that are being driven by the electric vehicle and energy markets.
So, there’s a lot that goes into a stackup, and it’s not only 2D in the vertical cross-section view (Z-axis), where we’re trying to figure out the copper layers and the dielectric thicknesses. We need to achieve designs that accommodate registration process limitations in lamination, drilling, plating, and etching, which drive the minimum design guidelines. Depending on the design and the components that the customer is using, we determine the minimum trace, space, pad and via geometries on the horizontal X–Y plane. But it can be kind of difficult for customers to accept minimum design rules that are slightly larger than they planned when they don’t understand all the trade-offs and tolerances that we have to consider in our fabrication processes.
Shaughnessy: It’s where the rubber meets the road.
Ellis: It’s totally where the rubber meets the road. It’s better to plan carefully before you start putting the rubber to the road.
Shaughnessy: That’s one of the things that we hear: Some designers are doing stackups as they go.
Ellis: Yes, and if they don’t have all the fabricator’s input, which they would not necessarily know from their experience and point of view, they can accidentally start a design with extreme features and end up with a finished product that can be manufactured in small prototype quantities, but not in production volumes.
Shaughnessy: I’ve heard some designers say that if they can get the stackup right, that’s 80% of the design, basically.
Ellis: It could be, especially if their fabricator also provides all the minimum design guidelines, before they start routing their circuits.
Shaughnessy: Give us a quick overview of how you work with a customer who has a potential problem with the stackup.
Ellis: With my regular customers, if they have a new product and it’s got special requirements, they call me before they even start routing their boards. For rigid designs, we begin by discussing project requirements to determine the subgroup, which establishes which fab sites should be considered, by the complexity of the stackup.
To read this entire interview, which appeared in the January 2021 issue of Design007 Magazine, click here.