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12 DESIGN007 MAGAZINE I JANUARY 2021 Feature Interview by the I-Connect007 Editorial Team In this interview with the I-Connect007 Edi- torial 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 proto- type 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 deliv- ery, and be planned for special requirements, such as laser microvias or thick copper for high current. For products des- tined for volume manufac- turing, stackups and mini- mum design guidelines should be verified with the final fabrication site capa- bilities in mind. We also take into account long- term reliability issues when we create our stack- ups 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-sec- tion view (Z-axis), where we're trying to fig- ure 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 TTM's Approach to Stackup Design: Train the Customer Julie Ellis

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