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52 DESIGN007 MAGAZINE I JANUARY 2021 In this column, I will discuss how to create the perfect board stackup, specify what you truly want to convey to the fabricator, and eliminate conflicting information about stack- ups. What Is a Board Stackup? The board stackup is the Z-axis stackup show- ing layer configurations, all specific dielectrics, copper weights, material types, and any infor- mation regarding controlled impedances. This is critical to make sure your design works as expected but is a frequently overlooked and underrated part of your output package that is often left up to the fabricator. This can work if you don't have any special dielectric or con- trolled impedance needs. Not sharing those needs with your desired fabricator sets you up for impedance mis- matches or performance variables in your design. Fortunately, a good fabricator has very experienced CAM folks who can recognize ser- pentine traces as single-ended (SE) structures, differential pairs as controlled structures, and CPWGs likewise as controlled structures. This should lead to an email, or at the very least, a phone call from your fabricator to clarify your stackup intent. 1. Where to Place This Information The Z-axis depiction of a board stackup is typically on the fab drawing, in a README file, or as a separate sheet depicting the Z-axis stackup, such as a PDF, DXF, spreadsheet, or some other type of document. It is important to remember if you call out material type, copper weights, and controlled impedance informa- tion on the fab notes, they should not conflict with what is shown on the separate stackup. In addition, the fab notes cannot conflict with the available space Stackups: Properly Conveying Your Info to the Fabricator The Bare (Board) Truth Feature Column by Mark Thompson, CID+, MONSOON SOLUTIONS

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