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38 DESIGN007 MAGAZINE I MAY 2023 Feature Article by Geoffrey Hazelett POLAR INSTRUMENTS Slash sheets can be confusing, and this is a big topic, so let's start big and drill down from there. Here's the big picture regarding slash sheet references: ey were designed to provide handy groupings of PCB materials (laminates, polyimides, etc.) that go into a stackup. ese groupings are designed around mechanical characteristics to provide insight for PCB fab- ricators to identify similar laminates with simi- lar properties. ese documents were not designed to be used by PCB designers to help select the cor- rect material for their job. For high-speed designers in particular, these group references can be a trap. While two material families may have similar mechanical processing char- acteristics for fabricators, the materials can have wildly dissimilar electrical characteris- tics. Dielectric constant (Dk), dissipation fac- tor (Df ), copper roughness values, and glass weave styles (106/1080/3313, etc.) are all sig- nificant for signal integrity and are not appro- priately distinguished within slash sheets. For example, due to this ambiguity of electri- cally significant values, two electrically dissim- ilar performing materials could be substituted for one another. e two materials may have similar processing temperatures and roughly similar characteristics; to savvy PCB designers with an eye for signal integrity, slash sheets fall short. For example, IPC-4101/126 has a maximum Dk value of 5.4, and a Df of 0.035. To use Isola Slash Sheets: Don't Fall Into the Trap

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