Slash Sheets: Don’t Fall Into the Trap

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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: They were designed to provide handy groupings of PCB materials (laminates, polyimides, etc.) that go into a stackup. These groupings are designed around mechanical characteristics to provide insight for PCB fabricators to identify similar laminates with similar properties.

These documents were not designed to be used by PCB designers to help select the correct 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 characteristics for fabricators, the materials can have wildly dissimilar electrical characteristics. Dielectric constant (Dk), dissipation factor (Df), copper roughness values, and glass weave styles (106/1080/3313, etc.) are all significant for signal integrity and are not appropriately distinguished within slash sheets.

For example, due to this ambiguity of electrically significant values, two electrically dissimilar performing materials could be substituted for one another. The 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 materials as an example, this lumps 185HR, 370HR, IS550H, IS415, FR408HR, and I-Speed all together. So, if a designer needed high-frequency performance with something like I-Speed, but allowed a substitution with /126, their fabricator would likely swap the material to something else. That other material could likely be a 185HR or 370HR, which have significantly differing Dk/Df values as well as differing glass weave types. 

A designer who wants to control the materials in their stackup is often pressed to communicate specific acceptable alternative supplier laminates, such as “Panasonic Megtron 6 (or Isola I-Speed),” to their fabricator.

To use shoes as an analogy: Shoes generally all have a sole, laces, breathable top, etc. A slash sheet reference would help identify “shoes,” but doesn’t help identify “running shoes” vs. "hiking boots." Nike and Adidas both make running shoes, so a designer could use “/shoes” (slash sheet reference), but they may be in trouble if they are running a marathon and want “Nike or Adidas running shoes.” A limitation of the slash sheets is that there are significant differences between the brands and the types of shoes, and unfortunately they could all be lumped together in the same slash sheet for “shoes.”  

A designer should clearly communicate with the fabricator regarding the desired glass weave, resin type, etc., going into their board if they wish to ensure their signal integrity characteristics will be met. Don’t take unnecessary short cuts when a few more words can save a world of headache.

Geoffrey Hazelett has a bachelor of science degree in electrical engineering, and was vice president of sales at Polar Instruments for several years. He is currently home with his new baby daughter, Sage. 

This article originally appeared in the May 2023 issue of Design007 Magazine.


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