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Slash Sheets: Don’t Fall Into the TrapJune 8, 2023 | Geoffrey Hazelett, Technical Sales
Estimated reading time: 2 minutes
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.
There has always been pressure to reduce line and space as we have seen the bleeding edge technology go from 8 mils to 5 mils and then to 3 mils. The difference between “then” and “now” is that the prior advancements, for the most part, used the same processes, chemistry and equipment going from 8 mils to 3 mils. But going from 3 mil to sub 1 mil trace and space is a quantum leap in printed circuit board (PCB) technology that requires a whole new set of processes and materials.
In a previous column, the critical process of desmear and its necessity to ensure a clean copper surface connection was presented. Now, my discussion will focus on obtaining a void-free and tightly adherent copper plating deposit on these surfaces. After the desmear process, the task is to insure a continuous, conductive, and void-free deposit on the via walls and capture pad. Today, there are several processes that can be utilized to render vias conductive.
Panasonic’s Darren Hitchcock spoke with the I-Connect007 Editorial Team on the complexities of moving toward ultra HDI manufacturing. As we learn in this conversation, the number of shifting constraints relative to traditional PCB fabrication is quite large and can sometimes conflict with each other.
MKS’ Atotech, a leading surface finishing brand of MKS Instruments, will participate in the upcoming IPCA Expo at Bangalore International Exhibition Centre (BIEC) and showcase its latest PCB manufacturing solutions from September 13 – 15.
Flexible circuit applications can be as basic as furnishing electrical interconnect between two conventional circuit board assemblies, or to prove a platform for placing and interconnecting electronic components. During the planning and pre-design phase of the flexible circuit, there will be several material and process related questions that need to be addressed. Most flexible circuit fabricators welcome the opportunity to discuss their customers’ flexible circuit objectives prior to beginning the actual design process.