We’ll get into these attributes along with bend regions and other design rules for flexes in another chapter. There will be at least two areas and often more depending on the nature of the flexible printed circuit. All of these segments should be specified on the fab drawing and a separate stack-up diagram would be used for each area. Where components and edge connectors reside, there are additional call-outs for more adhesive and stiffeners under the components as well as the solder mask and ink on the component side.įlexes and rigid-flexes are distinguished by the fact that the stack-up normally varies from one location to another. From there, adhesive and coverlay are added. In addition to the polyimide base material and copper, there is additional copper plating that is specified. When it comes to flexible printed circuits, a whole list of possibilities are in play. Anything more than two layers means that I will defer to the vendor for the actual materials. Even an eight-layer board can be stacked up a thousand different ways. Guessing the exact material layup that your vendor is going to use for a 24-layer board would be like winning the lottery. The variations are practically unlimited. Higher layer-count stack-ups may have multiple cores with prepreg layers used to combine them. A PCB stack-up is built around a core to which prepreg layers are added. Prepreg is basically a sheet of copper with an uncured layer of resin/fiber that is kind of like glue. The core material is a fully formed sheet with a cured layer of dielectric sandwiched between two layers of copper. Laminate materials come in two basic forms. The stack-up will be further described in a diagram that details the layer thickness of all dielectrics, copper, solder mask, and even ink if necessary. At that point, it becomes useful to spell out the attributes, especially the T g minimum and the RoHS requirement.įab shops in other countries will use locally sourced materials that are similar but may not meet the high-temperature requirements for lead-free processing or may contain the bromides that are prohibited for mass production in Europe. In an alternate universe of reference designs or globally sourced boards, the words “OR EQUIVALENT” would follow the slash sheet designation. That statement comes from doing defense projects. Material will meet the Reduction of Hazardous Substances (RoHS) rules or it won’t. Image Credit: EPEC - The basic details of PCB materials are catalogued in the various IPC slash sheets.Ĭalling these attributes out individually is more of a feel-good thing. SEE STACK-UP DIAGRAM FOR DETAILS.Įstablishing a set of dielectric materials in this way includes factors such as the maximum operating temperature and the coefficient of thermal expansion.įigure 1. LAMINATE AND PREPREG MATERIAL SHALL BE WOVEN E-GLASS / EPOXY IN ACCORDANCE WITH IPC-4101/126. If you want to have a clear note, avoid commercial terms like that and stick to specific terms like this. The most famous PCB material is called FR4. Flexible materials are catalogued in IPC-4102 and high density interconnect materials are covered by IPC-4104.Ĭalling out these materials is part of the fabrication drawing notes. (1) They have two main documents that are relevant to rigid stack-up materials IPC-4101 for typical materials and IPC-4103 for high speed/high-frequency materials. The nature of the resin and method of the weave are broken out into what is called slash sheets by the IPC. Just as the clothing industry weaves cotton or synthetic materials in different ways, so too does the PCB industry. While the home insulation uses air between the fibers, the stack-up employs certain types of resin and varying compositions of glass and other materials such as aramid paper. Glass resin has insulating properties much like the fiberglass that is used in home insulation. Most PCBs rely on dielectric material consisting of strands of glass woven into a resin material. A stack-up is made of alternating layers of conductors and dielectric materials. The first thing to understand about printed circuit boards is the stack-up. What about PC Boards With Multi-Layer Stack-ups?.This is the first section in the back-to-school series for PCB Designers and those who may want to know more about it.
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