Issue link: https://iconnect007.uberflip.com/i/1522641
32 PCB007 MAGAZINE I JUNE 2024 one of the first. Material manufacturers created competing products and these other catego- ries—mid-loss and low-loss material segments. We began using different resin chemistries to get to the electrical performance requirements in these materials. When you get down into the very-low, ultra-low, and extremely low-loss categories, the laminate suppliers are begin- ning to use resins that look more like what had always been used in RF microwave materials. ere were also other differences with RF microwave materials. Historically, they used a lot of fillers that were designed to maintain Dk stability over temperature, humidity, and fre- quency. One of the participants in that mar- ket had patents that limited others from devel- oping similar products. Most of those patents have now expired. Marcy LaRont: Because of these different polymers, adhesives, and blends, we can now potentially consider "FR-4" material as another option to traditional RF material. Yes, historically FR-4 was virtually all epoxy resins. But you just can't get to the Dk require- ments for high-speed digital applications using only epoxy. So, the first step was blending them with other resins, be it PPO, BT, SMA, etc. Eventually, you get to the lowest-loss mate- rials used today in high-speed digital, which usually use a resin called polyphenylene ether (PPE), and blends of PPE and other resins. Over the years, conventional copper-clad lam- inate (CCL) suppliers, particularly when lead- free assembly came on the scene, began to use a lot of inorganic fillers to control CTE prop- erties. So, you've had this convergence. e RF guys almost always used fillers. en, the conventional CCL suppliers became familiar with incorporating inorganic fillers into these other resin systems. Even the manufacturing processes look similar, with the exception of PTFE-type RF materials. For the RF/microwave CCL suppliers to get good copper peel strength to those res- ins, they had to use a fairly rough copper sur- face for good adhesion. But at very high fre- quencies, conductor losses begin to dominate, and copper roughness creates signal integrity problems. e conventional CCL guys got a bit more experienced along the way, working to get the right polymers, developing resin sys- tems, and using smoother, lower-profile cop- per foils to achieve electrical performance tar- gets. It's the combination of those things— resin system, fillers, and surface profile of the copper foil—that get you to the desired elec- trical performance target. Today's products designed for high-speed digital applications can perform well in RF applications because of these combinations. ere are a lot of different applications in RF, and it's not always lower Dk. Sometimes designers want higher Dk. It is really having a target Dk that's very stable over temperature, humidity, and frequency. If you look at the Dk of a resin system at low frequencies, it starts rel- atively high, then drops over frequency and sta- bilizes. So many high-speed digital products, along with fillers and controlled resin contents, can create that stability in what conventionally Ed Kelley