Importance of Controlled Impedance in Electronics

IMPEDANCE INTUITION. Achieving signal integrity (SI) in high-speed PCBs requires matching the impedance of physical structures. As data rates increase, the structures we care about become smaller – even down to several mils. Here, I’ll provide a simple way to increase your impedance intuition. The secret is understanding capacitance. Memorize this capacitance approximation equation: C = ƐA/d. These parameters directly translate to items in your PCB:  A (Area) is the size of your structure, d (distance) is the distance to ground/metal, and Ɛ is the dielectric constant of the material between the metal (sometimes called Dk or Ɛr). Next, understand that capacitance is inversely related to impedance (because Impedance=Z=sqrt[L/C]). ‘Inversely related’ means if capacitance goes up, impedance goes down. When you reduce capacitance, impedance goes up. In practical terms, if you make your trace wider, then A (Area) and hence capacitance get larger causing impedance to get lower. Move your trace further from ground, and d (distance) goes up thus lowering capacitance and increasing impedance. Use lower Dk material, C goes down and impedance goes up. See how this works? While understanding impedance guides design decisions, you can also use the concepts to grasp manufacturing changes. For example, if PCB fabrication substitutes a thinner core or presses your pre-preg layers thinner than plan, d (distance) becomes smaller, and impedance gets lower. Or, if traces are imaged or etched thinner, A goes down and impedance goes up. Though fab notes try to protect against these changes, cross-section images and/or measurements may reveal these problems. And what about vias? Think of vias as traces in the Z direction, where drill size defines A (area) and via barrel distance to antipad (metal) is d. Use a smaller drill, A and C go down, so impedance goes up. Widen your antipads and d goes up making C go down, also increasing your impedance. Because we typically have to raise via impedance, both these ideas are often deployed. Keep these concepts in mind, and you can expand into SMT pads, connector styles – you name it. Failing to match the impedance of relevant PCB structures causes impedance discontinuities, the #2 reason high-speed serial links fail. Impedance matters. While your 2D/3D design and simulation tools can help you get the impedance right, per Bogatin’s Rule #9, to believe what tools are telling you it’s important to grow your impedance intuition. Are my design changes having the effect I expect? For more concepts and guidance, be sure to check out both my ‘Signal Integrity, In Practice’ book and LIVE class: Book:  https://lnkd.in/guhndNJG LIVE Class:  www.siguys.com/training #siforees (filter using my name ‘Donald Telian’) #signalintegrity

𝐓𝐡𝐞 𝐍𝐞𝐞𝐝 𝐟𝐨𝐫 𝐏𝐂𝐁 𝐒𝐩𝐞𝐞𝐝 More and more of today’s PCB designs require faster processing of information. That means the bare printed circuit board must have what is known as controlled impedance—the elimination or minimizing of discontinuities in the signal that a trace will deliver. It is no longer a trace or track that connects a plated through-hole to a via or to another device on the PCB. Rather, transmission lines are designed to transport energy at high speeds, with little loss in signal shape, magnitude, or speed. This high-speed circuit is similar to a high-speed highway where controlled impedance prevents the formation of potholes or speed bumps in the road. The possible distortion of the original signal intended to be sent along a particular transmission line may cause the PCB to not perform as desired. A uniform controlled impedance is required for PCB signal traces to minimize signal distortions caused by reflections. Controlled impedance adds another level to the PCB’s design, artwork, material selection and manufacturing processes. Even solder mask, with its insulation properties, will affect impedance values. Upon receipt of an order, the PCB vendor will conduct a simulation to verify that the design will allow for the final impedance values, with a usual tolerance of +/- 10%. If the design calculations do not agree, the supplier will notify the customer to allow a change in the stack-up to meet the design’s needs. Artwork being plotted has to compensate for the plating tolerances of the PCB manufacturer. Material consistency is also important—glass style, resin content and resin flow affect the dielectric constant, which will affect the impedance results. Usually, the same materials will need to be used on repeat orders. All controlled impedance boards will require TDR (Time Domain Reflectometry) measurements that confirm the impedance values are within tolerance. TDR traces are placed on a coupon that is located on the manufacturing panel. The coupon usually contains several traces of some length (usually up to 8 inches) that run parallel to each other, with a plated through-hole at either end. Using a TDR measuring device, the coupon verifies the impedance values are met. The testing is done on the coupon and is considered the benchmark that both designer and fabricator agree upon. The PCB manufacturer has no control over discontinuities in the design, meaning if the coupon is correct, so–it is assumed–is the PCB. Upon request, the TDR report may be included in the shipment, along with the coupons. Need to know more about how PCBs are made? I can help! Reach out to me here on LinkedIn. Or visit DirectPCB.com, where you can get a free copy of my book, PCB Basics for Buyers.

𝐖𝐡𝐲 𝐜𝐨𝐧𝐭𝐫𝐨𝐥𝐥𝐞𝐝 𝐢𝐦𝐩𝐞𝐝𝐚𝐧𝐜𝐞 𝐦𝐚𝐭𝐭𝐞𝐫𝐬 "As signal speeds climb into the GHz range, #PCB traces stop behaving like simple wires, they become transmission lines. That’s why controlled #impedance is essential in high-speed designs." I'm going to share in the comments the link to the very first technical blog I wrote for Sierra back in 2017 (jeez, time flies!). My amazing team has edited it and republished it a couple of times since then. 𝐓𝐡𝐢𝐬 𝐛𝐥𝐨𝐠 𝐛𝐚𝐬𝐢𝐜𝐚𝐥𝐥𝐲 𝐛𝐫𝐞𝐚𝐤𝐬 𝐝𝐨𝐰𝐧: ► What controlled impedance really means ► When and why you need it (think DDR, HDMI, Gigabit Ethernet...) ► Key layout tactics, like length matching and diff pair routing ► Why relying on controlled dielectric alone might backfire ► How Sierra Circuits ensures impedance accuracy with in-house testing tools and calculators 𝑳𝒊𝒏𝒌𝒔 𝒊𝒏 𝒕𝒉𝒆 𝒄𝒐𝒎𝒎𝒆𝒏𝒕𝒔: • Why controlled impedance matters blog • Controlled Impedance Design Guide 😻 • Impedance Calculator 𝑾𝒉𝒂𝒕 𝒅𝒐 𝒚𝒐𝒖 𝒕𝒉𝒊𝒏𝒌? 𝑰’𝒅 𝒍𝒐𝒗𝒆 𝒕𝒐 𝒉𝒆𝒂𝒓 𝒊𝒇 𝒕𝒉𝒆𝒓𝒆’𝒔 𝒂𝒏𝒚𝒕𝒉𝒊𝒏𝒈 𝒚𝒐𝒖’𝒅 𝒕𝒘𝒆𝒂𝒌 𝒐𝒓 𝒂𝒅𝒅 𝒕𝒐 𝒎𝒂𝒌𝒆 𝒕𝒉𝒊𝒔 𝒎𝒐𝒓𝒆 𝒖𝒔𝒆𝒇𝒖𝒍 𝒇𝒐𝒓 𝒇𝒆𝒍𝒍𝒐𝒘 𝒅𝒆𝒔𝒊𝒈𝒏𝒆𝒓𝒔.