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How to design a high speed PCB!
It wasn’t that long ago when the word “high speed” didn’t exist in the vocabulary of PCB designers. Thinking back for example at the 90’s and 00’s, the speeds they used were much slower. In 2005, 3 Gbps was considered the typical high speed data rate, but today engineers are dealing with 10 Gbps, or even 25 Gbps.
It wasn’t that long ago when the word “high speed” didn’t exist in the vocabulary of PCB designers thinking back for example at the 90’s and 00’s, the speeds they used were much slower. In 2005, 3 Gbps was considered the typical high speed data rate, but today engineers are dealing with 10 Gbps, or even 25 Gbps. In the past the concern was all about putting the puzzle pieces together and strategizing your way through a physical board layout. But how about a high speed design? Now there’s a bunch of invisible forces to worry about, things like electromagnetic interference (EMI), crosstalk, signal reflection, weave of the PCB material and the list goes on and on. Here we discuss a couple of design tips to bear in mind when you are designing a high speed PCB. Start Your High Speed Design Process With a Plan Without a plan and a strategy for your high speed design project, you’ll likely encounter setbacks and unexpected issues. So before even laying down a symbol or connecting a net, you need some kind of a checklist in hand of what you can expect and what you want as a end product. Document Every Detail of Your Board Stackup for Manufacturing It’s important that you use enough time to define and document your layer stackup requirements thoroughly. This is a perfect moment to get together with your manufacturer to determine which materials or IPC spec you should use for your board, and which specific design rules you should follow. We have many customers who we help at the start of a project to guide them in the correct way, so they have the best starting point for a PCB which can be produced while having thought about the cost and quality drivers. IPC-4103 specifies materials for High Speed / High Frequency Applications. Underneath some examples: FR-4 This is a great material when you’re working with clock speeds of < 5Gbps and is classified as a low speed material. FR-4 has a decent ability to control impedance and is also known for its low cost depending of its characteristics. Nelco, SI or Megtron In the realm of high speed design, you’ll likely be working with these materials. Each is suited for 5-25 Gbps clock speeds. Also price and lead times are much better if you compare it to the Rogers material. Rogers If your first high speed design is pushing 56Gbps, then you’ll likely end up using a Rogers laminate. This is a high frequency, high-temperature material known for good impedance consistency, but it’s also expensive to produce and has long lead times. Match your impedance Impedance matching means that when energy is transmitted, the load impedance must be equal to the characteristic impedance of the transmission line. At this time, there is no reflection in the transmission, which indicates that all energy is absorbed by the load. On the contrary, there is energy loss in transmission. In high-speed PCB design, the matching of impedance is related to the quality of the signal. When Do You Need to Do Impedance Matching? It is not mainly to look at the frequency, but the key is to look at the steepness of the edge of the signal, that is, the rise/fall time of the signal. It is generally considered that if the rise/fall time of the signal (in 10% to 90%) is less than 6 times the wire delay, it is high speed. Signals must pay attention to the problem of impedance matching. The wire delay is typically 150ps/inch or ps/mm. Characteristic Impedance During the propagation of a signal along a transmission line, if there is a consistent signal propagation speed everywhere on the transmission line, and the capacitance per unit length is the same, then the signal always sees a completely consistent instantaneous impedance during the propagation process. Since the impedance remains constant throughout the transmission line, we give a specific name to indicate this characteristic or characteristic of a specific transmission line, and call it the characteristic impedance of the transmission line. The characteristic impedance refers to the instantaneous impedance value when the signal is seen along the transmission line. The characteristic impedance is related to the board layer on which the PCB conductors are located, the material (dielectric constant) used by the PCB, the trace width, and the distance between the conductor and the plane and has nothing to do with the trace length. The characteristic impedance can be calculated using software like speedstack and Si9000. In high-speed PCB layout, the trace impedance of a digital signal is generally designed to be 50 ohms, which is an approximate number. Generally, the coaxial cable base band is 50 ohms, the frequency band is 75 ohms, and the twisted pair (differential) is 100 ohms. Of course there are many other things to think about when you are designing a high speed PCB. Do not hesitate to contact NCAB group for your technical questions Randy Wessels, KAM NCAB