Engineers are beginning to face new challenges related to the design of printed circuit boards (PCBs) and their integration into an enclosure, either metal or plastic. These challenges will increase in the future with higher speed components and systems.
With the need to maximize functionality, while at the same time shrinking the physical size of a product to lower cost of development and production, we are discovering that signal integrity is becoming a greater concern than EMC. Although a system may radiate EMI in a broadband environment, will EMI be a concern in the future or should we enhance immunity protection against any and all electromagnetic threats that may occur?
To quote Eric Bogatin, signal integrity evangelist, “There are two kinds of design engineers, those that have signal integrity problems and those that will.” This means EMC engineers need to take a greater role in working with designers on creating products. The concern is that both system and PCB designers along with EMC engineers must understand multiple aspects of doing a board layout in a manner that ensures not only functionality but also compliance. What I tell students and clients is, “The magnitude of a signal integrity problem is the magnitude of common-mode RF current that is developed.” Basically I am saying that losses in a transmission line create common-mode EMI and that it is sometimes easier to work in the time domain instead of the frequency domain.
A transmission line, commonly called a trace on a PCB, allows an electrical signal to travel from a source to load through a dielectric as an electromagnetic field. During propagation, we must ensure there is no loss in any parametric value (voltage, current, propagation delay, timing, edge rate distortion, etc.). If the transmission line is perfectly lossless, the RF return current will be equal to the source path. This is called differential signaling. Any loss in the transmission line creates common-mode current. Thus, if there is significant loss in either the source or return path of the transmission line, common-mode EMI is developed and will propagate by any means available.
As frequencies get higher, we notice that second order effects cause loss to occur in a transmission line. While EMC engineers focus on making sure there are sufficient decoupling capacitors and that no traces cross a moat or is routed adjacent to a plane with a proper referencing along with other standard rules of thumb, there are now additional items of concern that can cause a signal integrity problem. In the future, engineers must learn about the following sample list of signal integrity concerns:
Incorrect transmission line routing; improper terminations; power and/or return plane bounce; rise time degradation; lossy lines at high frequencies due to board material usage; hidden parasitics (RLC); skin depth losses; dielectric loss in the board material; propagation delays due to high dielectric constant board material; crosstalk; excessive inductance in the transmission line routing; delta I noise; overshoot and undershoot; IR drops; copper roughness; anisotropic aspects of the board material; and RoHS (affects delamination and creates tin whisker), to name a few.
In essence, we can no longer consider only electromagnetic aspects of both time and frequency domain design, but must now become knowledgeable in material science to enhance signal integrity and minimize development of common-mode current. Almost all of the items mentioned above that affects signal integrity are caused by improper selection of the material used to create the PCB, generally FR-4. My definition of a PCB is “a physical structure used to mechanically support transmission lines”.
|Mark I. Montrose
is an EMC consultant with Montrose Compliance Services, Inc. having 30 years of applied EMC experience. He currently sits on the Board of Directors of the IEEE (Division VI Director) and is a long term past member of the IEEE EMC Society Board of Directors as well as Champion and first President of the IEEE Product Safety Engineering Society. He provides professional consulting and training seminars worldwide and can be reached at firstname.lastname@example.org.