[Editor's note : we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of Kimmel Gerke Associates. Note that there are links to all previous entries at the end of this item.]
After the critical circuits, we like to examine the printed-circuit board (PCB) stackup. Like the schematic, board stackup decisions are often made prior to routing and placement, so this is a good time to look at the proposed stackup. When routing and placement are complete, a second EMC review of the board is prudent. Stackup matters for EMC!
The first stackup question is the number of layers. If it is one or two layers (no power or ground planes), this raises serious EMI serious concerns about both the layout and the technology used. With today's devices, one- or two-layer designs are usually OK for embedded controllers with clocks under 10 MHz. But even here, EMI precautions must be taken.
We'll address EMC design recommendations for one/two layer guidelines in a future post. For now, we'll assume multi-layers. As a minimum, you can start with four layers: a ground plane, a power plane, and two signal layers. The multi-layer concepts can be extended to as many layers as you want or need (or can build.)
Multi-layer boards are preferred for high-frequency designs. This generally means RF (radio frequency) circuits or digital circuits with clocks over 10 MHz. But even low-frequency analog circuits (audio or instrumentation) can benefit from multi-layer designs when subjected to RF susceptibility requirements. Remember, low-frequency circuits and be affected by high-frequency threats.
Our experience has shown that multi-layer boards provide at least 10× reduction in radiated emissions, and 10× improvement in immunity (both RF and ESD.) We have repeatedly seen that when replacing simple two-layer boards with four-layer boards. The power and ground traces are now solid planes — everything else is the same. But as the old saying goes, your mileage may vary.
The first EMC miracle occurs due to proximity of the planes to signal traces. The image-plane effect provides a return path for high-frequency currents that greatly reduces loop size. Every trace is now a transmission line, instead of an unwanted loop antenna. This works as long as the adjacent plane is continuous all along the trace.
The second EMC miracle occurs due to reducing power/ground loops and impedances. These loops form additional hidden antennas for emissions and immunity. We refer to these loops as the “back door” for EMC. No, the clock Vcc CURRENT is NOT CONSTANT; rather, the clock Vcc current pulses at the clock frequency as the internal loads change.
So what do we look for in the board stackup? Here are four simple features to examine:
(1) Are all trace layers adjacent to a solid plane? Either power or ground planes are fine, since a well-decoupled power plane is just another high-frequency ground (return) plane.
(2) Are associated power and ground plane adjacent? For example, are the analog-voltage planes next to analog ground, and digital-voltage planes next to digital ground? Are there overlaps?
(3) If a plane is split (common for voltage planes), will traces run across these cuts? If so, you are just begging for EMI problems. It is best to define design rules prior to routing, and then examine the actual routing results. Additional split-plane issues will be addressed in a future post.
(4) Are the trace and solid planes symmetrical about the center of the board? While not an EMC issue, this can affect board construction and is worth a quick look.
For simple circuit boards, the entire process of examining critical circuits and the board stackup for EMC should not take more than a couple of hours . This time is well spent early in the design, to prevent EMI disasters later!
[Additional Editor's Note : for a somewhat “whimsical” look at printed circuit boards, click here .]
Previous entries in the series
EMC Basics #1: Welcome!; and Clocks: critical circuits for EMC
EMC Basics #2: Resets as Critical Circuits
EMC Basics #3: Voltage regulators as critical circuits
EMC Basics #4: Analog devices as critical circuits
EMC Basics #5: I/O as critical circuits
About the author
Daryl Gerke , an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still in relevant and in demand. He can be reached via http://www.emiguru.com or his other blog at http://www.jumptoconsulting.com/.