Real-world Impedance: It's All in the Build
October 15, 2007 | Polar InstrumentsEstimated reading time: 4 minutes
Regardless of their end product, engineers' objectives for PCB fabrication are always the same; they want a smooth transition from their simulated PCB layout to a real-world board which performs exactly as predicted. However, not all boards come up to initial expectations and, all too often, the fault can lie with a basic error in the calculations for the controlled impedances within the board.
As a fabricator of fast-turn, high-density interconnect and flex rigid PCBs, eXception manufacturers boards for some of the UK's leading Formula 1 racing teams as well as for clients in the semiconductor, communications, defence, avionics and medical industries.
Many customers come to eXception with a basic requirement such as a 12-layer PCB with controlled impedance on three specified layers. Typically, they will have worked their design based on calculations that they have performed themselves using any one of a number of tools. However, it is quite possible that, during the engineering of the PCB, these calculations can be shown to have provided a false result which makes the design unworkable in its original format.
It is hardly surprising that controlled impedance can challenge even the most skilled designer. A decade ago, there were few high-speed components and only around 20% of boards needed to control impedance. Today, the increasing number of high-speed interconnects mean that designers are using differential signals and balanced traces to improve noise immunity and reduce timing errors on boards. Impedance control is, therefore, now an imperative for up to 80% of boards.
Just as the boards have increased in speed, frequency and complexity, so impedance calculations have necessarily become more complex. A few years ago, impedance could be calculated using relatively simple equations to provide approximate trace dimensions for different impedances. The resulting calculations were accurate enough for line widths and spacing's in excess of 15 thousands of an inch but, crucially, fail to deliver the accuracy required by higher frequency circuits or more complex PCB technologies.
Controlling impedance is no longer simply a matter of calculating the length and width of the trace. As PCB technology has evolved, so has the number of possible trace configurations such as embedded or edge-coupled coated microstrip, offset stripline or edge-coupled offset stripline traces. Each of these configurations can have their own impact on the actual impedance of the trace.
Other factors play a part too. The characteristics of the materials from which the PCB is manufactured, and the manufacturing process itself, must also be included in the impedance calculation. For example, the dielectric constant of an FR-4 laminate is nominally 4.2, although this can vary between batches, and other materials have will have different electrical characteristics. In addition, the presence of solder, and the etching process, can affect the real-world impedance, particularly on narrower, higher-impedance traces.
Predicting impedance effectively means considering the configuration and geometry of the traces as well as the thickness of the PCB layers and the characteristics of different materials.
Fortunately for designers, there are a number of software tools available to help them to perform these complex calculations. The down-side, however, is that not all of these tools perform the calculations to the same degree of accuracy.
The difference between the predicted, theoretical impedance and the actual impedance of the physical board is usually highlighted in the early stages of fabrication. eXception uses Polar Instrument's CITS800 Time Domain Reflectometer (TDR) test system, to measure single-ended and differential impedance on test coupons, or on the boards themselves if the traces are accessible.
Where discrepancies are found between the customer's predicted impedance and the actual impedance, eXception has a number of options which can resolve the problem without sending the design back to the drawing board. Using Polar Instruments' Si8000 boundary-element method field solver eXception can verify and re-calculate the traces to achieve the required impedance.
The ease and flexibility provided by the Si8000 allows eXception to enter a number of parameters such as the trace's target impedance, track thickness, dielectric constant and core material thickness, and to goal-seek the correct track dimensions. Both nominal and worst-case scenarios can be plotted giving eXception an indication of process yield prior to the actual build. The Si8000 is used in conjunction with Polar's SB200a stack-up builder with which eXception can create and document an 8-layer board in a matter of minutes, and efficiently manage high layer-count builds. The SB200a provides graphical 3D images and 2D representations of the intended build and documents information on layers, materials and gerber files as well as the finished thickness of the PCB, and easily share this information with customers' designers.Polar's Si8000m allows eXception to goal-seek to the correct track dimensionsMany years of experience with Polar Instruments tools, have given eXception an extensive library of successful standard and special builds, such as controlled impedance boards for the Rambus architecture or boards with +/-5% impedance tolerance. Whilst this can help to fast-track corrections to customers' designs, the most sensible approach is for engineers to avoid designing their boards based on inaccurate calculations.
There are two options for ensuring that the initial impedance predictions will be workable in the real-world: Engineers can ask eXception to perform the calculations using Polar's field-proven tools, or they can invest in the tools themselves.
Basing designs on the solid foundations of calculations provided by eXception, or on calculations performed on the same tool-set as the fabricator, can make a significant contribution towards a clean process from the simulated board to the functional real-world PCB.Sharing build information with engineers increases build efficiency