A Fast and Inexpensive Method for PCB Trace Characterization in Production Environments

A fast and cost-effective method for performing loss measurements on differential traces in a printed circuit board manufacturing environment utilizing two-port structures and measurement instruments is provided. The method is an alternative to the SET2DIL method presented by Intel two years ago at DesignCon. Patent Disclosure Portions of this document are the subject of patents applied for. Biography Pete Pupalaikis was born in Boston, Massachusetts in 1964 and received the B.S. degree in electrical engineering from Rutgers University, New Brunswick, New Jersey in 1988. He joined Teledyne LeCroy (formerly LeCroy Corporation), a manufacturer of high-performance measurement equipment located in Chestnut Ridge, New York in 1995 where he is currently Vice President of Technology Development. He currently manages integrated-circuit development, signal processing technology development and intellectual property. Prior to LeCroy he served in the United States Army and has worked as an independent consultant in embedded systems design. Mr. Pupalaikis has twenty nine patents and numerous publications in the area of measurement instrument design and is a member of Tau Beta Pi, Eta Kappa Nu and the IEEE signal processing, instrumentation, and microwave societies. In 2013 he was elevated to IEEE Fellow for his contributions to high-speed waveform digitizing instruments. Kaviyesh Doshi received his Ph.D. in electrical engineering from University of California, Santa Barbara in June 2008. He joined Teledyne LeCroy, a manufacturer of high-performance measurement equipment located in Chestnut Ridge, New York in August 2008, as a Research & Development Engineer. At Teledyne LeCroy, he has been involved in design and development of SPARQ – a TDR based instrument for measuring s-parameters. He has filed patents in the area of de-embedding and signal processing for time domain network analyzer. Introduction LATELY, A LOT OF PERFORMANCE is demanded from critical traces on a printed circuit board (PCB). These performance demands require differential traces, low loss, and controlled impedance. PCB manufacturers produce high performance boards by offering improved materials and tighter dimensional tolerances. This allows board designers and engineers to design faster and denser on-board communications channels. The traditional language of communicating specifications between design and development engineers and the board manufacturers is the language of materials and dimensions. This means that boards are specified with various material stackups dictating material and thickness along with dimensions for widths, lengths and diameters of holes, pads and traces. As such, the specifications are mechanical and material in nature. As boards carried faster and faster signals, design engineers became focused on impedance and loss. Since these specifications are functions of materials and dimensions, it was incumbent upon them to make the correlation between impedance and loss and other electrical characteristics and translate these specifications into materials and dimensions that get communicated to board manufacturers. The board manufacturer might not know why or even care why certain materials, dimensions, and dimensional tolerances were being specified; they just knew that they were being relied upon to meet the specs. In the ideal world, from an electrical engineers perspective, the electrical characteristics of the board are specified to the board manufacturer. This is unrealistic in that this is not the board manufacturers area of expertise. To come closer to this ideal world, the design of the board comes from the board designer and certain electrical characteristics are measured and verified. Previously, this was done only in a design phase with the hope that the board manufacturer produces a consistent product. Lately, the goal has become to measure electrical characteristics on the production line as a board is being manufactured. Again, the main characteristics required are those of impedance and loss. Since the standard transmission medium today is differential, this requires the measurement of fourport trace structures. A reasonable plan is to produce these test structures in an area of each PCB panel produced. This area is called a test coupon and does not appear on the final circuit board. The testing requirements for boards produced in this manner create some difficult problems: 1. The popular method of measuring performance of microwave devices (which these differential traces have become) is to use the vector network analyzer (VNA). The VNA is a very expensive measurement instrument well suited for laboratory measurements. It is not well suited for production measurements. 2. The popular method of measuring performance of high-speed devices is to use time-domain reflectometry (TDR)1. TDR instruments are less expensive instruments, but are also not well suited for production measurements. 3. The measurement of four-port structures is slow using TDR or VNA techniques. Several solutions are in use today for the purpose of impedance measurement. Impedance measurement requires a one-port or two-port (actually viewed as a one-port differential-mode) instrument. Impedance measurements can be performed easily with TDR. Today, the requirement has been extended to loss and the speeds have increased. High-speed loss measurements would generally require a four-port VNA. 1We will use TDR and time domain transmission (TDT) interchangeably.