Introbotics High Frequency Testing Rise Time.

High Resolution Testing for Complex Printed Wiring Boards

Stephen Tobin, Vice President of Introbotics

It makes sense to get the most amount of information from each electrical test. Printed wiring boards (PWBs) are getting more complex and more difficult to test. The complexity is caused because: component pitch size is shrinking, board size is shrinking, board operating frequencies are increasing, circuit power levels are decreasing, and boards handle both digital and analog signals (mixed signals). We refer to this class of board as a Complex PWB. Complex PWBs are more difficult to test and more information is required from each electrical test to verify acceptable PWB operation. High resolution testing (the ability to gather a significant amount of meaningful data from each test) is the way of the future and is the subject of this paper.

Today's PWB testers don't provide enough relevant test data for the digital board designers of today and tomorrow. Bed-of-Nails and flying probe testers only report on DC electrical continuity. DC continuity tells nothing about how a small pitch PWB will perform at gigahertz (10⁹ Hz) operating speeds in a mixed signal environment. Hence, OEM's are beginning to require controlled impedance testing because it produces more relevant information. The best way to test how well impedance is controlled is by using Time Domain Reflectometry (TDR). TDR operates by sending a tiny burst of high-frequency signal into a PWB network and then listening for reflected signals. The reflected signals can indicate opens, shorts (just like DC testing), but also detect variations in characteristic impedance (Z_o), propagation delay, and stray capacitance or inductance that may cause PWB failure at operating speeds. We call this High-Resolution testing.

Today's controlled impedance testing techniques rely on manual probing using a TDR oscilloscope or comparable device and a hand probe. This is a slow technique with low repeatability and equally low accuracy. Manual probing is only applicable for coupon or sample testing. Besides the inherent repeatability/accuracy problems, samples and coupons do not ensure that each PWB will actually operate properly.

Introbotics has introduced the CI1000™ Controlled Impedance tester to overcome the problems associated with manual testing. The CI1000™ is a robotic system with extremely high accuracy and repeatability. The CI1000™ is designed for fast testing throughput so 100% of PWB's can be tested. The CI1000™ is the tester of choice for complex PWBs. A Tektronix TDS 8000 oscilloscope with an 80E04 TDR plug-in is used to make TDR measurements in the CI1000™. Single-ended or differential impedance measurements can be made on a routine basis. Additional CI1000™ software records the TDS 8000 readings and creates reports. A WaveForm viewer is also available from Introbotics to view the detailed test results of each trace, including impedance, propagation delay and dielectric constant.

The CI1000™ uses a patented "high-resolution" probe with extremely fast rise and fall times on the order of 35 pico-seconds (10^-12). The high-resolution probe allows the CI1000™ to generate far more information about each electrical test than other TDR/controlled impedance testing techniques. To demonstrate the superior performance of the CI1000™ high-resolution probe we selected a single ended microstrip (picture following). The entire microstrip is 3.95 inches long, not including the end-launch connector. The microstrip was connected to a Tektronix TDS 8000 Oscilloscope that was set for TDR measurements. The Oscilloscope was set to record at a resolution of 35 pico-seconds on channel C1 (shown below in green) and at 200 pico-seconds on channel M1 (shown below in white). Both channels were set at 10 ohms per vertical division. Channel C1 (35 pico-seconds) represents the sensitivity of the CI1000™ high-resolution probe, while channel M1 (200 pico-seconds) represents the sensitivity of most of today's hand probes.

The discontinuities deliberately shown on the microstrip illustrate the ability of TDR to detect and locate over-etching and under-etching. In production a high-resolution probe can detect and pinpoint many other types of PWB discontinuities such as: mouse nibbles, misplaced vias, misaligned solder masks, etc. Most importantly TDR can also identify design flaws and potential problems associated with impedance mismatches, spurious reflections, and ringing.

The following is a picture of the microstrip and a screen shot of the Oscilloscope traces and settings.

We have exaggerated the size of the microstrip (shown above) to better correlate discontinuities with the oscilloscope traces. The screen of the Tektronix TDS 8000 is condensed to accommodate the graphics format.

We observed the following microstrip characteristics on the TDS 8000:

A	The first peak shows the rapid change in impedance caused by a small air gap between the end-launch connector and the microstrip. The second peak shows the effect of an indentation in the trace ground plane. To the left of the twin impedance peaks are small ripples showing the good impedance match between the end-launch connector and the coaxial cable test lead. I
B	Shows the reduced impedance caused by the increased width of the trace. In production this could be caused by under-etching or poor design.
C	Shows the increased impedance caused by the narrower width of the trace. In production this could be caused by over-etching.
D	Shows the even further increased impedance caused by the even narrower width of the trace.
E	Shows the reduced impedance caused by the increased width of the trace.
F	Shows a dramatic increase in impedance at the termination of the trace.

Key findings from this test are:

	A High-Resolution probe gives substantially more detailed information about the electrical properties of a trace. In this example it is possible to derive information on trace lengths as short as 1/2", with "warnings" at dramatic continuities (as shown at point "A").
	The impedance measurements provided by the high-resolution probe allow a manufacturer to diagnose the characteristics of trace segments and to decide if the electrical properties are satisfactory.
	The low-resolution (200 pico-second) probe is not capable of providing significant data on the characteristics of trace details for Complex PWBs.

Do you have comments or questions on this paper? Please click on "stephen.tobin@introbotics.com" to send an email to Stephen Tobin, VP Engineering at Introbotics.

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