CircuiTree - July 2008 - (Page 23)

This test is designed to give fabricators a ﬁrst indication of a change in loss characteristic on a given transmission line structure/ layer stackup. This change in characteristic loss may be due to a number of factors including incorrect material used in a stackup. It may also be used in conjunction with other techniques (more suited to lab/ QA use) to ﬂag the need to escalate problem builds for more detailed laboratory analysis. The RIE method has been presented in previous papers and presentations.1 This paper looks at some of the more practical implementations of deploying this method in a conventional production environment. TDRs from two manufacturers were used for the acquisition of results in this paper. probe coupon interconnect are not as critical as with higher speed systems. Overall, it presents a more repeatable and reproducible test solution in the current factory environment, compared to laboratory equipment with faster risetimes. For the purpose of this study, instruments from two different vendors were used. TDR1 hardware has a reﬂected risetime of 250 ps. TDR2 has a faster risetime, so the waveforms were mathematically ﬁltered to simulate the results of using a 250 ps risetime using software features embedded in that equipment. ning with the warp or weft of the material, thus minimizing the effect of dielectric variation sometimes caused by trace alignment with the ﬁber weave.3 Test Board Design A test board was designed to identify the answers to some practical problems: 1) Can RIE discern the difference in signiﬁcant changes in dissipation factor (Df, aka loss tangent)? 2) How long must the RIE trace be in order to attain suitably precise RIE results? 3) Is an HVM-compatible probe footprint (2.54 mm, or 0.1in) adequate, or do we need a more precise micro-probe footprint? 4) If serpentine routing is necessary (to accommodate a lengthy trace), what is the minimum separation required between legs of the serpentine? 5) Is there a signiﬁcant difference between microstrip and stripline? 6) Do vias have a signiﬁcant effect? 7) What are the environmental (relative humidity, temperature) effects on loss measurements? The following board (Figure 1 and 2) was manufactured in several materials: Isola IS410 (1080), IS410 (2116), IS408, and Nelco N4000-13. The board was designed to be ~2.34 mm (0.092 in) thick to mimic thick server boards, in which via effects are worst. Figure 2 Test Board Layout RIE—Root Impulse Energy The RIE method employs a modiﬁed version of the standard impedance test coupon, which contains a short reference line, and a longer test line. The RIE method compares the reﬂected TDR signal of the short trace with the longer sample, and by differentiating the resultant reﬂection and taking the root of the area under the resultant curve, calculates a measurement proportional to the high-speed losses encountered on the structure.1 Practical Considerations for the Implementation of RIE Testing Goals Different length traces were included on this board to see if reliable results could be obtained from shorter lengths. In addition, the 300 mm (12 in) traces were folded back on themselves to try to eliminate the need for a large amount of linear board space. Probe Layout A consistent interconnect to the coupon under test is key to achieving good R&R. For production use this has to be easy, repeatable, and, ideally, suited to both hand and automated test systems. A back to back layout of coupon was investigated (Figure 3) so that both the long and short reference lines could be addressed in a single probing action if required. An ideal RIE measurement system would: • Ensure robust and reliable contact; • Occupy minimal room on a production panel; • Generate repeatable measurements; • Use reliable instrumentation; • Be instrument manufacturer agnostic; • Be tolerant of local environmental conditions; • Produce measurements which are predictable; and • Be suitable for high volume manufacturing (HVM) conditions – Cost-effective equipment; – Robust equipment; and – Straightforward operation/analysis. Figure 3 Probe Connections RESULTS Different length traces In a production environment there is signiﬁcant pressure to minimize board panel area consumed by coupons and other nonproductive artifacts. To this end, two coupon styles were evaluated. Both would have the 25 mm (1 in) reference trace but the second trace would be 300 mm (12 in) long in one case and 200 mm (8 in) long in the other. It is interesting that there is a signiﬁcant difference in the magnitude of the returned RIE result between the two TDR systems used. Although there is a block shift in the data magnitude of TDR results, the underlying differences between materials are simcircuitree.com • July 2008 23 Instrumentation For practical purposes the robustness of a 250 ps TDR is preferred on the PCB fabrication ﬂoor. A 250 ps system, while still a sensitive RF measurement tool, is not as susceptible to ESD performance degradation as can be the case with faster risetime systems. In addition, at 250 ps, the inﬂuences of the Figure 1 Test Board Stackup STACKUP Board Layout The test traces were placed at a slant of 10 degrees (Figure 2) to avoid the trace run- http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - July 2008

CircuiTree - July 2008 Contents My Line Industry Review Tech Talk Flexible Thinking Toward a PCB Production Floor Metric for Go/No Go Testing of Lossy High-Speed Transmission Lines Intelligent Design 20-Year Retrospective Ask the Flexperts Environmentally Speaking BPA Growth Curves Considering Design Variants to Maximize Process Efficiency Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - July 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.