Miroslav Kotzev

Accuracy of Physics-Based Via Models for Simulation of Dense Via Arrays

This paper studies the accuracy of the physics-based via model, specifically when applied to dense via arrays. The physics-based model uses Greens functions for cylindrical waves in radial waveguides to model the via return current paths and the coupling between vias. The effects of approximations made in this model are studied with regard to four types of modes based on an eigenmode expansion for the radial waveguide. It is found that for the mode conversion in the vicinity of the via, an accurate consideration of nonpropagating modes becomes critical with an increasing cavity height. For the interaction between vias in dense arrays, anisotropic modes have an impact for small pitches, whereas the coupling by nonpropagating modes is small for practical printed circuit board dimensions. For a data rate of 20 Gb/s, conclusions with regard to the applicability of the physics-based via model to a multilayer structure are drawn. For 80-mil pitch, a good agreement to full-wave results can be observed. Measurements have been carried out to validate this finding. For 40-mil pitch, the accuracy of the physics-based via model is not sufficient for data rates of 20 Gb/s or higher.

Crosstalk analysis in high density connector via pin fields for digital backplane applications using a 12-port vector network analyzer

In this paper the authors present results from the crosstalk analysis of a high density single ended connector and its associated card via array obtained with 12-port vector network analyzer (VNA) measurements in the bandwidth from 10 MHz up to 20 GHz. The device under test used for this paper is typical for a high end mainframe processor node to node link scenario consisting of daughter cards plugged into a backplane card by using a multipin connector. In previous studies the authors have shown that mainly the connector via pin field is impacting the electrical link performance. Here, the measurements have shown that the via pin field constitutes a complex crosstalk problem depending on the orientation and the distance between victim and aggressor via, the common coupled via lengths, and the local power/ground environment.

Electrical Performance of the Recessed Probe Launch Technique for Measurement of Embedded Multilayer Structures

This paper explores the electrical performance of a recessed probe launch (RPL) technique for measurement of embedded (internal) printed circuit board (PCB) structures such as striplines and vias (plated through holes). The RPL uses high-frequency microprobes in combination with a milling technique that removes upper board layers and exposes internal probing points. A two-tier calibration technique is applied for the extraction of the S-parameters of the RPLs with the help of thru-reflect-line calibration standards designed on PCB. Full-wave models are used to corroborate and validate the obtained S-parameters. Good model-to-hardware correlation is obtained up to 40 GHz. Furthermore, the launch performance with respect to microprobe positioning and variations of the cavity size is investigated using the full-wave models. These simulations suggest techniques for launch optimization and probe modifications that improve the launch measurement bandwidth.

Extraction of broadband error boxes for microprobes and recessed probe launches for measurement of printed circuit board structures

This paper presents results for the error boxes (two-port scattering parameters) of ground-signal-ground (GSG) mini-coaxial probes (microprobes) and recessed probe launches up to 20 GHz. The recessed probe launch uses internal GSG pads for the access of embedded printed circuit board (PCB) structures without the need for signal vias. The error boxes were obtained by using two-tier calibration techniques where the first tier was based on short-open-load-through (SOLT) and the second tier on short-open-load (SOL) or through-reflect-line (TRL) calibration algorithms. Results from different calibration set ups are compared with each other and agree well. The obtained error boxes can be used for deembedding or analysis of the microprobe and probe launch performance.

Bandwidth Study of Recessed Probe Launch Variations for Broadband Measurement of Embedded PCB Structures

In this paper the recessed probe launch technique for measurement of embedded (internal) printed circuit board (PCB) structures such as striplines and vias (plated through holes) will be reviewed and its bandwidth will be studied in experiments. The recessed probe launch (RPL) uses high frequency microprobes in combination with a milling technique that removes upper board layers and exposes internal probing points. By providing direct access to the PCB layers of interest a broadband launch into embedded structures is thus provided. Here, measurements of different RPL designs show that reflections of less than -25 dB from DC up to 40 GHz can be achieved. First investigations show that this level of reflection is mostly due to a mismatched stripline impedance in the 52 Ohm range and not due to the recessed probe launch itself. Full-wave simulations are underway to corroborate the results obtained by measurements.

Multiport measurement and deembedding techniques for crosstalk study in via arrays

In this paper the authors present techniques for crosstalk measurements in via arrays using broadband microprobes and a multiport vector network analyzer. The desegmentation method is used for probe deembedding in the bandwidth from 10 MHz up to 50 GHz. In contrast to the common approach which relies on T-matrices the desegmentation procedure ensures flexible fixture extraction using directly the S-parameter or Z-parameter matrices of the multiport network that has to be deembedded. The main advantage of the deembedding by desegmentation technique is that it does not require any S- to T-parameters conversion and if desired allows stepwise extraction of the embedded error networks. A typical application of the desegmentation method is presented by the authors where in the case of microprobe based measurement the error boxes of the microprobes are obtained at first by the use of a two-tier calibration technique and in the next step the desegmentation is applied to remove their detrimental effect on the measurement data.

Novel multiport probing fixture for high frequency measurements in dense via arrays

In this paper the authors present a novel multiport probing fixture for high frequency measurements of dense via arrays. The probing fixture consists of a 36 mm by 36 mm multilayer printed circuit board plugged into an LGA socket clamped down to the device under test using custom hardware. The test signal is launched from top surface mounted coaxial connectors and fanned in with striplines to a 1 mm via pitch on the bottom side. Custom calibration substrates are used to deembed the probing fixture from measurements by applying multiport deembedding techniques. The effect of contact pressure on obtaining reproducible measurements is investigated.

Lessons from applying IEEE standard 1597 for validation of computational electromagnetics computer modeling and simulations

In this paper guidelines and benchmark examples from IEEE standard and recommended practice 1597 for validation of computational electromagnetics computer modeling and simulations are applied with the aim of evaluating their practicality and consistency. Benchmark examples include a thin dipole antenna, a loop antenna, and a rectangular cavity with two apertures. In addition, a fourth example—consisting of a box-shaped monopole antenna on a finite plate—is measured and simulated. All examples are investigated by means of three different numerical techniques commonly applied by EMC engineers using two commercially available software tools and one self-developed software tool. Numerical results for input impedance, electromagnetic field values, farfield pattern, power budget, and shielding effectiveness obtained by the different techniques are compared and discussed with regard to the validation procedure outlined in IEEE standard 1597. Applicable simulation results are compared to results from closed-form equations and measurements. Observations with regard to differences between the techniques are outlined, and recommendations for a pragmatic application of IEEE standard 1597 are given.

High frequency measurement techniques for vias in printed circuit boards

This article provides an overview of some multiport measurement techniques for dense via arrays, which play an important role in high-speed links as package and connector footprints in multilayer printed circuit boards. Advantages, challenges, and limitations of three approaches for high-frequency measurements up to 50 GHz are discussed: launches with fan-out traces and on-board coaxial connectors, double-sided direct surface probing with RF microprobes, and recessed probe launches with extension traces and microprobes. The measurement results obtained by the presented measurement techniques are then compared to simulation results computed using full-wave and physics-based via models. Finally, the multiport probing approach with interposers is addressed through a case study of simultaneous access for up to 12 vias in a 1 mm pitch via array that can be applied for measurements in the frequency range up to 10 - 15 GHz.

Electrical performance of a multiport interposer for measurements of dense via arrays

In this paper the bandwidth limitations of two types of multiport probing fixtures for measurements of high density via array structures are investigated. Using a two-tier calibration technique and custom-made calibration substrates, error parameters for the probing fixtures were extracted and used to evaluate variability in measurements and substrate correction parameters. Results show adequate repeatability of the obtained error parameters up to 20 GHz. Digital links were measured utilizing these probing fixtures and results were compared to S-parameter measurements using RF microprobes. Good correlation up to 10 GHz – 15 GHz was established when compared to microprobe based measurements of link structures with stub access vias. For link structures with through access vias the adequate correlation bandwidth was founded to be 5 GHz – 10 GHz.