There was a time when resistors, capacitors, filters, FETs and other discrete devices seemed to be the Rodney “I don’t get no respect” Dangerfields of the component world. But all that has changed, since advancements in IC technology have impacted these components.
Discrete components have become smaller, operate at ever higher frequencies and exhibit operational characteristics approaching theoretical limits. Component test equipment has kept pace. Some new models measure resistance values as low as a microhm or as high as a teraohm, filter characteristics in the gigahertz region or MOSFET on-resistance in the milliohm range. Many of these are easier to use than ever before and cost less to own.
The Venerable RLC Meter
The Wheatstone bridge has been used to accurately determine resistance values since the middle of the last century. Later, it was modified to determine capacitor, inductor and complex impedance values.
More recently, the bridge measurement technique has been supplemented by measuring the current flowing through the component in response to an excitation voltage, followed by an automatic calculation of impedance and other parameters. Hence the familiar term “RLC bridge” has been superseded by the more general “RLC meter” nomenclature, covering either technique.
For most component measurements, one AC excitation source, usually 1,000 Hz, is sufficient. For some applications, however, it is necessary to determine the impedance of a component in its actual operating frequency range. As a result, a number of RLC meters provide variable test frequencies extending to 100 kHz, several megahertz and even into the gigahertz region.
If the characteristics of nonlinear components must be evaluated, it may be necessary to vary not only the frequency but also the magnitude of the excitation signal. To perform these tests efficiently, some RLC meters provide automatic sweeping and sequencing features.
“The 7000 series allows sweeping voltage, current or frequency over any portion of the available ranges,” said David Warnock, Chief Engineer at QuadTech. “Test frequencies extend from 10 Hz to 2 MHz, allowing component testing at exactly the frequency of interest, not a compromise forced by older designs.
“Test sequences, consisting of up to six separate tests, may be programmed,” Mr. Warnock continued. “Each test is totally independent and may have its own set of test conditions. “For example, a capacitor could have C and DF tested at 120 Hz, ESR at 100 kHz, and Z and Phase at 1 MHz, all at the single push of a button. This feature is extremely useful in production testing as well as QA because it assures uniform test conditions and reporting,” he said.
Many of today’s RLC meters also provide data links to PCs, and some contain built-in disk drives or PCMCIA facilities. This allows you to utilize additional PC analysis tools to benefit more from your measurement or quality-analysis activities.
Testing at Higher Frequencies
When testing at higher frequencies, several problems prevent us from making measurements directly with bridges or from deriving results from voltage/current ratios. The wavelength of the excitation signal becomes small compared to the physical dimensions of the device, and signals arriving at two closely spaced points can be at significant phase differences. Also, losses in instruments and cables become larger and parasitic reactances cause additional errors.
So instead of wires, we use high-quality cables as transmission lines to connect to the device. Rather than voltage/current ratios, we measure the ratio (including phase) between the incident signal and the reflected one which traveled down the line and back from the DUT. From these measurements, we can calculate the impedance of the device.1
High-frequency measurements must often be performed on two-port components, devices having an input and an output, such as filters and transmission lines. The behavior of these devices is most efficiently described through s-parameters.
S-parameters are easy to measure and the s-parameters of individual devices may be cascaded to help model and predict the overall system response.
To determine s-parameters, we apply a stimulus to a two-port device in both the forward and reverse directions and measure the reflected as well as transmitted signals. A set of equations (Figure 1) relates the resulting input and output signal behavior to the circuit’s s-parameters which, in turn, can be used to derive all other inherent circuit characteristic parameters.
Scalar and vector network analyzers are best suited to make these measurements. These instruments measure a variety of parameters plus the transmission and reflection characteristics of linear, nonlinear and passive or active devices. Generally, scalar analyzers can only be used to determine magnitude information, while vector analyzers determine magnitude and phase.
Most network analyzers provide swept-frequency facilities, making it possible to determine impedance, amplitude, phase and group delay as a function of frequency. The performance/cost ratio of network analyzers has continuously improved, with prices dependent on the frequency range covered.
Function-Dedicated Component Testers
The bare PCB–which provides the mounting surface for all other components–is itself a component affecting circuit operation. Transmission characteristics–determined by the board material, printed tracks and ground plane–affect high-speed digital-circuit, RF or microwave performance. Insulation resistance may also affect analog circuit performance.
Manufacturing processes as well as material properties can influence PCB performance. The phaseout of fluorocarbons as a cleaning agent for PCBs has required other alternatives, such as water-based cleaning. “The effectiveness of these new processes must be monitored to maintain a high resistance across any insulating material,” said Mark Hoersten, Strategic Marketing Manager at Keithley.
Keithley developed the 6517 Electrometer to allow these high-resistance measurements to be made under a range of voltages and soak times. “With special built-in test sequences, the 6517 is automatically set up to make the required test on capacitors, circuit boards or any other insulating material,” explained Mr. Hoersten.
The Hewlett-Packard HP 4291A Impedance/Material Analyzer addresses all aspects of high-frequency testing of passive components in SMT packages. It is a device family-specific product used in conjunction with a dedicated family of SMD test fixtures. Other function-specific component testers are typified by the HP 4338A Milliohmmeter and the HP 4279A C-V Meter.
Interfacing between the test instrument and the DUT can present special problems for very high or very low resistance measurements or high-frequency applications. All these component testers provide internal compensation features to “calibrate out” the effects of the interconnecting leads or fixtures.
Discrete Semiconductor Test Systems
Advancements in semiconductor technology are also impacting discrete device testing. Devices are faster, handle more power and perform more intricate functions. Some have capabilities approaching theoretical limits.
“The continuing trend in power MOSFET development is to reduce RDS(ON) specifications to very low milliohm levels,” said George Kurtz, President of FET/TEST, Inc. “With many device specifications now approaching 5 m(ohms) as a maximum limit, even the handler-contactor-to-device lead connection can introduce errors approaching 20% of the 5-m(ohm) limit.
“Guaranteeing performance within such tight limits in a production test environment presents some interesting testing challenges,” Mr. Kurtz continued. “To cope with this problem, our testers provide accurate force and sense V/I power supplies and precision pulsed-measurement circuitry to minimize error contributions associated with the force and measurement paths.”
Other major challenges posed by today’s active devices include dynamic test of IGBTs and verification of ancillary functions, such as diagnostic, protection and programmable circuits of smart power devices. “To satisfy their special needs, we developed a new generation of discrete testers,” said Hans Gieszibl, Product Manager at SZ Testsysteme.
Some of the discrete power semiconductors now handle very large amounts of current, voltage and power. Several semiconductor device testers, such as the 5300HS from Scientific Test, feature current levels to 1,200 A and voltages to 5 kV, and still provide the resolution needed for sensitive devices at 1 pA and 1 mV, respectively.
More dynamic tests of discrete components are being performed at the wafer level to save costs that would be incurred if bad devices were packaged. Since discrete devices have relatively few leads, wafer probers can readily make simultaneous contact with several die, setting the stage for parallel wafer testing of these devices.
“The M3020 is the first discrete tester to perform tests of four dies in parallel,” said Mr. Gieszibl. “Multi-die testing quadruples the output of one prober, and saves both clean-room floor space (same throughput in 30% of the space) and costs by more than 50%.”
Alternatives
While some component tests are best carried out on a function-specific tester, many multiproduct test requirements are best served by general-purpose or multifunctional test systems. Alternatively, modular systems may provide the best solution. For instance, Wayne Kerr offers building-block systems that allow the customer to add features when required, according to Mario Masto, Product/Application Engineer.
References
1. RF and Microwave Device Test for the ’90s, Seminar Papers, 1995, Hewlett-Packard Co.
Component Tester Products
Multifrequency RLC Meters
Offer High Precision
The 7400 and 7600 Precision RLC Meters simultaneously measure and display 2 of 14 parameters. The 7400 operates from 10 Hz to 500 kHz and the 7600 from 10 Hz to 2 MHz, both with an accuracy of (+)0.05%. Component parameters may be displayed as a function of conditional (or swept) measurement variables, such as test frequency, voltage or current. Automated test sequencing, calibration and binning are provided. Up to 50 test setups may be stored internally. A 3-1/2(“) disk drive, IEEE 488.2, RS-232, a printer port and a handler interface are standard. 7400: $8,600, 7600: $10,600. QuadTech, Inc., (800) 253-1230.
LCR Meters Perform Over
Range of 100 Hz to 100 kHz
The SR700 Series LCR Meters measure R+Q, L+Q, C+R and C+D at frequencies ranging from 100 Hz to 100 kHz. The SR715 has a basic accuracy of 0.2% and the SR720 accuracy is 0.05%. Three drive voltages and five source frequencies may be selected. Up to nine instrument setup configurations can be stored and recalled. Automatic binning and limit features and RS-232, optional GPIB and parts-handler interfaces facilitate production testing. SMD and Kelvin clips are optional. SR715: $1,295; SR720: $1,995. Stanford Research Systems, (408) 744-9040.
247-1241. Circle 197
Catalog Describes Component
Test Instruments, Techniques
A new 24-page catalog outlines Wayne Kerr’s line of precision component testing instrumentation. The catalog includes LCR meters, component analyzers and precision inductance analyzers with options for DC bias and high AC drive inductance applications. A section defines measurement terms and techniques. Wayne Kerr, Inc., (800) 933-9319.
System Tests
Power Components
The Model ACDC Automatic Magnetics Test System permits determination of DC/DC-converter, power-supply, transformer, battery-charger, coil and winding parameters with one connection. It verifies AC hipot breakdown and insulation resistance; measures input and output voltages, currents, power, resistance; and derives regulation and efficiency. Internal sources supply 0 to 600 VAC or 0 to 60 VDC up to 3 kVA. A PC stores test programs per part number, makes pass-fail decisions, and produces failed-item data tickets. Statistical process control software is available. $29,000. Optimized Devices, Inc., (914) 769-6100.
System Tests Multiple
Die and Packaged Components
The M3020 Advanced Discrete Test System offers high throughput for DC and AC tests of packaged discrete devices and enables multi-die testing at the wafer level. IGBTs, MOSFETs, power FETs, thyristors and triacs can be accommodated. An integrated flexible connection to the test head permits interfacing to wafer probers or automatic handlers. Parallel instrumentation allows simultaneous testing of up to four die. Starting at $140,000. SZ Testsysteme, Inc., (408) 744-0793.
Electrometer Simplifies
High-Resistance Measurements
The Model 6517 Electrometer contains a low-current amplifier supporting input bias current of <2 fA, <0.75-fA p-p noise and <20-uV burden voltage. The input impedance is 200 T(ohms). A built-in +1 kV voltage source with sweep capability simplifies leakage, breakdown and resistance testing as well as volume and surface resistivity measurements. The 6517 offers built-in IPC-compliant test sequences and autoranging over the full span of ranges for current (100 aA to 20 mA), resistance (100 m(ohms) to 1016(ohms)), voltage (1 uV to 200 V), and charge (1 fC to 2 uC) measurements. Keithley Instruments, Inc., (800) 552-1115.Component Tester Features
Throughput of 60,000 Devices/h
The Model 5300HS tests a variety of discrete components, including MOSFET, IGBT, transistor, diode, zener, J-FET, MOV, opto, triac, SCR, quadrac, regulator, SSOVP, diac, STS and sidac devices, and relays. Features include 3-station multiplex operation, fast single test/measure, throughput to 60,000/h, real-time math, a handler/prober interface, self-test, auto calibrate, datalog, binning, lot summary, intuitive programming, 1-kHz ZZ test, a programmable soak, a scanner and built-in statistics. It provides current to 1,200 A and voltage to 5 kV, and has 1-pA/1-mV resolution. Scientific Test, Inc., (214) 487-9421.
Hand-Held Bridge Suited
For Sorting, Matching
The Model 878 hand-held LCR bridge measures inductance (0.1 uH to 10,000 H), capacitance (0.1 pF to 10 mF), and resistance (1 m(ohm) to 10 M(ohm)) with an accuracy of 0.7%. Dual digital readouts facilitate simultaneous display of capacitance and the dissipation factor or inductance and Q. A minimum/maximum/average function records the running average and the highest and lowest readings. The relative mode stores the value of a reference part, then shows (+) differences. An audible alert can be set to indicate when a component is outside of tolerance. B+K Precision, (312) 889-1448.
Fifth-Generation Tester
Provides High Throughput
The 201C Component Test System accommodates high-volume testing of components, such as transistors, diodes, FETs, relays, resistors, capacitors and inductors. A new measurement module features a 16-bit ADC, a high-speed I/O module and bus scheme. Operating software includes pull-down menus and windows with a test language optimized for 486 and higher performance PCs. Software filters enhance system capabilities, enabling performance of fast, low-level leakage measurements. Real-time processing of delta calculations speeds datalogging. Testronics, (214) 542-3111.
Copyright 1995 Nelson Publishing Inc.
April 1995