Measurement of High Ohmic Value Resistors

High ohmic valued chip resistors exhibit characteristics of both insulators and normal resistors. As a result, a chip resistor may have a significant voltage coefficient of resistance, i.e., the resistance may vary with the applied voltage. This mandates that a fixed voltage source rather than a fixed current source be utilized when measuring these devices. Resistances can then be computed as volts/amperes. The applied voltage for a high ohmic value chip resistor is often specified as appropriate for the application to assure correlation. Typical values of applied voltage are 1 and 5 volts DC. This requirement leads to the measurement of very low currents and the attendant problems of such measurements. The problems of low current measurements include: 1. Noise currents generated in the connecting cables when flexed (triboelectric effect) 2. Noise from sources other than the cables 3. Cable, fixture and probe leakage currents 4. Cable, fixture and other stray capacitances 5. Radio Frequency Interference (RFI) 6. Characteristically high Temperature Coefficients of Resistance (TCRs)   These problems can be minimized by: 1. Properly connecting the resistor to the measurement system. 2. Properly guarding and shielding to reduce or eliminate electrostatic noise. IMS recommends that all meters be calibrated to [...]

2017-11-02T14:25:48+00:00January 24th, 2017|Tech Info|

Passive SMT Mounting Techniques: Face-up vs Facedown and Performance Trade-offs.

The choice of mounting orientation, termination styles, pad dimensions and sizes used for surface mount passive resistive components are some of the factors that contribute to the device’s performance in an RF circuit layout. Determining the trade off between these factors in addition to ease of component-to-PCB assembly, solderability, ruggedness and other design optimization for power and RF performance could be an onerous task for RF engineers. The purpose of this technical note is to shine light on the advantages and/or disadvantages of these mounting techniques and termination styles in high frequency applications. Fig. 1A & 1B above are simple illustrations of an SMT resistor wraparound termination style. At DC, the resistor is simply governed by ohm’s law’s Current, Resistance and Voltage relationship (R=V/I) whose impedance Z is simply equal to R. However in an RF environment, certain parasitic effects and contributions must be understood and accounted for as shown in Fig 2 below. The equivalent circuit diagram of Fig 2 below is a more appropriate representation of the simple resistor described above. From the equivalent circuit, C1 is the shunt capacitor that is formed between the input and output pads of the full wraparound termination style device. This capacitor [...]

2017-10-25T15:56:28+00:00December 25th, 2016|Tech Info|

SZG: Where High Power and High Frequency Join Forces

The number of high frequency, high power applications is rapidly increasing each year. The need for resistors with both high frequency performance and high power handling capability is growing. In most cases, the smaller the resistor, the better it will perform at high frequencies. At the same time, the larger the resistor, the better the thermal properties are. Many designers are making compromises when both power and performance matter. IMS has solved this dilemma with an innovative resistor that offers the best of both worlds called a SZG style termination device. Power Thermal management is becoming much more important as the density of electronic components and the applied power increases. Both factors lead to higher temperatures produced by not only the individual components, but also the entire assembly. Heat dissipation in high power electronics creates challenges for integrating material selection together with thermal designs. As a result of the power and signal combinations running through resistors, heat is generated. When more power is applied to a resistor, more heat is generated. If this heat is not properly transferred out of the part, the heat build-up can cause the part to shift in value and eventually fail electrically and mechanically. When [...]

2017-11-02T19:34:03+00:00November 15th, 2016|Tech Info|

ThermaBridge™ and HIPOT

HIPOT: What is it? The Hipot test (short for High Potential test), or sometimes called Dielectric Withstand test or Insulation breakdown test, is a test used to verify the strength of the insulation between the current-carrying path of a device and its chassis or enclosure. This is done by applying a high voltage (significantly higher than the device’s nominal operating voltage) from the input lines to the chassis of the product and measuring or monitoring the resulting leakage current flowing through its insulation. Why HIPOT? The Hipot test is the most important safety test for electronics and electrical devices. The test is done on finished goods prior to shipment to detect or verify the quality of the insulation system of the product as well as detect any latent manufacturing faults. It can also be used to verify the installation correctness or otherwise of a system for field or end user applications. The theory behind the test is that if a voltage much higher than the product would typically see is applied across the insulation of the system or product without a breakdown, the product will be able to operate safely at nominal operating conditions. The Hipot basically stresses the insulation [...]

2017-11-02T12:53:00+00:00June 24th, 2016|Tech Info|

High Mega-Ohm Resistor Performance for Low Voltage Applications

Resistors are common devices required for today’s electronics. It is so prevalent in design, we often tend to think they do not require close attention to their inherent properties. While many applications can be designed using performance assumptions gained from years and years of experience, this is not the case for all resistors, all of the time. When using very high value resistors, there are a number of common resistor behaviors that may not apply, or in some case are backwards of traditional thinking. One such counter-intuitive relationship is Voltage Coefficient of Resistance (VCR). VCR is the relationship between the voltage seen by the resistive element and the corresponding resistance measured by a given device. One of the most important parameters of precise high-ohmic resistors is the VCR. The goal of resistor manufacturers is to try to create a resistor device that has consistent stated value of resistance, but does not have varying resistance values across broad voltage ranges. The Voltage Coefficient is the change in resistance with applied voltage over a specific voltage range. We measure the stability of the resistor with respect to changes in voltage. A resistor with a VCR of 100 ppm/V will change 0.1% over [...]

2017-10-25T13:49:31+00:00May 25th, 2016|Tech Info|

Solder Leach Resistance Profile for Aluminum Nitride (ALN) Chip Components

1. The terminals of IMS’ ALN chips are constructed using proprietary processes and materials, which were specifically formulated to partially fulfill the requirement of a non-magnetic (no nickel), solder leach resistant material (but whose leach resistance would still approach that possessed by “nickel barrier” type products). 2. According to ana analysis of testing performed at ims, these ALN chips will withstand soldering conditions characterized by a maximum temperature of 260 °C for a duration of 10 seconds. 3. Due to the relatively high thermal conductivity (TC) of ALN ceramic, contact style soldering methods (such as soldering irons and hot air column heating) may be difficult to implement. Best results occur with hot plate reflow, belt or chamber reflow methods. The TC of ALN ranges from 170-180 (W/m°C), which can result in significant cooling of soldering iron tips during contact with the component. Subsequently, this often results in the operator compensating the effect by either unduly elevating the temperature of the iron or by over-extending the contact duration, thus potentially damaging the part. 4. Typical Five-Zone Infrared Belt-Furnace profiles are characterized by the following graph: Although specific applications will vary, each with many factors to consider (such as board type and [...]

2017-10-25T16:19:33+00:00March 24th, 2016|Tech Info|

Specifying a Low Pass Filter

The graph shown here illustrates a typical low pass filter (LPF) function. It indicates how much attenuation is achieved at a given frequency for, in this instance, the IMF2293. All other LPF graphs will be similar to this one, but will have different frequency spans and different gradients depending on the design of the filter. In order to quote/specify a low pass filter, the following information must typically be provided: 1. The extent of the passband (i.e.; what frequencies does the low pass filter PASS?). 2. The passband insertion loss (how much power is dissipated by the filter). 3. The stop band attenuation levels and frequencies (which frequencies does the filter reject and by how much?). Generally, IMS filters will have passband insertion losses ranging between 0.5 dB and 1.0 dB with a few exceptions, depending on design. If a customer needs less than 0.3dB loss, it is unlikely that IMS can meet that requirement. Conversely, if the customer can live with greater than 2 dB insertion loss, then they may be able to use an off the shelf low cost LTCC device. The extent of the passband is usually described as “DC to 1.5GHz” or “DC to 2.4GHz” or [...]

2017-11-02T13:50:27+00:00February 24th, 2016|Tech Info|

Heat Sinking Relative to Alumina Chips and Aluminum Nitride Chips

Frequently, one needs more power in a chip of a given size. The question regarding IMS chip resistors then becomes whether or not to choose aluminum nitride (ALN) chips, or alumina (Al2O3) chips in the Super RCX family. In most cases, the answer depends on whether or not the power requirement is a couple of watts, or much more – e.g., 10-20W. Before ALN is considered, however, we must first ask whether or not the customer is thermally managing the chips that need upgrading. If the answer is “yes” and 20 watts or more are needed in a small package, then ALN is the answer. If the customer is not thermally managing (or cannot), choosing ALN will not help, since ALN does not radiate heat any more readily than Al2O3. ALN is a much better conductor of heat than Al2O3, which makes it a great solution when a lot of power is needed and it also makes ALN a great alternative to BeO, especially when BeO is unacceptable for use, an ever more frequent occurrence. ALN has a thermal conductivity (abbreviated: k) of about 170-180 W/meter°C, whereas Al2O3 has a k of about 25-28 W/meter°C. The value of k actually [...]

2017-10-24T20:31:55+00:00November 24th, 2015|Tech Info|

Dictionary – Know Your Resistors

Adhesion: The ability of dissimilar metals or particles to cling together as a joint is referred to as adhesion. Adhesion in the thick film is created by either a chemical (oxide bonded) or mechanical (Frit) bonds. Peel tests can be used to measure adhesion between lead and substrate and dielectric. Alpha coefficient: In a thermally sensitive resistor or Thermistor applications, the alpha (?) coefficient is a material characteristic that defines the percentage resistance change per degree centigrade. It is also known as the temperature coefficient and it's calculated by the following relationship; ? = 1/RT x dR/dT Where RT is the resistance of the component at the relevant temperature in (°C), dR/dT is the gradient of the R vs. T curve at that temperature point and alpha is expressed in (%/°C) Annealing: Annealing is a heat process whereby a material (metal or glass) is heated to a specific temperature (annealed point) and then allowed to cool slowly in order to maintain ductility and prevent crack and internal stress. Aspect Ratio (AR): The ratio of resistor length to resistor width is the aspect ratio. It's also known as the number of squares. Amplitude Balance: The maximum deviation or difference in amplitude [...]

2017-10-24T19:02:04+00:00October 24th, 2015|Tech Info|

Understanding The Basic Thermal Properties of SMT Devices

Ensuring optimal RF performance (that is, mechanical, thermal and electrical performance) of a surface-mount technology (SMT) ceramic device requires the careful procurement and use of consistently well-behaved materials, the use of proper and rigorous design principles, and proper attention paid to the appropriate parameters (on the part of the manufacturer in its design, and on the part of the user in its operation). In this way, the reflection, dissipation, and transmission signatures of the device in question remain consistently and predictably well behaved. Some instances of devices that are designed to maximize dissipative loss are terminations, loads, absorptive filters and dividers, etc. Typically, these devices must minimize reflective loss (regardless of the nature of the signals at their inputs), which, when combined with proper dissipative design, results in an optimal transmission signature as well. SMT devices that are non-dissipative by design are hybrid couplers, power dividers, filters, etc. Filters, of course, are designed such that some signals are to experience a minimum of reflection and dissipation while maximizing transmission, whereas other signals are to experience a maximum of reflection while minimizing transmission. Other dissipative devices are attenuators, voltage dropping resistors, resistive splitters, etc. Attenuators must be designed such that reflective [...]

2017-10-25T14:12:34+00:00September 25th, 2015|Tech Info|
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