Why Choose Bulk Metal® Foil Resistors for Your Circuit ?
July 1st 2010
Bulk Metal Foil Resistors
Vishay Precision Group (VPG)
 Vishay Precision Group (VPG) is a leading designer, manufacturer, and marketer of resistance strain gages, ultra‐precision foil resistors, Resistance Foil current sensors, load cells/transducers, instruments, weigh modules, and systems for a wide variety of applications.
 VPG operates as a part of Vishay Intertechnology. From July 2010, it will be an independent, publicly traded company.
 Bulk Metal Foil® technology was first introduced in 1962 and still remains the most precise resistor technology available today.
Bulk Metal Foil Resistors
There is More to Resistor Precision than Meets the Eye
 Resistors from different technologies may seem alike on the surface, and may often have similar published specifications (initial TCR, Initial Tolerance etc).  However, each is made of a different resistive material and produced differently. Resistance material, design and processing variations strongly influence electrical performance, leading to different behaviors after installation.  Resistor stability should be judged by real‐life performance under load and temperature through short‐term and long‐
term exposure to different electrical and mechanical stresses.
Bulk Metal Foil Resistors
The Ideal Resistor
 When current passes through a resistor it generates heat, (Joule effect), and the thermal response induces relative mechanical changes (stresses) due to differential thermal expansions in the different materials comprising the resistor.
 Ambient temperature changes have a similar effect. Therefore the ideal resistor would employ materials and designs that minimize manufacturing stresses for a precise thermo‐mechanical balance to eliminate the need to compensate for the effects of heat and stress during use.
 Furthermore, the ideal resistor should have no inductance and no capacitance.
Bulk Metal Foil Resistors
Cornerstone In Any Analog Circuit – The Resistor
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* Vishay Foil Resistors (VPG) estimates
Bulk Metal Foil Resistors
Main Reasons For Precision Resistor Malfunction
Stress Factors on a Typical Resistor *
During Operation *
* Vishay Foil Resistors (VPG) estimates
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Bulk Metal Foil Resistors
What Factors Do Design Engineers Look For?
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* Vishay Foil Resistors (VPG) estimates
Bulk Metal Foil Resistors
Main Resistor Technologies
 Wirewound.
 Thin Film.
 Thick Film.  Bulk Metal® Foil. Bulk Metal Foil Resistors
Wirewound Resistors
 Precision wirewound resistors consist of resistance wire wound on a plastic or ceramic bobbin. The wire is spotwelded to outside copper leads and epoxy molded to protect it from the outside environment.  Winding produces permanent deformation (not elastic) in the wire rendering it unstable. Prior to molding, the resistor is adjusted to required ohmic value by abrading some of the wire. This operation and the prior winding method introduce potential failure sites and reduce reliability.  Furthermore, abrading of wire may result in opens during resistor exposure to power, time, temperature. To stabilize the resistor, it is necessary to subject it to temperature cycling. However, this operation is only a partial cure and does not produce uniform results.
Bulk Metal Foil Resistors
Thin Film
 Thin Film resistors consist of an extremely thin layer of resistive material (50 to 250 Angstroms) deposited on a ceramic or silicon substrate.
 Thin Film resistors have relatively limited surge capabilities such as ESD and short time overload due to the low mass of resistive material.
 For low values <10[Ω] stability issues arise with deposition and trimming of increased thickness of the alloy, for high values >1M[Ω] problems arise with uniformity and protection of reduced thickness.
Bulk Metal Foil Resistors
Thin Film ‐ cont
 Various and complex factors contribute to the instability of thin‐film resistors. These include lattice distortion, discontinuous aggregate formations, occlusion of gas at crystal interfaces, oxidation of the film to form oxide semiconductors and mechanical strains.
 Lateral laser trim produces “hot spots,” hence instability. The principal virtue of thin film resistors is that they can be used in high speed applications where precision may not be a major factor, and where small size (high Ohm/square) and price are considerations . Protective Lacquer
Resistive Layer
Electroplated Termination
Base Contacts
Ceramic Substrate
Laser Trimming
Bulk Metal Foil Resistors
Thick Film
 Thick film resistors are comprised of ceramic based materials combined with metallic particles.
 The process involves firing the thick film resistive material to a solid ceramic substrate, thus providing a rugged base for resistors to withstand high surge conditions.
 Lateral trim produces current crowding and “hot spots.”
 It is the cheapest resistor on the market but it is relatively unstable with time, power and temperature as compared to Thin Film, Foil or Wirewound resistors.
 Thick Film resistors are not available in tight tolerances.
Bulk Metal Foil Resistors
Foil Resistor
 The Bulk Metal Foil resistor is based on a special concept where a proprietary bulk metal cold‐rolled Foil is cemented to a ceramic substrate. It is then photoetched into a resistive pattern (no mechanical stress introduced). Later, it is laser adjusted to any desired value and tolerance. Because the resistive metal used is not drawn, wound or mechanically stressed in any way during manufacturing process, the Bulk Metal Foil resistor maintains all its design, physical and electrical characteristics while winding of wire, or sputtering of Thin Films, or thick film glazing do not.
Mass of Resistance Element
Bulk Metal Foil Resistors
Foil Resistor‐ cont
 Foil resistors achieve maximum stability and near‐Zero TCR. This performance is built‐in for every unit, and do not rely on screening or other artificial means for uniform performance.
Ruggedized Construction
Surface-mount wrap-around chip
Foil resistor construction
Bulk Metal Foil Resistors
Unique Trimming Method of Foil Avoids Hot Spots
 How a resistor is trimmed has a significant effect on stability.
Most resistors are trimmed or adjusted to achieve the desired resistance value, usually by having a laser evaporate some resistance material in the element. Unfortunately this can create a potential failure point where the current path is crowded around the laser’s trim kerf.
 For wirewound resistors abrading of wire for calibration purposes contributes to instability and may produce opens.
Foil element‐ Trimming operations increase resistance in precise steps but from remote locations so that the etched grid in the active area remains reliable and noise‐free. Furthermore the resistive pattern minimizes inductance and capacitance.
Bulk Metal Foil Resistors
The Real Facts Behind TCR Parameters As Shown on Datasheets
 Temperature Coefficient of Resistance (TCR) is a measure of change of resistance with change of temperature , usually expressed in parts per million per degree centigrade (ppm/°C). It is a figure of merit for the resistor’s stability through temperature variations.  An increase in resistance with an increase of temperature is a positive TCR, while a decrease in resistance with an increase in temperature is a negative TCR.
 Resistors may have a positive TCR over one temperature range but a negative TCR over a different range.
Bulk Metal Foil Resistors
The Real Facts Behind TCR Parameters As Shown on Datasheets
 Often the TCR is specified over a commercial temperature range (usually 0°C to 60°C) as well as over military range (‐55°C to +125°C), with both ranges using the resistance at +25°C as the initial resistance and reference temperature.
 The typical TCR’s over military range for resistors are:
TECHNOLOGY
TYPICAL
COMMONLY AVAILABLE
ACHIEVABLE
Bulk Metal Foil
2ppm/°C
0.5ppm/°C
0.2ppm/°C
Precision wirewound
15ppm/°C
10ppm/°C
5ppm/°C
Precision Thin film
25ppm/°C
10ppm/°C
5ppm/°C
100ppm/°C
50ppm/°C
35ppm/°C
Thick Film
Bulk Metal Foil Resistors
The Real Facts Behind TCR Parameters As Shown on Datasheets
 TCR of a given resistor is not a constant value ‐ it changes with temperature to some extent, dependent on technology used.  While normally specified over the complete temperature range, Thin film resistors often neglect to note that crafting (and specifying) a Thin Film resistor with lower TCR over a very narrow range actually increases the TCR beyond that range .
Bulk Metal Foil Resistors
The Real Facts Behind TCR Parameters As Shown on Datasheets
 For example to achieve a lower TCR in the +25°C to +125°C range, the normal parabolic resistance change versus temperature curve might be rotate to be more horizontal in +25°C to +125°C range but at the same time it increases the TCR in the ‐55°C to +25°C range. This fact is often obscured because full and complete specifications are often avoided to show only favorable characteristics while obscuring the concurrent unfavorable changes. The following figure shows 3 typical curves of resistors made with Vishay Z foil, Nickel‐Chromium Thin Film and a Nickel‐Copper alloy. Bulk Metal Foil Resistors
How Much Performance Do We Need ?
 Naturally, not all engineers need an entire high‐performance package for their circuitry. Resistors of lesser precision and/or stability can be used satisfactorily in many applications, so the question of need is divided into six basic categories:
1. Existing applications that can be upgraded by relying on a total performance package of Bulk Metal Foil Resistor.
2. Existing applications that require one, or more, but not necessarily all of the Bulk Metal Foil performance parameters.
Bulk Metal Foil Resistors
How Much Performance Do We Need ?
3. State‐of‐the‐art circuitry that can only be developed now because of the availability of the improved performance of precision resistors.
4. Purposeful over‐specifying of resistors to allow for future upgrading.
5. Cost savings by having the circuit accuracy maintained by the resistors rather than by the active devices, which would greatly increase cost for only slightly better levels of performances.
6. Enhanced safety margins in life‐critical and mission‐critical applications or in performance demonstrations necessary to win new contracts.
Bulk Metal Foil Resistors
How Much Performance Do We Need ?
 In group (2), for example, the need for a single parameter must be weighed against the economics of the entire package. It could cost less to use a resistor with superior overall performance specifications because it may eliminate the need for compensating circuitry (plus assembly cost)
 Another question that might be posed is: “ Would using an improved resistor performance for upgrading equipment performance enhance market acceptance of the equipment?”
Bulk Metal Foil Resistors
Foil Process
 The standard foil‐process production flow includes several 100% tests which are vital for precision resistors in order to simulate unexpected stresses that may appear during the working conditions:
 Thermal shock: a major temperature change with specified rate change imposed on a device.
 CLT (Component Linearity Test): an effective in line test to detect and eliminate potential infant mortality failures, the CLT is capable of identifying resistors with a potential for excess parametric changes.
 Power shot: above rated power pulse for a short period of time to assure reliability under actual load life conditions. Bulk Metal Foil Resistors
Post Manufacture Operations (PMO)
for Better Load‐Life Performance
 These PMO are uniquely applicable to resistors made of Bulk Metal® Foil.
 They take the already superior stability of Bulk Metal Foil devices one step further.
 Operations described are not applicable to Thick Film, Thin Film, or Wire resistors.
 They exercise the bonding resin that holds the foil to the substrate, remove any etching anomalies, and further stabilize the resistance element.
 The exercises that are employed are:
(1) temperature cycling (2) short time overload
(3) accelerated load life
Bulk Metal Foil Resistors
PMO
 Temperature Cycling
This exercise is done initially in the chip stage of all production and will eliminate any fallout in the PMO cycling. The cycling exercises the bonding resin and relaxes the Foil without reducing the bonding strength. A small reduction in resistance is tolerable during this PMO operation.
 Short Time Overload
The STO operation is performed on all resistors during manufacturing. Its function is to eliminate any possible weak spots, thus adding an extra measure of reliability.
Bulk Metal Foil Resistors
PMO contd.  Accelerated Load Life
The standard load‐life curve of a Foil resistor exhibits most of its expected change in the first 100 hours of operation and limited change thereafter. Accelerated load‐life testing progresses the resistor beyond this initial change so that the user’s in‐circuit resistance change is greatly reduced in actual operation. This allows the user to get a known and dependable initial tolerance while reducing the necessary load‐life error budget for end‐of‐life planning. How much acceleration is a function of the application and should be worked out between VPG’s Applications Engineering department and the user’s design team. These same operations when applied to resistors of Thick Film, Thin Film, and Wire have vastly different consequences and are applicable to Bulk Metal Foil Resistor only. They are an enhancement to Foil performance but can drive other devices out of tolerance or open. The failure mechanisms in these other devices are too numerous to discuss here but suffice it to say, Foil is the least affected by these PMO operations and they should be considered when the level of stability required is beyond the published limits for standard products.
Bulk Metal Foil Resistors
Inherent Characteristics of Foil Resistors
 Vishay Foil Resistors are not restricted to standard values, specific “as‐required” values can be supplied at no extra cost or delivery (e.g., 100.1234Ω vs 100Ω)
 Electrostatic discharge (ESD): up to 25 kV
 Rise time: 1 ns, effectively no ringing
 Thermal stabilization time <1s (nominal value achieved within 10ppm of steady state value)
 Non‐inductive, non‐capacitive design
 Voltage Coefficient: 0.1ppm/V  Current noise: 0.010μVRMS/V of applied voltage (<‐ 40dB)  Thermal EMF: <0.1 µV/°C
Bulk Metal Foil Resistors
Metal Bulk Foil Unique Features:
 Resistance Value Range
 Rated Power/ Current / Voltage
 Temperature Coefficient of resistance absolute and tracking:
from 0.1ppm /°C
 Resistance Tolerance absolute and match: from 0.001% (10ppm)
 Load Life stability: from 0.002% after 2000h at rated power
 Power TCR (PCR): from 5ppm at rated power
with Z‐Foil resistors
 End of Life Tolerance (Total error budget): <0.05%
 Shelf Life stability : 2ppm for at least 6 years
 No minimum order quantity Bulk Metal Foil Resistors
LONG TERM STABILITY OVER 22 YEARS (0.1W AT 70C)
50 samples (S102C, 10 K )
200
150
100
R/R (ppm)
50
0
-50
-100
-150
-200
0
5
10
15
Time (Years)
20
Bulk Metal Foil Resistors
How to Order and Prototype Service
Bulk Metal Foil Resistors
Foil Door2door ‐ Service For Rapid Prototyping Samples
 When you need to evaluate 1‐5 standard precision resistors for your prototype, you want them in a hurry. Vishay Foil resistors now guarantees a 5 working day delivery on any value from 0R002 to 1M and any tolerance to 0.005%, per individual product specifications.  There is no need to stock a wide array of R&D precision resistors at high minimum order prices when you can buy only what you need and get them within a few days. And, because the foil resistor is the most precise resistor available, it should satisfy all your R&D requirements.  VPG will send the resistors directly from the main facility or via one of its precision centers or “Foil resistor quick delivery sources” that are spread around the world. Bulk Metal Foil Resistors
For further information please
contact : foil@vishaypg.com
Visit us in our new website:
www.foilresistors.com