4 IRE TRANSACTIONS ON INSTRUMENTATION June Electronic-Type Resistor ReliabIlity Measurements Study* B. F. LATHANt, H. G. HAMREt, AND R. M. BERGSLIENt T HE PROBLEM of reliability in electronic equip- ment. The latter portion of the program (Part II) was ment, particularly that used by the military initiated in order to exploit these newer approaches, and services, is very serious and because of the large to expand the investigation to cover additional resistor number of components involved in specific equipments, types and designs. a simple solution is not possible. The examination of nondestructive methods for de- The magnitude of the problem is so great that rela- posited-carbon film and for metal-film resistors, of tively reliable components such as the common types of several ratings and sources, comprised the two phases resistors are continuously subjected to intensive design, covered in the Part IS program. The initial tasks were research, production, and testing surveillance. How- planned with the objective of pointing out the test ever, in spite of these measures, there is a definite need methods which would be most applicable toward further for further improvement in reliability to the extent that investigation of carbon-film resistors and of metal-film critical resistive properties are insured in specific units resistors. In addition, the selected techniques could be finding key application in electronic equipment. Since refined somewhat toward the basic goal of a 100 per cent resistors often comprise a major portion of the compo- acceptance test. nents used in complex equipments, the over-all reliability The initial evaluation of the carbon-film resistors is often is dependent upon a few unreliable resistors. As being followed up in a final test series with a more ex- one approach to identifying unreliable resistors, the haustive appraisal of the performance of units of this Armour Research Foundation has been carrying out an type, under advanced application of the techniques investigation, under Air Force sponsorship, of non- which were screened as being the most qualified. Also destructive measurement techniques. some of the test schedule is being devoted to corroborat- Present methods for evaluating resistors largely em- ing initial evaluations of the metal-film and the carbon- ploy statistical techniques along with destructive test- composition resistors. At the conclusion of the final ing, and thus preclude vital inspection of 100 per cent testing series, data will have been obtained for nearly of the accepted components. Test methods in this pro- 20,000 resistor samples over the entire investigation. gram have been designed for fairly rapid and economical This paper endeavors to present some of the measure- inspection of the individual resistors in a group with the ment techniques, and their implications, for selected aim of utilizing the nondestructive testing criteria for tests among those meeting preliminary requirements for the rejection of all unreliable units. Promising tech- this reliability evaluation. Since data for the entire niques have arisen out of these investigations with vari- sample grouping are as yet incomplete, this paper does ous resistor types, based upon approaches which should not present final results and conclusions. be expandable to cover electronic components which are relatively more complex than resistors. CHARACTERISTICS OF THE RESISTOR SAMPLES During the course of the program, several candidate D g te c e of tvan- Resistor failures may be attributed to one or more of tests have been extensively investigated, including var-three general causes. These are: ous modifications and applications to different cate- gories of resistors. Part I of the program was divided 1) Improper design of equipment so that component into three project phases: a survey of potential ap- resistors must operate outside of their design proaches with respect to the governing requirements, an limits. exploratory investigation of selected test methods, and 2) Failure in a resistor as a consequence of a previous the refinement of those techniques which appeared most failure of another component in the equipment. promising. On the basis of the evaluation of this initial 3) Defects in the resistor itself. work, techniques exhibiting the most applicability to resistor samples did not fully meet the nondestructive This study is restricted to the consideration of failures requirements. However, test approaches were found arising from the third cause. This type of failure is ap- which held sufficient promise to merit further develop- parent if the resistor has had an uncharacteristically short life and the other two factors cannot be blamed. * Maucrp reevdb. h G,Noebr9 90h A failure will either manifest itself as a catastrophic work described in this paper was carried out under Contract No. failure, resulting from a radical breakdown in the resis- AF 33 (600)-31331 AMC Project 7448. tor more or less without warnitng; or a drift failure, t Armour Research-FRoundation, Illinois Institute of Technology, whr th eitnegrdal -agsutli ece {Chic~go 16, Ill.'whrthreitneluly-a;suiltracs
T HE PROBLEM of reliability in electronic equip- ment. The latter portion of the program (Part II) wasment, particularly that used by the military initiated in order to exploit these newer approaches, andservices, is very serious and because of the large to expand the investigation to cover additional resistor
number of components involved in specific equipments, types and designs.a simple solution is not possible. The examination of nondestructive methods for de-The magnitude of the problem is so great that rela- posited-carbon film and for metal-film resistors, of
tively reliable components such as the common types of several ratings and sources, comprised the two phasesresistors are continuously subjected to intensive design, covered in the Part IS program. The initial tasks wereresearch, production, and testing surveillance. How- planned with the objective of pointing out the testever, in spite of these measures, there is a definite need methods which would be most applicable toward furtherfor further improvement in reliability to the extent that investigation of carbon-film resistors and of metal-filmcritical resistive properties are insured in specific units resistors. In addition, the selected techniques could befinding key application in electronic equipment. Since refined somewhat toward the basic goal of a 100 per centresistors often comprise a major portion of the compo- acceptance test.nents used in complex equipments, the over-all reliability The initial evaluation of the carbon-film resistors isoften is dependent upon a few unreliable resistors. As being followed up in a final test series with a more ex-one approach to identifying unreliable resistors, the haustive appraisal of the performance of units of thisArmour Research Foundation has been carrying out an type, under advanced application of the techniquesinvestigation, under Air Force sponsorship, of non- which were screened as being the most qualified. Alsodestructive measurement techniques. some of the test schedule is being devoted to corroborat-
Present methods for evaluating resistors largely em- ing initial evaluations of the metal-film and the carbon-ploy statistical techniques along with destructive test- composition resistors. At the conclusion of the finaling, and thus preclude vital inspection of 100 per cent testing series, data will have been obtained for nearlyof the accepted components. Test methods in this pro- 20,000 resistor samples over the entire investigation.gram have been designed for fairly rapid and economical This paper endeavors to present some of the measure-inspection of the individual resistors in a group with the ment techniques, and their implications, for selectedaim of utilizing the nondestructive testing criteria for tests among those meeting preliminary requirements forthe rejection of all unreliable units. Promising tech- this reliability evaluation. Since data for the entireniques have arisen out of these investigations with vari- sample grouping are as yet incomplete, this paper doesous resistor types, based upon approaches which should not present final results and conclusions.be expandable to cover electronic components which arerelatively more complex than resistors. CHARACTERISTICS OF THE RESISTOR SAMPLES
During the course of the program, several candidateD g te c e oftvan- Resistor failures may be attributed to one or more oftests have been extensively investigated, including var-three general causes. These are:ous modifications and applications to different cate-gories of resistors. Part I of the program was divided 1) Improper design of equipment so that componentinto three project phases: a survey of potential ap- resistors must operate outside of their designproaches with respect to the governing requirements, an limits.exploratory investigation of selected test methods, and 2) Failure in a resistor as a consequence of a previousthe refinement of those techniques which appeared most failure of another component in the equipment.promising. On the basis of the evaluation of this initial 3) Defects in the resistor itself.work, techniques exhibiting the most applicability toresistor samples did not fully meet the nondestructive This study is restricted to the consideration of failuresrequirements. However, test approaches were found arising from the third cause. This type of failure is ap-which held sufficient promise to merit further develop- parent if the resistor has had an uncharacteristically
short life and the other two factors cannot be blamed.* Maucrp reevdb. h G,Noebr9 90h A failure will either manifest itself as a catastrophic
work described in this paper was carried out under Contract No. failure, resulting from a radical breakdown in the resis-AF 33 (600)-31331 AMC Project 7448. tor more or less without warnitng; or a drift failure,
t Armour Research-FRoundation, Illinois Institute of Technology, whr th eitnegrdal -agsutliece{Chic~go 16, Ill.'whrthreitneluly-a;suiltracs
l160 Lathan, Hamre, and Bergslien: Electronic-Type Resistor Reliability Measurements Study 5
an intolerable value. There are significant differences in population. Correspondingly, it is important that thethe frequencies at which each of these kinds of failure are handling of the sample and data be designed into themanifested, and in the general life characteristics, experiments in a manner to obtain maximum sig-among the various resistor types. Basically, the fixed nificance for each evaluation. The aim of this study im-resistor types which are widely employed in military posed different requirements on the sample than if it hadelectronic and communication equipment fall within two been intended to evaluate comparatively the productionclassifications, fixed composition and fixed film. These of the manufacturers. Therefore, the samples obtainedresistors are covered by the Military Specifications were typical of the more important types of resistors andMIL-R-11B, and MIL-R-10509C, respectively. methods of construction, with the source of supply being
secondary.Composition TypesThe fixed composition resistor is available in three Sample Procurement
principal types of construction and consists of a resistive Before the results of this investigation could be fullycore, an insulating envelope, and terminals. As simple implemented, there was a need to evaluate more cate-in concept as this may be, it is found that pertinent gories of fixed film resistors than were included withincharacteristic differences exist in the three types as a the scope of previous work.'consequence of their methods of manufacture. A hot Thus, it was planned to test samples of additionalmolding process is used in making one of these types. In power ratings, resistance values, and manufacturer'sthis process, the binder of the resistive element and the types of deposited-carbon film resistors. Also, the in-insulating jacket are composed of the same resin ma- vestigation was expanded to cover the more recentlyterial. The resistor is molded at a high pressure and developed metal-film resistor. The ratings and quanti-temperature and finally cured in a high-temperature ties of samples which were procured for the initial testingoven. Another type is similarly constituted but is series are shown in Table I. At the then current stage ofmolded at room temperature. In this "cold-molded" development, metal-film resistors were relatively noveltype, the insulating jacket is porous until it is impreg- and considerably more expensive than the carbon-filmnated by molten wax. The third type of composition units. Due to this relation, the initial test groups ofresistor has a carbon-ink film deposited on the surface metal-film samples consisted of smaller unit quantitiesof a small-diameter glass tube as its resistance element in order to gather preliminary data upon their charac-and molded jacket to provide insulation. teristic responses to the candidate experiments. Also, a
limited number of carbon-composition resistors were
Film Types added to the samples undergoing this initial testing, inorder to corroborate previous trends and to appraise theSeveral types of fixed film resistors have been ap- .
proved for the applicable Qualified Products List. Use of applicability of new or modified test methods to these,deposited carbon-film resistors has grown in prominenc resistor types.
prominence,.. In order to reflect any variation due to normal factorsespecially in some precision applications; this is pri-manly~~~ ~du to reenl imrvdmnfatrn .eh associated with the manufacture of different batches of
resistors, the cooperation of the suppliers was obtainedniques. In this resistor, a crystalline carbon film is de-for shipping the samples in accordance with a produc--posited from gaseous hydrocarbons at high temperatures* * * r r r . . ~~~tion lot scheme. With three lots and two lots procured(onto a ceramic or glass rod. Strips at the ends of the rod for the carbon-film and carbon-composition resistors,are silvered and metal caps, from which the leads pro r
.. r .sS r r rw1 ~~~~~respectively, it was requested that for each order of.ect,are pressed over the silvered area. Other fixed fil identical' resistors, the lots consisting of 100 units each,types utilize a thin layer of a metal oxide or a metal alloy be randomly withdrawn from consecutive productionwhich is deposited on a glass substrate. An insulating
runs spaced at least two weeks apart. For the deposited-coating, such as an epoxy resin paint, is then applied, carbon film resistors, the three resistance values (TableHowever, in one variety, the film is deposited on the I) were selected as representative of three discreteinner surface of a glass tube. In this variety, a precious orders of magnitude in the resistance range.metal film is deposited in a spiral form. Other fixed filmresistors are spiralled by cutting a nonconducting helical General Measurement Techniquespath through the continuous film. The resistance valuemay be precisely determined by the length of the spiral Upon the delivery of each shipment of identical re-per substrate area. sistors, the package holding the units was designated
with the appropriate lot number and stored in a desic-SAMPLE AND DATA HANDLING
The purpose of a sampling method is to obtain a ' D. F. Gilbert, "Non-destructive tests to predict the reliability of1 r * 1- * r *s * or ^ ~~~~~resistors," Proc. Third Natl. Symp. on Reliability and Quality Controlnumber of individlual units which are suffciently repre- in lElectronics, pp. 204-210; January, 1957.
sentative of the statistical population of interest that it 2 Resistors of a particular type and manufacture having the same* * *r 1r r r s r , ~~~~~~~~powerrating, nominal resistance, and no history of application, areit iS possible to generalize from the sample to the total regarded as "identical."t
6 IRE TRANSACTIONS ON INSTRUMENTATION JuneTABLE I
CHARACTERISTICS OF THE SAMPLES PROCURED FOR INITIAL TESTS
Sample Applicable Military Rated Nominal Resistance SampleResistor Type Design Military Specification Characteristic Watts Resistance Tolerance Quantity
Carbon compositionHot molded A MIL-R-111B GF 2 220K ±20 200Film B MIL-R-11B GF 1 220K ±20 200Cold molded C MIL-R-11A BF 1 220K ±20 200
* The sample design code, as used consistently throughout this program, refers to an identical category of resistors with symbol desig-nations as follows, respectively:
Symbols Specific DesignationD, J, A, B, or C The resistor type4, 5, 6, 7, 8 Manufacture of the resistor typeC or M Ambient temperature and power rating2, 6, or 8 Nominal resistance value.
cator. When all lots of identical samples were on hand, be reasonably handled. It was recognized that certain ofpreprinted cloth tags bearing a sequence of numbers the nondestructive tests were nondestructive arising outwere serially affixed to the units in accordance with a of their definition, and thus, a series of these methodsprocedure for randomly distributing the sampled lots could be applied to the same test group prior to its de-within a given test group of 100 resistors. This was ac- structive test. Other prospective methods, such ascomplished by choosing each resistor from boxes marked short-time overload, were known to be only largely non-by appropriate lot numbers, to be tagged in random destructive. Therefore, it became necessary to verifyorder as given in a prepared table of permutations for actual nondestructiveness in these candidate methods.these designations. As the resistor was tagged with the This was done by subjecting, for a given instance, twoserial number, this number was recorded on a data groups of identical resistors to the same standard testsheet as the part of the unit's identification number fol- after having exposed one group to a test of known non-lowing the first digit (which designated the resistor's destructiveness and the other to one of questionableorigin and type). The corresponding lot number was also character.recorded, and the other columns on the data sheet were In conducting a series of measurements in this study,reserved for the test entries and other information relat- care was taken to provide reproducible test conditions.ing to each resistor. In subjecting the samples in a group Resistor units were removed from storage and mountedto test in random order, any biases that might have upon test racks of ten samples each, prior to any meas-been introduced by the production lots were counter- urement of the group. This served to minimize theacted. effects of manual handling of the resistors.
In allotting the sample groups to the various test com- Although earlier in the program use was made ofbinations, there was the primary problem of screening mark-sense cards to record data which could be directlyeach identical category of resistors with each combina- sensed by the machine for transcribing, this proceduretion of a prospective nondestructive method and a was abandoned in favor of the use of the data sheetstandard destructive test. In view of the many pro- form. Even though the data had to be subsequentlyspective methods and resistor designs considered, and punched onto IBM cards for processing, recording thethe dlesirability of effecting statistically significant sam- original data by this means offered less possibility forple sizes, it was found that the test arrangements could operator and mechanical errors and afforded more ac-be designed without involving more samples than could curacy when cross checked after processing.
1960 Lathan, Hamre, and Bergslien: Electronic-Type Resistor Reliability Measurements Study 7
DESTRUCTIVE TEST STANDARDS FOR subsequently studied in the laboratory, as the experi-RELIABILITY STUDY mental evaluation of the more promising methods was
Little detailed and systematic information is available being refined.regarding the mechanism of failure in resistors under Reist Noiactual "in service" conditions. The isolated effects ofrated load-temperature, moisture, and temperature- Several studies have been made of the electrical noisecycling environments on resistors may be determined generated in composition3 and deposited-carbon film re-by standard destructive tests. However, even though it sistors when a direct current flows through the com-appears that load life and environments such as mois- ponent. The electrical noise level has been shown to beture are not independent, it can be assumed that, col- extremely sensitive to the potential barrier existing be-lectively, these standard tests realistically simulate tween conducting materials, and to microscopic imper-actual conditions on an accelerated basis. The validity fections. Also, it has been found for carbon resistorsof this premise has been experienced in the high reli- that noise at a given frequency depends upon the squareability of resistors procured from the industry where of the current, or the power, in the unit. Thus, it isquality control is measured by employing the specified logical to suspect that exceptional noise magnitudes, asstandard tests along with statistical sampling. have been observed in departures from the square-lawThe three tests-load life, moisture resistance, and dependence, may be the results of defects in the resistor.
temperature cycling-as given in the applicable military Preliminary efforts in this investigation set out tostandards, were adopted as comparative standards for evolve a simple and rapid procedure for a test of resistorthe nondestructive experiments on the test sample noise. Plots of the mean-square noise voltage per cyclegroups. The results of a given series of nondestructive of bandwidth over a number of midfrequency pointstests and one of these destructive tests on the sample were analyzed for several resistor specimens. The resultsgroups were examined with the object of finding if cor- indicated that a specimen roughly obeyed a relationrelation existed. equating this unit of noise voltage, squared, to a constantThe load-life test is the most important of the en- factor times the reciprocal of the effective midfrequency.
vironmental tests. This test involved cycling the re- Significant variation was observed in the magnitudessistors on and off full load for 1000 hours while they were but not in the distribution of noise in the frequency plotssubjected to the rated ambient temperature. for individual resistors. Thus, it was concluded thatThe resistors' relative independence of humidity ef- single measurements of the relative noise magnitudes in
fects is determined by the moisture-resistance test. Ten samples would satisfy the test requirements.cycles of ambient conditions of 90 to 98 per cent relative The effects on the noise characteristic due to varioushumidity with temperatures of 250 to 650 C and - 10° C, treatments of the resistor were studied. Some of eachare applied to the samples for a total of ten days. The type were overloaded for a period of time without af-usual vibration subcycle was omitted due to the diffi- fecting the magnitude or distribution of the noise overculty of handling several hundred samples at a time. the range of frequencies measured. A force of five poundsTemperature cycling tests evaluate the ability of re- exerted axially to the leads had no effect on the distri-
sistors to withstand rapid fluctuations in temperature. bution but did increase the magnitude of noise of theThe test is performed in separate hot and cold chambers hot molded types. Heating the samples to 1500 C for awith temperatures of 850 and -55° C. The resistors are half hour had no effect except to increase slightly thesubjected to five cycles for a total of seven and one-half noise of the hot molded type. Finally a test was per-hours. formed on resistors physically mulitated by scraping or
sawing. Even this had no effect on the distribution ofSELECTED NONDESTRUCTIVE TEST PROCEDURES frequencies and slight effect on the magnitude.
In order to acranmxThe equipment for making noise measurements uponelay quantities of test samples was ultimately assembled in
resistors procured by the military services, suitable non- a form adapted from the experience gained during mostdestructive tests are required to inspect production of the full noise investigation. The basic measuring cir-quantities individually. Consequently, such tests must cuit for the noise'test is shown in Fig. 1. The dc powerbe rapid, easily interpreted, inexpensive, and non- source consisted of a series of batteries, the output ofdestructive to good resistors. They must show results which was made variable to a maximum of 500 volts andwhich highly correlate with the standard destructive filtered. Means were provided for inserting a decoupling
tets resistor (wire wound) of a value approximately equal toSeveral approaches were found for nondestructive the test sample's resistance. This served to prevent ex-
tests during the complete investigation. The simpler andmre sraihfrwr of ths aprace wer de3 G. T. Conrad, Jr., "Noise in composition resistors," Proc. Nati.
veloped for initial exploratory tests. Other tests were Electronics Conf., vol. 10, pp. 791-802; February, 1955.
8 IRE TRANSACTIONS ON INSTRUMENTATION JuneDecoupling
r. Resist. .iRfW
| =1 F 1 1 RM~~Vaiabe olts
Fig. 1--Noise measurements circuit.
cessive loss of noise into the dc supply. The series corn- sistors which are prone to failure involves several at-bination of capacitor-filtered supply and decoupling re- tendant measurement problems. Conventional tests re-sistor, in parallel with the detector circuit, could be ap- quire measurements of resistance at rated voltage andplied across the test sample through a selector switch. at 0.1 rated voltage, from which the voltage coefficientThe noise detector circuit included the smnall capacitor is calculated. However, in applying rated voltage, it iswhich blocked passage of dc current and passed the noise important that heating does not occur to introducesignal into the battery-powered amplifier stage. The efects of temperature in measurement of voltage co-amplifier used had adjustable frequency limits of 0.2 cps effcient. One of the standard test procedures recognizesto 40 kc. In order to reduce outside interference to toler- this by restricting the application of rated voltage to noable levels, it was necessary to operate the sample and more than 1 second in 15-second intervals. It becomesits supply ungrounded into the balanced input of the apparent, then, that the need for accuracy and repeat-amplifier, and enclose these components in a heavy ability in measurement as well as reasonable speed inaluminum case. The amplifier output was measured by taking data requires something less cumbersome thana specially constructed microvolt-squared meter. The the standard test procedures.oscilloscope was useful for detecting periodic inter- A method described in the literature4 appeared to beference such as that from 60-cycle pickup. adaptable for measurements of voltage coefficient duringThe test circuit was initially calibrated; however, the this program. Briefly, the described method utilizes the
quantity of interest in the full-scale measurements of deviation from an ideal resistor to generate harmonics ofsamples was their relative noise magnitudes. These were the ac applied voltage. Voltage coefficient is shown to beobtained using specific test current, amplifier gain fac- directly proportional to harmonic content and, since ittor, time constant, and frequency band-pass for a given is possible to test at any voltage up to rated voltage, theresistor design. test parallels actual operation. There is also a negligible
Extensive testing was performed with various modi- effect due to temperature since the resist~or is allowed tofications of the noise measurement. These comprised stabilize at the test voltage.measurement of rms noise in a frequency bandwidth of A circuit employing this method has been assembled80 cps to 40 kc, measurement of peak noise in the same and operated satisfactorily. A diagram of the circuit isbandwidth, measurement of rms noise with a five-pound shown in Fig. 2 and a photograph of the operating equip-pull on the sample leads, measurement of peak noise ment is shown in Fig. 3. For purposes of establishingwith this terminal pull, and finally, measurement of rms correlation with destructive tests, it was not necessarynoise in a bandwidth of 8 to 50 cps. to perform the actual calculation of voltage coeffcient
since the harmonic voltage indication served as an indexVoltage Coefficient of Resistance which is directly proportional to voltage coefficient.
The cnsidratio of he vltagecoeficien of re- 4 L. A. Rosenthal and A. S. Louis, "New resistor voltage coef-sistor as a nondestructive test capable of selecting re- ficient tester," Tele Tech., vol. 13, pp. 62-69; September, 1954.
1960 Lathan, Hamre, and Bergslien: Electronic-Type Resistor Reliability Measurements Study 9
I: C"7s
> O 5e0k.o g
AUIrO TRASFO4AMN
/80 - 0~~~~~~~~~~~~~~~(. F
,__ .o o_~~~~~~~~~~~o
50k.~~~~V
Fig. 2-Circuiit for determininig voltage coefficienit.
Fig. 3-Equipment for measuirement of voltage coefficient.
Temperature Coefficient of Resistance- myent each other in predicting performance of carbon-Short-Time Overload filni samples and permit expectation that a combined
The tests of temperature coefficient of resistance and test designed to apply simultaneously the effects of eachshort-time overload, which were applied as separate nondestructive method of the resistor would be an ef-tests during the initial investigation, exhibited useful ficient predictor.prediction for specific test com-binations of destructive Equipment has beeni designed and assembled for de-tests. For some of the test groups of deposited-carbon termining an index of temperature coefficient with a
film resistors, promising trends were noted for tempera- controlled application of overvoltage. This test was theture coefficient vs load life and temperature coefficient nonidestructive test applied to miost of the carbon-filmvs moisture resistance. In other test groups which were samiples. For metal-film and carbon composition, onlysubjected to overload, promisinig trends were shown in~ the short-timie overload showed promising trends in thethe cases of short-time overload vs load life and short- initial evaluationis. Thus, an adaptation of the conven-
time overload vs temperature cycling. These results tional overload alone was made for samples of theseshow that together the two nondestructive tests comple- types.
10 IRE TRANSACTIONS ON INSTRUMENTATION JuneStep Sw. Bank 4
B / nt,\ [1flietex F Coter 1
Start Sw.
)> mplifierl Stop Start | On-Off Sw.
I aes10ChanncsIN_-Xz 1 / L L--- tLfi
o- XX i|115
T--2-aW-No
~_
Stepping Switch Step Sw. ControLBanks' Oatlin.e
12
Meg1607Fig. 4-Circuit for measuring initial resistance and index of temperature coefficient with overload applied.
Fig. 5-Equipment for measuring initial resistance and index oftemperature coefficient with overload applied.
The circuit for determining the temperature coef- Short- Time Humidityficient index and applying short-time overload voltageis~~.shwi g. 4. It i bascall a Weatsone ridg Subjecting the samples to an environment of the givencirshownithFig.stepping baswitchl atedstone same humidity conditions for a short term (one hour) has been
the bridge balance alternately and apply a given vale shown to exhibit marginal usefulness as a nondestructivethebridge balance alternately and apply a given value test for specific carbon-film and carbon-composition re-of overvoltage to the unit. The stepping switch ener-gizes alternate contacts over 50 steps for a maximum sistors. The absence of a trend in the case of metal-film
duato of on.eod hs h etsml'eit samples illustrates the relative mildness of this testduration of one second. Thus, the testsample'sresist-when resistors of military characteristics A and B,ance would be monitored every 1/25 second. Fig. 5 is a having a more moisture-resistant insulation, are exposedphotograph of the operating equipment. itiThe test procedure begins with a measurement of the to it.
initial resistance of the sample. A signal then simultane- The test was modified somewhat in an effort to adaptously energizes the "start" channel of the electronic it for use with metal-film samples. Since it would be un-counter and the "stress and sampling" operation of the desirable to lengthen the test time over one hour, thestepping switch. When the test sample's response results ambient or imposed conditions had to be modified. Onein a predetermined unbalance voltage in the detector modification consisted of an application of a polarizingbranch, the "stop" channel is energized. The counter voltage across the resistor's insulation. The effect of thisreading in milliseconds is indicative of the temperature modification was observed in a preliminary experimentcoefficient of resistance. using ten 500-ohm resistors.
1960 Lathan, Hamre, and Bergslien: Electronic-Type Resistor Reliability Measurements Study 11TABLE II
COMPARISON OF TEMPERATURE-COEFFICIENT INDEXES WITH RESULTS OF LOAD-LIFE TEST
Range of Ind exes Per Cent of Samples Changing Within Given Limits of Percentage Resistance for Load Life*for Temperature Test Test
* Per cent change, initial resistance to resistance after temperature cycling or 1000 hours of load life.
12 IRE TRANSACTIONS ON INSTRUMENTATION June
Measurements were made of the functional and in- index with the results of load life. It may be seen thatsulation resistance of the subject specimens before and useful trends are evident in Groups 24, 27, 21 and to aafter exposure to the given humidity test. Specimens lesser degree in Group 15.were subjected to the short-time humidity, with a dc Table III compares changes in resistance due to over-potential of 6 volts applied between the polarizing strap load with changes due to temperature cycling or loadof the rack and the resistor terminals. The final half hour life. Here, Test Group 17 shows a distinct positive cor-of this test was conducted without application of the relation. Groups 8 and 15 also show some correlation.polarizing voltage. After the resistance measurements, These data are part of the results obtained for carbon-the specimens were subjected to the standard moisture- film resistors. Data obtained for carbon composition andresistance test which includes a determination of final re- metal film resistors generally showed that only the over-sistance values. The results were examined for a possible load test and the short-time humidity test were appli-trend toward association of per cent resistance changes cable.of either the functional or insulation resistances due to The exploratory results from the Part I, Phase 2 test-humidity with corresponding changes incurred in mois- ing revealed some merit for rms noise in deposited car-ture resistance. Such a trend was generally absent. How- bon-film resistors. However, extensive investigation inever, the significance was mitigated by the lack of large the following phase failed to establish useful correlationdeviation in the specimens' resistances. Nevertheless, a of noise with the performance of sample groups in thecomparison of these results with those observed in a standard destructive tests. It was demonstrated thatgroup of identical resistors which underwent the initial this noise test was not a suitable nondestructive methodevaluation showed that there is no advantage for the for the tested resistor designs and did not merit furtherapplication of the trial amount of polarization to resis- study. Although it was not the purpose of this investi-tors during the short-time humidity. gation to compare the merits of resistors from the vari-
In the short-time humidity test adopted for evalua- ous manufacturers, it was also noted that there weretion, the samples were exposed to a combined environ- large differences in the noise characteristics of resistorsment of 650 C at 90-98 per cent relative humidity for between manufacturers.one hour in an environmental chamber, with subsequent Results from preliminary tests using voltage coef-measurement of resistance a half hour after exposure. ficient indexes were inconclusive. However, an examina-
tion of the 14 test groups to which this test was appliedin later phases demonstrated conclusively that the testis not a suitable predictor for any of the types of resistors
Results of the preliminary investigation have show tested.the temperature coefficient index and the short-time Again, it should be noted that processing of data ob-overload test to be the more promising of the non- tained from measurements of 6400 resistors to which thedestructive tests. However, there was some association more promising tests were applied is as yet incomplete.noted for the short-time humidity test with moisture It is anticipated that analysis of these data will yieldresistance. Table II illustrates data representative of results which will definitely determine the ultimate suc-test results and correlation of temperature coefficient cess of these tests as predictors.
