IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 3, AUGUST 2005 503
Environmental Qualification Testing and FailureAnalysis of Embedded Resistors
Lawrence John Salzano, II, Chris Wilkinson, and Peter A. Sandborn, Senior Member, IEEE
Abstract—Embedding passive components (capacitors, resis-tors, and inductors) within printed wiring boards (PWBs) is one ofa series of technology advances enabling performance increases,size and weight reductions, and potentially economic advantages inelectronic systems. This paper explores the reliability testing andsubsequent failure analysis for laser-trimmed Gould subtractivenickel chromium and MacDermid additive nickel phosphorousembedded resistor technologies within a PWB. Laser-trimmedresistors that have been “reworked” using an inkjet printingprocess to add material to their surface to reduce resistancehave also been considered. Environmental qualification testingperformed included: thermal characterization, stabilization bake,temperature cycling, thermal shock and temperature/humidityaging. In addition, a pre/post-lamination analysis was performedto determine the effects of the board manufacturing process onthe embedded resistors. A failure analysis consisting of opticalinspection, scanning acoustic microscope (SAM) and environ-mental scanning electron microscope (ESEM) imaging, and PWBcross-sectioning was employed to determine failure mechanisms.All the embedded resistors were trimmed and the test samplesincluded resistors fabricated both parallel and perpendicularto the weave of the board dielectric material. Material stabilityassessment and a comparison with discrete resistor technologieswas performed.
D ISCRETE passive components are continuing to increasein use in electronic systems even though manufacturers
are increasing the degree of system integration. In 2001, pas-sive devices accounted for 91% of components, 41% of boardarea, and 92% of all solder joints in an electronic system, butonly 2.6% were integrated in some fashion [1]. Driven by perfor-mance, size, and economic concerns, embedded passives wereintroduced to the market in the early 1980s. Embedded passives,also known as integral passives, are passive components buriedin interconnecting substrate materials. The potential advantagesoffered as a result of embedding passives include: increased cir-cuit density, improved electrical properties, cost reduction, in-creased product quality, and improved reliability.
Embedded resistors are manufactured from both thin- andthick-film technologies by depositing and pattering layers of
Manuscript received June 28, 2004; revised December 9, 2004. This workwas supported in by the Computer-Aided Life Cycle Engineering (CALCE)Electronic Products and Systems Center and, specifically, the members of theCALCE Consortium.
The authors are with the CALCE Electronic Products and Systems Center,Department of Mechanical Engineering, University of Maryland, College Park,MD 20742 USA.
Digital Object Identifier 10.1109/TADVP.2005.848387
Fig. 1. TV-1R embedded resistor test vehicle [13].
resistive material in conjunction with interconnect lines withina substrate. The primary driver for replacing surface-mountresistors with embedded resistors is to enable faster bus speeds.As bus speeds increase, the electrical termination requirementsincrease, and thus the number of resistors required increases. Inorder to reduce transmission times and improve performance,embedded resistors can be buried in the substrate to minimizethe parasitics (inductance and capacitance) generated by inter-connects. In addition, embedded resistors free valuable boardsurface area and potentially improve reliability through theelimination of solder joints and plated through-holes character-istic of surface mount and through-hole technologies.
One significant factor governing the applicability of em-bedded resistors is their tolerance level. The tolerance to whicha resistor can be fabricated determines the applications forwhich it can be used. Tolerances of 10 or larger are readilyachievable with today’s embedded resistor technologies, how-ever, achieving 1 is a challenge [2]. While surface-mountresistors can be presorted by value, or even replaced duringassembly when their value is not within the required range,embedded resistors provide no such opportunity and mustbe within design tolerance value before the board fabricationprocess is completed. One possible impediment to the wide-spread use of embedded resistors is the ability (and expense) oftuning or trimming the resistors to the appropriate value range(as defined by the design tolerances) prior to the lamination ofthe layer pair containing them into the board [3].
Laser trimming of film resistors has been performed for manyyears with application to resistors on silicon and trimming ofsurface mount discrete resistors prior to packaging, e.g., [4].
504 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 3, AUGUST 2005
Fig. 2. Reliability test plan. The numbers located in the arrow paths indicate the quantity of each type of board subjected to the proceeding test.
However, only recently, highly automated laser trimming tech-nologies have been developed and demonstrated for trimmingof embedded resistors during the board fabrication process [2].Resistors are normally trimmed by micromachining a troughin the resistive element. The length and path of the trough de-termine the resistance change obtained. As the laser cuts thetrough, the resistor value is measured and used as feedback tocontrol the trimming process. Several differently shaped pathscan be used depending on the specific trimming requirements.High-precision laser trimming of buried resistors can tailor re-sistance values to within less than 1% tolerance of target values[2].
Laser trimming is limited to only increasing the resistancevalue of embedded resistors—the value of a resistor cannot bedecreased by removing material. If laser trimming is a manu-facturing option, embedded resistors are usually designed sothat the distribution of resistance values resulting from theirmanufacturing process is centered on a value that is lowerthan that required by the application—trimming is then usedto increase the resistance to the desired value. Nonetheless,some fraction of manufactured resistors will have higher thanrequired resistance values due to the distribution of the orig-inal manufacturing process, material voids encountered duringtrimming, or trimming errors. Prior to completion of the boardfabrication process, it is also possible to perform a material-ad-
dition “rework” process on embedded resistors that have toohigh a value. One method of reworking embedded resistors isto print conductive ink on the surface of an embedded resistor,thus, adding a parallel resistor that effectively “trims down”the resistor value [5].
A. Embedded Resistor Reliability
Since embedded passives cannot be replaced after the board orsubstrate is completed, long-term reliability and yield are majorconcerns for manufacturers. One of the key reliability issues forembedded resistors is how well the resistive materials are ableto adhere to the surface of the substrate onto which they aredeposited [6]. Furthermore, delaminations between the resistorand copper leads are possible in addition to chemical interactionsbetween resistor and copper electrode materials [7]. Thesefailure mechanisms are a result of manufacturing processesand application-specific environmental and mechanical loadsthat subject the embedded resistors to temperature variations andsubstrate deformations. Testing embedded passive componentsfor these particular failure mechanisms can be achieved throughtemperature shock, temperature cycling and various bendingand torsion loading tests [6]. Zhou et al. [8] conducted thermalcycling and electrostatic discharge (ESD) tests on laser-trimmed,embedded ceramic paste resistors measuring 100, 1000, and
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 505
Fig. 3. Pre/post lamination analysis. These results are for all 40 fabricated boards.
10 000 and ceramic capacitors of large and small sizes buriedin FR4 boards. No induced failures were observed in any ofthe tested materials. Gould Electronics, Inc. reviewed in-housereliability assessments of two different types of subtractivethin-film nickel–chromium alloy resistors subject to humidity,thermal cycling, conventional reflow, and soldering heat tests[9]. The effect of power dissipation on embedded thin-filmnickel–chromium resistors has been touched upon; however, noenvironmental qualification was performed [10]. Felten et al.[11] conducted thermal cycling and ESD testing on ceramicembedded resistors and capacitors; however, their researchwas limited in the same manner as Zhou et al. Fairchildet al. [12] conducted reliability tests on flexible thin-filmembedded resistors and electrical characterization of thin-filmembedded capacitors and inductors; however, laser-trimmed(and material-addition reworked) embedded resistors were notincluded within the study.
Generally, all the previous reliability assessment work onembedded resistors has only treated subtractive technologiesand not considered reworking. The previous studies do notconsider thermal characteristics, long-term stability, thermalshocking, and humidity exposure, and since no failures wereobserved, subsequent failure analysis was not performed. Thispaper presents more extensive environmental qualificationtesting results for both additive and subtractive embeddedresistor technologies than previously reported and includesdetailed failure analysis.
For embedded passives to insert themselves into the main-stream market, performance characteristics and materialstability must be comparable to, or out perform existing dis-crete passive technologies. Therefore, long-term operationand storage reliability needs to be assessed for tight-toleranceembedded resistors. Section II of this paper summarizes thetest vehicle and test plan. Section III summarizes the results
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TABLE ITCR COMPARISON BETWEEN GOULD AND MACDERMID EMBEDDED RESISTORS IN ppm= C (�25 C TO 125 C RAMPED OVER A 4-h TIME PERIOD)
obtained from the testing, and Section IV provides the resultsof the associated failure analysis.
II. ENVIRONMENTAL TESTING
A. Test Vehicle
The TV-1R test vehicle is a multilayer printed wiringboard (PWB), approximately 5-in square, manufactured frometched FR-406 copper panels originally designed by DelphiAutomotive Systems for the NIST Advanced EmbeddedPassives Technology Consortium [13]. The FR-406 materialis an epoxy–glass specifically selected for its characteristichigh glass transition temperature (T 170 C) enabling theTV-1R board to withstand severe environmental regimens.Layer 2 of the PWB contains an array of 20 cells containingapproximately 600 embedded resistors of the two distinct ma-terial/fabrication approaches described in the next paragraph.The 20 cells are equally divided into ten horizontal and tenvertical orientations. Fig. 1 shows the TV-1R test vehicle withone of the resistor cells magnified. The TV-1R test vehicle isconstructed with only square aspect ratio resistors; however,an assortment of different size resistor squares and I/O trackwidths exist throughout the board. Resistor sizes range from10 to 50 mils, and the I/O track widths vary from 5 to 20 mils.Cell arrangements contain two distinctive internal patterns;either a daisy-chained configuration consisting of four stringsof nine resistors (36 resistors total) or isolated resistors. A 100contact edge connector is located at the top of the PWB throughwhich the resistance of daisy-chained and/or isolated resistorconfigurations can be measured.
Fabrication of the inner layer pairs and final board lamina-tion was completed by Merix Interconnect Solutions. TV-1Rtest vehicles were fabricated using two types of embeddedresistor materials/approaches, provided by Gould Electronicsand MacDermid Inc. The MacDermid M-Pass nickel–phos-phorous (NiP) material is plated directly onto the inner layer
pairs of the FR-406 PWB using an additive process, [14].Meanwhile, Gould (TCR Thin Film Embedded Resistor Foil)uses a dedicated layer pair in conjunction with a subtractivetechnique starting with a copper foil coated with resistivenickel–chromium (NiCr) material that requires a multistagephotoresist and etching process to fabricate the required pattern[9].
Electro Scientific Industries (ESI) conducted the embeddedresistor trimming process for the TV-1R test vehicle. MicroFabTechnologies, Inc. performed the embedded resistor material-addition rework for the TV-1R test vehicle by inkjet printinga proprietary conductive polyimide-based ink onto the surfaceof embedded resistors to lower individual resistor resistancevalues.
B. Manufacturing Process and Test Plan
Forty printed wiring boards of each type of embedded resistortechnology/material (MacDermid and Gould) were fabricatedand 27 of each type (54 boards total) were subjected toenvironmental testing. All of the reliability tests were conductedaccording to the process flow in Fig. 2. Testing commencedwith initial metrology to determine baseline resistance values.Then, two boards of each type were used to establish upperand lower bound temperatures of both the TV-1R PWB testsamples and the environmental testing chambers. Then, subsetsof samples (five boards of each type, ten boards total) weresubjected to linearly increasing temperature, stabilization bake,thermal shock, temperature cycling, and temperature/humidityaccelerated aging profiles. During predetermined, periodic timeintervals, the TV-1R boards were measured to extract resistancevalues. Finally, after running each of the qualification tests,data and failure analysis was performed on the test samples todetermine material stability, operating performance, reliabilityattributes, and failure mechanisms characteristic to Gould’ssubtractive NiCr and MacDermid’s additive NiP laser trimmed,and material-addition reworked and nonreworked embedded
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 507
Fig. 4. Gould embedded resistor stabilization bake analysis after 2100 h at 105 C.
resistors in daisy-chained or isolated configurations, orientedhorizontally or vertically to the board weave of various sizeresistive elements and I/O tracks.
No preconditioning of the boards was performed prior totesting in order to avoid compounding the results with ad-ditional unknown variables. It would, however, be beneficialto conduct a series of experiments with preconditioned testvehicles subjected to a reflow profile to simulate a full elec-tronic system manufacturing process before environmentalqualification commenced. It should also be noted that thequalification testing reported in this paper (as with the previouswork referenced in Section I) is for unbiased resistors, i.e.,all testing was performed with no current passing through theresistors—biased qualification testing could yield differingresults.
III. TEST RESULTS
Each test was conducted as described by the reliability testplan presented in Section II. Before environmental testingcommenced, every TV-1R PWB was tested for initial baselineresistance values. Then, each board was subjected to a particularenvironmental qualification test and periodically retested asprescribed in the appropriate test procedure. Finally, the acquireddata was grouped and sorted according to individual embeddedresistor characteristics (technology, material-addition reworked,connection, and orientation) in order to generate time-dependentand statistical representations presenting aggregate resistancefluctuation within the sample population.
In order to present the acquired embedded resistor results ina compact fashion, histograms were generated for each of the
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Fig. 5. MacDermid embedded resistor stabilization bake analysis after 2100 h at 105 C.
qualification tests. For all of the tests except the thermal char-acterization regimen, a percent change in resistance for eachtest location was calculated. A percentage increase indicatesan increase in resistance while a decrease reflects a decreasein resistance. Vertical lines are provided on each histogramrepresenting zero change in resistance. The total number ofdata points used to create a histogram appears in the corner ofeach distribution and represents the total quantity of measuredembedded resistors, which is a combination of isolated anddaisy-chained measurements. Justification for combining iso-lated and daisy-chained measurements into single data countswas the result of analyzing each environmental qualificationtest’s data sorted into isolated and daisy-chained configurations.No quantifiable shift occurred in any portion of the distribu-tions as a result of segregating the data; therefore, combinedisolated and daisy-chained resistor results are represented inthe histograms.
Since trend data demonstrates dynamic behavior over time,time-dependent plots showing dynamic resistance change areincluded for each of the qualification tests in addition to thehistograms presenting static final resistance change.
A. Pre/Post Lamination Analysis
In order to determine the effect on the embedded resistors dueto the PWB lamination process, each individual resistor on theresistive layer was measured before the layer pairs were lam-inated together. The resistors were then measured again afterlamination.
Fig. 3 presents the aggregated resistance percent change dis-tribution for both Gould and MacDermid resistor technologies.The results are ranged over 50 for Gould and 10 forMacDermid in order to show the histograms clearly. 195 valuesor 1.23% (Gould) and 325 values or 1.27% (MacDermid) of per-cent change values are outside these ranges.
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 509
Fig. 6. Gould embedded resistor temperature cycling analysis after 500 cycles between �65 C and 125 C.
Gould’s subtractive embedded resistors exhibited a positiveshift in mean resistance (3.85%), while MacDermid’s additiveresistors showed a negative mean shift ( 2.68 ). Gould’s man-ufacturing process has a larger standard deviation, 7.46 ,compared to MacDermid’s, 1.94 .
B. Thermal Characteristics
To determine the effective temperature coefficient of re-sistance (TCR) for both embedded resistor technologies, fivesamples of each type were subjected to a linearly increasingtemperature profile ranging from 25 C to 125 C rampedover a 4-h period. The value of the TCR (ppm C) was deter-
mined using the method in [15] using a reference temperatureof 25 C.
Table I provides a comparison between the mean, stan-dard deviation, and median values for both manufacturers’embedded resistor technologies for the two orientations andreworked/nonreworked. Gould embedded resistors exhibitedan aggregate mean TCR of 190.81 ppm C, while Mac-Dermid embedded resistors revealed a more even distributionbetween negative and positive TCR with an aggregate meanof 6.28 ppm C. The Gould TCR is more than an order ofmagnitude greater than MacDermid’s. The standard deviationof TCR for MacDermid is greater than that of Gould (26.23and 7.76 ppm C, respectively).
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Fig. 7. MacDermid embedded resistor temperature cycling analysis after 500 cycles between �65 C and 125 C.
We also noted that the TCR profile of the Gould resistorswas essentially positive linear, while the overall trend line ofthe MacDermid characteristic was marginally negative.
C. Stabilization Bake Analysis
To determine the sensitivity of the TV-1R test vehicle tostorage at elevated temperatures without electrical stress ap-plied, a stabilization bake at 105 C for 2100 h was performed.Table II provides a comparison between the most frequentlyoccurring peak interval of the distribution and standard de-viation for both manufacturers’ materials/approaches. Gould
embedded resistors exhibited a symmetric distribution of resis-tance change, while MacDermid resistors are weighted towardincreases in resistance (positively). Figs. 4 and 5 provide per-cent change versus time for a median case resistor as well asaggregate percent change after 2100 h for all embedded resistormeasurements.1 Gould’s resistors stabilized more quickly thanMacDermid’s at 750 h to MacDermid’s 1500 h. MacDermidembedded resistors have one less annealing procedure duringthe manufacturing process than Gould embedded resistors, thismay be the reason why they take longer to stabilize.
1The bar labels on the histograms in Figs. 4–11 represent the value of the endof the interval associated with the bar.
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 511
Fig. 8. Gould embedded resistor thermal shock analysis after 500 cycles between �40 C and 120 C.
D. Temperature Cycling Analysis
To determine the sensitivity of the TV-1R test vehicle toextremes of high and low temperatures and the effect ofalternate exposures to those extremes, temperature cyclingbetween 65 C and 125 C for 500 cycles was performed.Table III provides a comparison between the mean, median, andstandard deviation of the distribution for both manufacturers’embedded resistor materials/approaches. Figs. 6 and 7 providepercent change versus cycles for a median case resistor as wellas aggregate percent change after 500 cycles for all embeddedresistor measurements. Gould embedded resistors exhibited asymmetric distribution between negative and positive response
to the temperature cycling profile while MacDermid resistorswere weighted toward increases in resistance (positively). Bothmanufacturers’ technologies responded with an approximate,most frequently occurring 1% change after 500 cycles. Inaddition, Gould’s embedded resistors stabilized immediatelyafter 20 cycles, meanwhile MacDermid’s took almost 450cycles.
E. Thermal Shock Analysis
To determine the sensitivity of the TV-1R test vehicleto sudden exposure to extreme changes in temperature and
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Fig. 9. MacDermid embedded resistor thermal shock analysis after 500 cycles between �40 C and 120 C.
the effect of alternate exposure to those extremes, thermalshocking between 40 C and 120 C for 500 cycles wasperformed. Table IV provides a comparison between the mean,median, and and standard deviation of the distribution for bothmanufacturers’ embedded resistor materials/approaches. Gouldembedded resistors exhibited a negatively weighted distributionwith a positive peak interval, while MacDermid resistors wereweighted positively except for a few measurements falling intothe negative region. Neither technology stabilized after 500cycles. Figs. 8 and 9 provide percent change versus cycles for amedian case resistor as well as aggregate percent change after500 cycles for all embedded resistor measurements.
F. Temperature/Humidity Analysis
To determine the sensitivity of the TV-1R test vehicle tohumid environments, temperature/humidity accelerated agingwas performed at 130 C with 85% relative humidity under2.325 atm. Table V provides a comparison between the mean,median, and standard deviation of the distribution for both man-ufacturers’ embedded resistor materials/approaches. Figs. 10and 11 provide percent change versus time for a median caseresistor as well as aggregate percent change after 1244 h for allembedded resistor measurements. Both Gould and MacDermidembedded resistors exhibited positively weighted distributionswith a few measurements falling into the negative region.
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 513
Fig. 10. Gould embedded resistor temperature/humidity analysis after 1244 h at 130 C with 85% relative humidity under 2.325 atm.
Gould’s resistor technology stabilized after 800 h, meanwhileMacDermid’s resistors continued to increase beyond the 1244-htest time.
G. Material Stability Assessment
The stability of a resistor refers to how its resistance changeswith time under stressed conditions, e.g., temperature. Thesechanges may be due to recrystallization, hydration, oxidation,and/or other chemical alterations of the resistor material as wellas effects at the conductor–resistor interface [16]. Traditionalresistor technologies are classified according to several parame-ters depending on the specific needs of electronic system manu-facturers including: resistance value, tolerance, stability during
storage or operation, and dissipation [17]. Table VI provides anoverview of several resistor technologies as a function of the re-quired precision.
After performing an extensive environmental qualificationregimen on the TV-1R test boards, Gould and MacDermidembedded resistor technologies performance characteristicsappear to fit within the semi-precision classification for tra-ditional resistor technologies. Yet, for high-speed electronicsystems, resistor tolerance is not as important as the eliminationof the inductive reactance of surface-mount chip components,vias, and traces. Actual tolerance is a combination of the initialmismatch of the device value and line impedance, the devicetolerance, and the series inductance and inductive reactance
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Fig. 11. MacDermid embedded resistor temperature/humidity analysis after 1244 h at 130 C with 85% relative humidity under 2.325 atm.
of the device. Embedded resistors have been found to elim-inate almost all of the inductance normally associated withdiscrete resistors and their vias, therefore enabling a 10%–15%embedded resistor to exhibit significant signal integrity im-provements over chip and discrete resistors of a 1%–2%tolerance [18]. Table VII provides a breakdown of both Gouldand MacDermid embedded resistor technologies and com-pares them against Ohmega Technologies’ well-establishedOhmega-Ply 50 square planar resistor technology [19].
IV. FAILURE ANALYSIS
In this paper, we defined a failure as a shift in embeddedresistance value greater than 50 of the initial unstressed
resistor condition, or the development of an open or shortcircuit as a consequence of a particular qualification test. Afterconducting data analysis, failed embedded resistor cell locationswere identified within the TV-1R test vehicle’s 20-cell matrix(see Fig. 1). Table VIII provides the quantity and configurationof failed resistor measurements for a given test. Two numbersappear below each embedded resistor configuration in TableVIII. The number to the left of a backslash denotes the quantity offailed embedded resistors, and the number to the right signifiesthe quantity of resistors adhering to the specified tolerancelimit (nonfailed resistors).
Visual, scanning acoustic microscope (SAM) imaging, andcross-sectioning of representative failed resistors was per-formed. Two characteristically different failure characteristics
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 515
TABLE IISTABILIZATION BAKE COMPARISON AFTER 2100 h AT 105 C (PERCENT CHANGE)
TABLE IIITEMPERATURE CYCLING COMPARISON AFTER 500 CYCLES BETWEEN �65 C AND 125 C (PERCENT CHANGE)
were apparent upon visually inspection of Gould and Mac-Dermid boards. Failed MacDermid embedded resistors con-tained light discolorations within the resistor material asillustrated in Fig. 12(a). Meanwhile, failed Gould embeddedresistors contained dark discolorations in thin and thick jaggedlines as illustrated in Fig. 14(a). SAM revealed possible delami-nation between the embedded resistors and the FR-406 materialwithin MacDermid test vehicles as shown in Fig. 12(b)–(d). Inorder to identify the failure mechanism, the failed embedded re-sistor was cross sectioned and observed with an environmentalscanning electron microscope (ESEM). The ESEM picturesindicated that delamination occurred between the embeddedresistor and the FR-406 inner layer pair. Fig. 13 shows imagesof both “good” and “bad” embedded resistors making contact
with either a left or right trace. As a result of the delamination,the value of the resistor cross sectioned in Fig. 13 increasedfrom 52 to 705 .
The SAM also revealed material inconsistencies within theGould test vehicles; however, they have a different nature thanthose found within the MacDermid boards. Fig. 14(b)–(d) illus-trate a failure instance within the Gould sample population. Theboxed region in the optical image provided by Fig. 14(a) sug-gests the occurrence of an anomaly; however, upon examiningthe through transmission C-scan of the site, no additionalinformation could be determined about the failure mechanism.Meanwhile, conducting a pulse echo, peak amplitude C-scanat the resistor level disclosed the possibility that the darkeningregion within the resistor was a metallic growth between copper
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TABLE IVTHERMAL SHOCK COMPARISON AFTER 500 CYCLES BETWEEN �40 C AND 120 C (PERCENT CHANGE)
TABLE VTEMPERATURE/HUMIDITY COMPARISON AFTER 1244 h AT 130 C WITH 85% RELATIVE HUMIDITY UNDER 2.325 ATM (PERCENT CHANGE)
TABLE VIBREAKDOWN OF ESTABLISHED RESISTOR TYPES AS A
FUNCTION OF PRECISION [17]
traces either above or below the embedded resistor. When apulse echo, phase inversion C-scan was performed and the
TABLE VIIBREAKDOWN OF EMBEDDED RESISTOR TECHNOLOGIES
corresponding darkened region in the optical image was over-laid with the various colorations generated, the result suggestsmaterial inconsistency at the resistor level. Moreover, the same
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 517
TABLE VIIIFAILURE SUMMARY
coloration patterning was present throughout the copper traceson the rest of the TV-1R board suggesting the metallic growthwas possibly copper.
In order to confirm the metallic growth hypothesis, theembedded resistor was cross sectioned and observed underan optical microscope. Metallic growth appeared to occurbetween traces on the upper surface of the embedded resistor.Fig. 15(a) provides an optical image of a “good” resistor whileimages b, c, and d illustrate the “bad” resistor investigatedwith the optical microscope. Likewise, Fig. 16 provides furtherinvestigation with ESEM imaging. Looking more closely atthe left and right traces, it is difficult to conclude that themetallic growth started from both traces equally since more ofthe copper builds at the right trace and thins as it approachesthe left. In addition, since the cross-sectioned resistor was theproduct of the temperature/humidity qualification test and nobias was used during testing, it cannot be concluded that acurrent flow starting from the right trace and ending at theleft caused the thinning effect on the growth. One possibilityis that during the measurement phase of the test plan, thecopper traces were still malleable and the resulting current flowproduced by the data acquisition equipment caused the transferof copper as the resistance dropped and the current increased.In the cross-sectioned case, the value of the embedded resistordecreased from 52 to 0.01 representing a short circuit.
The example resistors shown in Figs. 12–16 were from tem-perature/humidity testing; however, the remainder of the Gouldand MacDermid sample populations (from the other environ-mental tests) exhibited the same types of failure mechanisms.While MacDermid embedded resistors exhibited delaminationabove the devices between the nickel phosphorous resistormaterial and FR-406 inner layer that increases resistive value,
Fig. 12. Optical and SAM imaging of a failed MacDermid resistor. The failedresistor is located in the box (good resistors appear to the left and right of thebox).
failed Gould embedded resistors have decreasing resistivevalue as the result of metallic growth on the upper surfaceof the nickel–chromium resistor material between coppertraces. Accordingly, (see Section III), the majority of theGould histograms represent negatively weighted distributionssignifying decreases in resistance. Conversely, the majority ofMacDermid histograms represent positively weighted distribu-tions signifying increases in resistance. A correlation betweenthe distribution of resistance values illustrated within the dataanalysis histograms and the observed failure mechanisms onthe embedded resistors can be concluded.
518 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 3, AUGUST 2005
Fig. 13. ESEM images of a “good” and “bad” MacDermid embedded resistor delaminating from an FR-406 inner layer.
Fig. 14. Optical and SAM imaging of a failed Gould resistor. The failedresistor is located in the box (good resistors appear to the left of the box).
V. DISCUSSION AND CONCLUSION
The experimental results obtained through selected environ-mental qualification experiments were used to determine theoperating characteristics and failure mechanisms associatedwith Gould’s subtractive nickel–chromium and MacDermid’sadditive nickel–phosphorous embedded resistor technologiesused in printed wiring boards. Gould and MacDermid embedded
resistor technologies performance characteristics were foundto be comparable to the semi-precision classification for tradi-tional resistor technologies. In addition, the embedded resistorswere inspected for and found to be statistically insensitive toconfiguration in horizontal/vertical orientations, material-addedreworked/nonreworked conditions, and isolated/daisy-chainedconnections. While the majority of Gould’s resistors respondedto environmental stressing with decreasing resistive values, themajority of MacDermid’s resistors responded with increasingresistive values. Gould failures appeared to be the result ofmetallic growth on the upper surface of the embedded resistor,meanwhile MacDermid failures were the result of delaminationbetween the upper surface of the embedded resistor and theFR-406 inner layer. Both failure mechanisms could possibly bediminished using tighter tolerance manufacturing constraintsto eliminate unnecessary defects that historically lead to de-lamination and metallic growth. Cleaner, smoother substratesurfaces, more adhesive bonding methods, additional annealingprocesses, more pure, untarnished materials, etc., could alsohelp to alleviate these unwanted defects. Even though Gouldand MacDermid’s technologies have less tolerance adherencethan high-precision surface-mount discrete resistor technolo-gies (approximately two orders of magnitude less), embeddedresistors will undoubtedly establish themselves in the highspeed electronic systems market since they have proven toeliminate almost all of the parasitic inductance, normallyassociated with discrete resistors and their vias.
SALZANO et al.: ENVIRONMENTAL QUALIFICATION TESTING AND FAILURE ANALYSIS OF EMBEDDED RESISTORS 519
Fig. 15. Optical images of a “good” and “bad” Gould embedded resistor.
Fig. 16. ESEM images of a “good” and “bad” Gould embedded resistor illustrating metallic growth.
ACKNOWLEDGMENT
The authors would like to thank the NIST Advanced Em-bedded Passives Technology (AEPT) Consortium – NCMS,ITRI, 3M, Compaq Computer, Delphi Delco Electronics,
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Lawrence John Salzano, II received the B.S. andM.S. degrees in mechanical engineering from theUniversity of Maryland, College Park, in 2002 and2003, respectively. He is currently pursuing thelaw degree at The George Washington UniversityLaw School, Washington, DC, where his interestsconcentrate on litigation in intellectual property andtechnology law.
At the University of Maryland, he worked on em-bedded resistors reliability analysis and co-developeda GPS-based intelligent tracking system harnessing
both location and data management technologies for use in the criminal justicesystem as a part of TRX Systems.
Chris Wilkinson received the M.I.E.E. (electronicsengineering, B.Sc.) from the University of London,London, U.K., in 1971.
He is a Member of the Research Faculty of me-chanical engineering at the University of Maryland,College Park. His research experience and interestsare in avionics design, maintenance-free operatingperiod, electronics prognostics, parts obsolescencemanagement, and uprating. He has contributedpapers in the areas of reliability assessment, partsobsolescence and management, and uprating. He
was previously with Smiths Aerospace, Cheltenham, U.K., most recently as aPrincipal Research Engineer in the Corporate Research Department.
Peter A. Sandborn (M’87–SM’01) received the B.S.degree in engineering physics from the University ofColorado, Boulder, in 1982, and the M.S. degree inelectrical science and Ph.D. degree in electrical engi-neering, both from the University of Michigan, AnnArbor, in 1983 and 1987, respectively.
He is an Associate Professor in the CALCEElectronic Products and Systems Center (EPSC),University of Maryland, College Park, where hisinterests include technology tradeoff analysis forelectronic packaging, embedded passive component
analysis, system life cycle economics, and virtual qualification of electroniccomponents and systems. Prior to joining the University of Maryland, he wasa Founder and Chief Technical Officer of Savantage, Austin, TX, and a SeniorMember of Technical Staff at the Microelectronics and Computer TechnologyCorporation, Austin. He is the author of over 100 technical publications andbooks on multichip module design and part obsolescence forecasting.
Dr. Sandborn is an Associate Editor of the IEEE TRANSACTIONS ON
ELECTRONICS PACKAGING MANUFACTURING and a Member of the EditorialBoard for the International Journal of Performability Engineering.
