Effect of Solder Joint Degradation on R-F Impedance

Daeil Kwon, Michael H. Azarian, and Michael G. PechtCenter for Advanced Life Cycle Engineering (CALCE)

University of MarylandCollege Park, MD 20742, USA

Phone: +1-301-405-5323, FAX: +1-301-314-9269

Abstract of the signals is contained [4]. As shown in equation (1), theThe trend for many types of electronic products is toward skin depth, 6, is directly related to the frequency, f and the

higher operating frequencies or digital bit rates. At high resistivity of the conductor, p:frequencies, signal propagation is concentrated at the surfaceof interconnects, a phenomenon known as the skin effect. p (1)Degradation of interconnects, such as cracking of solder joints i= owdue to fatigue or shock loading, also usually initiates at the where, p denotes the material's permeability in a vacuum.surface and propagates inward. Therefore, even a small crackat the surface of a solder joint may change RF impedance andadversely affects the performance of high speed electronic Signal propagation in high frequency electronic assembliescircuits. Traditional methods used to monitor interconnect is more sensitive to interconnect degradation than it is in lowreliability are based on measurement of DC resistance. More frequency electronic assemblies due to surface concentration.accurate and sensitive alternatives are required for monitoring In other words, even a small crack at the surface of solderthe reliability of current and future electronic products. RF joints can directly influence signal integrity, which mayimpedance analysis offers an improved means of sensing reduce the performance of high speed electronic products. Ininterconnect degradation. monitoring solder joint reliability for high speed applications,

This study demonstrates the value of RF impedance traditional low speed measurements, such as DC resistance,measurements as an early indicator of physical degradation of are of limited value since they are not able to sense smallsolder joints compared to DC resistance measurements. cracks that can affect the performance of high speedMechanical fatigue tests have been conducted with an assemblies. As a reliability monitoring tool, DC resistanceimpedance-controlled circuit board on which a surface mount often responds too late; for example, after the crack is largecomponent was soldered. Simultaneous measurements were enough to result in a DC open circuit. RF impedance,performed of DC resistance and the time domain reflection however, should be capable of detecting small cracks. Due tocoefficient, as a measure of RF impedance, while the solder the skin effect, RF impedance should show an increase injoints were stressed. The RF impedance was observed to response to physical degradation at the surface earlier than DCincrease in response to cracking of the solder joint while DC resistance. Figure 1 shows a conceptual representation of theresistance remained constant. Failure analysis revealed that increased sensitivity of RF impedance measurements tothe RF impedance increase resulted from a physical crack physical degradation of interconnects. In this study, cyclicinitiated at the surface of the solder joint, which had shear stresses were used to cause fatigue cracking of solderpropagated only partway across the solder joint. joints. Results of failure analysis and in-situ electrical

monitoring will demonstrate how an intermediate stage ofIntroduction solder joint failure affects RF impedance, and compare its

As clock speeds and communication frequencies rise, sensitivity to solder joint degradation with that of DCperformance of electronic products and their reliability are resistance.becoming increasingly sensitive to the integrity of theinterconnects across which signals travel, such as solder Interconnect degradation measurementjoints, printed circuit board traces, and connectors. Solder Figure 2 describes the relationship between the frequencyinterconnects are exposed to a variety of load conditions from and the skin depth for 1-oz copper and eutectic tin-lead soldertheir operation and storage, such as temperature cycling, compared to typical dimensions of these two kinds ofvibration, shock and humidity. Each of these conditions can interconnects. According to Figure 2, the skin depth for bothlead to solder joint failure by specific failure mechanisms; for copper and eutectic tin-lead becomes less than about a tenth ofexample, differences in the coefficient of thermal expansion the interconnect thickness above the 500MHz. Therefore, at(CTE) between components, solders and substrate materials frequencies above 500 MHz one can expect enhancedresult in fatigue failure of solder interconnects [1-3]. Most sensitivity of RF impedance to intercoimect degradation.fatigue failures are initiated by cracks in the circumferential Since many current commercial products are operating in thearea of solder joints where the strain range is maximized. frequency range of a few gigahertz, in this study theCracks propagate inward, leading to the failure of solder joints monitored frequency window was chosen to be 500 MHz to 6and eventually to the electronic system itself. GHz.

At high operating frequencies, signal propagation is S-parameters are commonly used to characterize electricalconcentrated at the surface of interconnects. This phenomenon properties in high frequency applications. The S-parametersis known as the skin effect. The skin depth refers to the can be measured by using a network analyzer to send highthickness of the conductor within which approximately 63% speed signals through the circuit and measure reflection (Si,

978-1-4244-2318-7/08/$25.00 ©2008 IEEE SPI 2008

S22) and transmission (S21, S12) coefficients over either thefrequency or time domain. A frequency domain measurement V-UQINetrXQI2I.shows the effect of discontinuities present in the circuit as the itd'lyzi (VNJA)amplitude of the reflected (SI,) or transmitted (S21) signal pi INt; il

across the frequency spectrum. On the other hand, a timep

domain measurement shows any discontinuities as discrete hqi 1"peaks with respect to their locations in the circuit. Any ____discontinuities due to impedance mismatches within thecircuit are seen as discrete impedance discontinuities, which isuseful in identifying fault locations. Since this study focuses Don the solder joint degradation that occurs at specifiedvsiDlocations in the circuit, a time domain analysis was conducted. Figure 3 - Schematic of the circuit for simultaneous

monitoring of the RE and the DC responsesFailue critefion In order to allow simultaneous monitoring of the RF

/ ~~~impedance and the DC resistance, bias-tees were incorporated/ ~~~into the test circuit. The DC and the RF measurement/ ~~~~instruments were connected to the DC and the RE ports of the

Z bias-tees, respectively. The composite ports were connected to<'RDCI both ends of the circuit board. All connections were made

using RE cables, which also have a characteristic impedance/ ~~~~of50 Ohms.

A Keithley 2010 multimeter and an Agilent E8364AIncreasedsensitintv ~vector network analyzer (VNA) were used to monitor the DCIncremeSenSiij7itN A tDCresistance and the RE impedance, respectively. The VNA has

AtRF a bandwidth of 45 MHz to 50 GHz, and can be used for signaltcAt" tr'~~'DC t,

reflection or transmission measurements in the frequency1f~A1RPt~~Arc tf Tinie domain or for the time domain reflectometry. In this study,Figure 1I Conceptual representation of increased time domain analysis is presented.sensitivity of RI impedance Test conditions

1000 Mechanical shear force was directly applied to the lowDiameter of solder ball pass filter to degrade the solder joints connecting the filter to

100 ~~~~~~~~~~~~thecircuit board. A sinusoidal shear force was applied using100 ~~~~~~~Thickness of i-ozcopper an MTS Tytron 250, in order to generate fatigue failures. An10 Eutectic~.. .. .. tin-lead. offset force of 40 N was applied in order to maintain contact

10 __--_.Futectic tin-lead ~throughout the entire fatigue cycle. The oscillatory force waschosen to be 10 N, which corresponds to the amplitude of the

1 ~~~~~~~~~~~~sinusoidalforce superimposed on the offset force at afrequency of 0.25 Hz. As a result, the cyclic shear force varied

0- ~~~~~~~~~~~~between30 N and 50 N in a sinusoidal wave form with a

100k iM lOM loom IG lOG period of four seconds. A ceramic material was insertedFrequency (Hz) between the metal tip of the force transducer and the filter to

Figue 2 Comarisn beweenskindept anddimesionavoid electrical contact at the metal tip. Both the RE and the

ofiguere2-comparisonbtensi ephaddmnin DC responses were collected every 30 seconds.Instrumental control software was used to instruct the

Experimental setup multimeter to collect the DC resistance measurementsA test circuit for simultaneous measurement of the RE and periodically. At the same time, the TDR reflection coefficients

the DC response has been developed, as shown in Figure 3. over the entire time domain of the test circuit were collectedThe test circuit consists of the following: an impedance- as a measure of RE impedance. For comparison to the DCcontrolled board with an SMT low pass filter, two bias-tees, resistance measurements, the TDR reflection coefficients fromRE cables, mechanical load unit, and measurement the failure site were extracted and displayed in one plot. Theinstruments. The circuit board has a controlled characteristic actual values of initial TDR reflection coefficient and DCimpedance of 50 Ohms to match that of the test equipment, resistance depend on the amount of solder and vary somewhatcables, and other components. On this circuit board, an SMT from sample to sample, although similar trends were observedlow pass filter was soldered. The cut-off frequency of the low in multiple trials. Each experiment was conducted until itpass filter is 6.7 GHz. Since the monitored frequency span in resulted in a DC open circuit or until TDR reflectionthis study is between 500 MHz and 6 GHz, the filter acts as a coefficient increased significantly.conductor with the same characteristic impedance of 50 Results and discussions

Ohms. ~~~~~~~~~~~~~TheTDR reflection coefficient at particular points ofinterest needed to be extracted from the overall time domain

plot. Figure 4 shows two measurements of TDR reflectioncoefficient over the signal transit time domain: before andafter the experiment.

The physical locations corresponding to the peaks wereidentified experimentally through association with knownfeatures in the circuit. By comparing the initial and final TDRresponses, the peak was visually confirmed to be a responsefrom the failure site, a cracked solder joint.

700

Fn2alTDRresponse[

ITDRreflection coefficienlt30 ig s i at the failu site

F 299 Figure 6 SEM image of the degraded solder joint

= Initial TDR responselr Plane of observation

8 8 9 9.5 1 19.5 c tCross-sectioning= 9 9m5 degraded solder joint. A crackwhiciniiateathesrfacdirectioneSignal transit time (ns) SMT low pasr flter soserjoinFigure 4 -lThe TDR response before and after theexperiment IFi (lder ade

169 8 Board

1587

12 Th tes wasrdstope -etr ossetwt hedrcino h ple156este 60wefoFigure 7 - Cross-sectioning direction and plane ofFigure 8 shows cross-sectional SEM images of the

TDR reflection coefficient at0 the failure se30 adegraded solder joint. A crack which initiated at the surface

~~~~Tstne(i)TDR reflection coefficient aa nraey4m,btthe faluC-1iue54 - CoprsnbtenR n Crsoss rssand eproatedainwad wais cntilevarlyeob iserved.dAssleenri

Figure 5 sow te rsutsofonefaige estcopain Figure 8(b), the left hand side solder joint began to open1 e2 4 towards the center, consistent with the direction of the applied

DC resistance shear force, while the right hand side solder joint was still159 3 intact. From the figures the crack growth was found to be

9 59 199 159 299 259 399 350 about a half of the solder joint in length. By this time, theTest time (Min) TDR reflection coefficient had increased by 4 mU, but the DC

Figure 5 - Comparison between RF and DC responses resistance remained at its initial value. This provides clearduring the fatigue test evidence that RF impedance is more sensitive in detecting

Figure 5 shows the results of one fatigue test comparing interconnect degradation due to the skin effect. From otherthe TDR reflection coefficient at the failure site with the DC experiments, it was shown that the test results wereresistance. In order to study the effect of solder joint failure on qualitatively repeatable, though their initial values wereRF impedance changes, the test was stopped after 308 minutes slightly different depending on the applied force and theof operation when the TDR reflection coefficient showed amount of solder.about a 4 milliunit (mU) increase from its initial value. At the This study showed that the combination of an impedancebeginning of the test, both DC and RF responses remained controlled board with SMT low pass filter and two bias teesaround their initial values. As the test proceeded the RF was an appropriate test vehicle for monitoring RF and DCresponse began to increase while DC resistance remained responses simultaneously. Also, TDR reflection coefficientalmost constant. was a useful parameter for monitoring circuit impedance

A partially degraded solder joint was obtained and used to changes to detect interconnect degradation. The test resultsrelate the failure mechanism and extent of damage to the R1F showed that the RF response exhibited earlier sensitivity thanresponse changes. From the SEM (Scanning Electron the DC resistance to solder joint degradation. The failureMicroscope) image seen in Figure 6, an externally visible analysis of the degraded solder joint revealed that the crackcrack was found along the interface between the component which initiated from the surface of the solder joint resulted inand circuit board. In order to allow further investigation, this the RF impedance changes. Therefore, RF impedance analysissample was cross-sectioned along the direction of the signal can serve as a non-destructive early indicator of solder jointtrace as shown in Figure 7. degradation.

at its initial value. The physical crack was found to haveextended about halfway across the solder joint. The directionof crack propagation coincided with that of the applied shearforce. It was confirmed and demonstrated that RF impedance

low pass filter exhibits earlier sensitivity to interconnect degradation thandoes DC resistance. These results imply that reliability

Shear forceassessment based on DC resistance measurements maydire ~~~~~~~~~~~~~overestimate the lifetime of high speed electronic assemblies.

Solderjoint Co__e RF impedance can provide a more accurate assessment of theSolderjint Coper padreliability of high speed electronic products in response to

solder joint degradation. Furthermore, this technique showspotential as a prognostic tool that can provide advancedwarming of impending interconnect failures.

Currently, these RF measurements have been performedwith laboratory equipment, such as a vector network analyzer,

*ll Z3 -_*- _ which is multi-functional and very high performance, and(a) _-consequently may be two orders of magnitude more expensive

than the equipment needed for resistance monitoring.Eventually, an improved understanding of the correlationbetween RF characteristics and interconnect degradationshould allow simplification of the RF monitoring tools. Thismay involve measurement of specific parameters, such as the

|directlon s _ | signal strength at a specific frequency or the bit error rate. Theequipment needed for this purpose may be an order of

Low pass filter magnitude less expensive, more widely available, and evenfeasible to implement in the field as opposed to the laboratory.Ultimately, one can envision reducing the RF-based detection

Solderjoint of interconnect degradation to the board or die level. ThisCopper pad would require an investment of time and expense for

development, but would drastically reduce per-unit cost.Furthermore, it would create the opportunity for real-timedetection and early warning of interconnect degradation, allow

(b) condition-based maintenance, and reduce unplanned down-time. This could bring potentially substantial savings inoperationa and repair costs, hel to redue te incidece of"no trouble found" failures due to intermittent contactbehavior, and improve product safety and availability.

Lwpass file AcknowledgmentsThis work was funded by the over 50 members of the

CALCE Electronic Products and Systems Consortium at the

erjoint University of Maryland.References

Cper pad [1 Cuddalorepatta, G., and Dasgupta, A., "Creep and StressRelaxation of Hypo-Eutectic Sn3.OAgO.5Cu Pb-freeAlloy: Testing and Modeling," 2007 ASME IMECE,Seattle, WA, (2007)

[2] Pecht, M., McCluskey, P., and Evans, J., Failures inamm_ _ a Electronic Assemblies and Devices, Springer-Verlag,(c) (London, 2001), pp. 204-232.

Figure 8 - Cross-sectional SEM image of (a) overview (b) [3] Lau, J., Solder Joint Reliability - Theory andleft hand side close-up, and (c) right hand side close-up Applications, Van Nostrand Reinhold, (New York, 1991),

Conclusions pp. 545-587.A technique for detecting solder joint degradation using [4] Thierauf, S. C., High-Speed Circuit Board Signal

RF impedance has been presented in this study. A test vehicle Integrity, Artech House Inc., (Massachusetts, 2004), pp.was developed to allow direct comparison of RF and DC 17-30.measurements to monitor interconnect degradation. Failureanalysis of the degraded solder joint was performed when theTDR reflection coefficient, as a measure of RF impedance,showed an increase of 4 mU while the DC resistance remained