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Effect of Selected Process Parameters on Durability and Defects in Surface-Mount Assemblies for Portable Electronics Leila J. Ladani, Abhijit Dasgupta, Idelcio Cardoso, and Eduardo Monlevade
Abstract—This paper presents a systematic approach to study the effect of manufacturing variables on the creation of defects and the effect of those defects on the durability of lead-free (Pb-free) solder joints. An experiment was designed to systematically vary the printing and reflow process variables in order to fabricate error-seeded test assemblies. The error-seeded samples were then inspected visually and with X-ray, to identify different types of defects, especially voids, and then test for electrical performance. The specimens were subjected to an accelerated thermal cycling test to characterize the durability of these error-seeded specimens and to study the effect of each manufacturing variable on the durability of the solder joints. The response variables for the design of experiments are thermal cycling durability of the solder joints and void area percentage in ball grid array (BGA) solder joints. Pretest microstructural analysis showed that specimens produced under inadequate reflow profiles suffered from insufficient wetting and insufficient intermetallic formation. Statistical analysis of the response variables shows that waiting time, heating ramp, peak temperature, and cooling rate have nonlinear effects on thermal cycling durability. Two variables in particular [peak temperature and waiting time (the time waited after the solder paste barrel was opened and before print)] appear to have optimum values within the ranges investigated. Statistical analysis of void percentage area for all design of experiment (DOE) runs show that higher stencil thickness results in higher void percentage and that void percentage increases as time above melt and peak temperature increases. Index Terms—Defects, design of experiment (DOE), manufacturing process, portable electronics, reflow, statistical analysis, thermomechanical durability.
I. INTRODUCTION ANUFACTURING defects can affect the in-service durability of miniature components that are becoming increasingly popular because of the demand for small hand-held products such as cell phones, personal organizers, and other
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Manuscript received August 21, 2006; revised July 30, 2007 and August 10, 2007. This work was supported by the members of the CALCE Electronic Product and System Consortium at the University of Maryland. This work was recommended for publication by Associate Editor N.-C. Lee upon evaluation of the reviewers comments. L. J. Ladani is with the Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322 USA (e-mail:
[email protected]). A. Dasqupta is with the Department of Mechanical Engineering, University of Maryland, College Park, MD 20742 USA. I. Cardoso and E. Monlevade are with the Instituto Nokia de Tecnologia (INdT), Manaus 69048-660, Brazil (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEPM.2007.914222
small devices. The majority of defects in the assembly of surface-mount technology (SMT) electronics assembly is introduced during the printing and reflow processes. Studies have shown that about 60% of defects identified after reflow originated during the solder paste printing process [1]. Manufacturing defects and their effects on durability are reasonably well understood for Sn37Pb solder joints because of their relative maturity in the electronics industry. The assembly processes have been optimized to improve the quality of the process and the durability of products made with Sn37Pb solders [1]–[9]. The manufacturing process for Pb-free solders has not yet undergone the same level of scrutiny for quality issues. Due to different melting temperature, wettability, and solder surface tension, Pb-free solder joints are more prone to some types of defects than Sn37Pb solder joints. The difference in wettability and surface tension can cause certain kinds of manufacturing defects such as voids [10], reduced spread on copper [11], and wicking in assemblies with Pb-finished surfaces. A higher melting temperature causes other kinds of defects such as secondary reflow effect, fillet lifting, intermetallic growth, increased oxidation, and popcorned components. Therefore, the optimal processes of solder paste deposition and reflow in Pb-free solders are different from those for Sn–Pb solders. Many variables affect the quality and reliability of solder joints. Some of these variables have been investigated in the past. Harrison et al. [12] investigated reflow profile variables and the durability of joints under thermal shock cycling, power cycling, and vibration loading for Pb-free solders. Their experiment showed that the reliability of Pb-free solder joints is comparable with that of Sn–Pb solder joints and that the addition of a small percentage of Bi can allow for the reduction of peak reflow temperature without reducing thermal cycling reliability. Nurmi et al. studied the effect of multiple reflow cycles on solder joint voids for Pb-free ball grid arrays (BGAs) [13]. Their study showed that a longer time in the molten state caused bigger voids. The percentage of voids also increased as the number of reflow cycles increased. Other researchers have also investigated the effect of manufacturing variables on quality and durability of Pb-free solder joints such as Melton [14], Maattanen et al. [15], Jackson et al. [16], Li et al. [17], and Ryan et al. [18], [19] . All the researchers mentioned above show sensitivity of the quality and reliability of joints to printing and reflow variables. The shortcomings of most of these studies are that in most cases the process variables are considered to have a linear effect on durability, and hence only two-level experiments have been designed and conducted for parameter analysis of factor effects
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Fig. 1. Typical ramp-to-spike profile [21].
TABLE I DOE MATRIX, L18 [20]
Fig. 2. Specimen used in accelerated temperature cycle loading.
[18], [19]. In some cases, the result are conflicting [12], [17] and some variables have not been investigated at all. A comprehensive matrix of variables and a three-level DOE is proposed in this study, to systematically explore the nonlinear effects of selected process variables on thermal cycling durability. II. DESIGN OF EXPERIMENT Several factors in the printing and reflow processes affect the quality of solder joints. Studying all of these variables requires a huge test matrix, especially when the nonlinearities in the response necessitate more than two levels for each variable. Due to limitations in resources, a total of six variables (two from the print process and four from the reflow process) have been selected in this study. The variables chosen in the print process include: stencil thickness ( ) and waiting time ( ). The reflow process in this specific application uses a ramp-to-spike profile [21] (Fig. 1) and parameters varied in the reflow process include: heating ramp rate ( ), peak temperature ( ), time above melt temperature ( ), and cooling ramp rate ( ) (Table II). Responses are considered to be nonlinear, except for stencil thickness, since the paste volume is expected to be a linear function of stencil thickness. Therefore, the stencil thickness is studied at two levels, while all other variables are studied at three levels. Since the test matrix for a conventional factorial or fractional factorial DOE is very large for such a study, a Taguchi orthogonal array [20] was selected. Taguchi arrays allow the use of smaller test matrices, but the penalty is that obtaining interaction
effects is very difficult, because many of the interaction effects are confounded into main factor effects and cannot be separated. A mixed L18 orthogonal array, which can handle one twolevel variable (variable A) and up to seven three-level variables (variables B to H), was selected. Variables G and H are not assigned in this study. The L18 test matrix is shown in Table I. The first column in Table I shows the run number and the DOE variables are placed in the subsequent columns. “1” refers to the “low” level of each variable, “2” represents the “intermediate” or “nominal” level, and “3” refers to the “high” level. The only interaction effect that can be identified correctly in this L18 array is that between variables A and B. The six variables for this study and the levels selected for these variables are listed in Table II. Based on the terminology presented here, variable A represents stencil thickness, and variable B represents the waiting time during printing (the time waited after the solder paste barrel was opened and before print). Thus, their interaction effect can also be assessed. The response variables in this study are 1) thermal-cycling durability under the accelerated test condition discussed in the next section and 2) void percentage in BGA joints. III. SPECIMEN AND TEST DESIGN The test specimen used in this experiment is shown in Fig. 2. The printed wiring board (PWB) material is FR4 and PWB finish is organic solderability preservative (OSP) (106A). The board is an eight-layered board with thickness of 1 mm and dimensions of 132 mm 77 mm. The assembly contained three different surface-mount components: 1) CTBGA132 (0.5-mm pitch) with the ball composition of SAC405 (Sn4Ag0.5Cu), 2) MLF68 (0.5-mm pitch), and 3) ceramic chip resistors (1210). The coating on micro lead-frame (MLF) and chip resistors was 100% Sn. The board pad dimensions for MLF was 7.6 7.6 mm . The pad dimensions for chip resistors 2.5 mm and the pad dimensions for BGA was was 1.5 300- m diameter for each solder ball. The solder type used was Lead Free Loctite Multicore 96SCLF300A6588.5. The solder composition is Sn3.8Ag0.7Cu. The stencil opening for MLF was 4 opening for each MLF with dimension of 1.5 1.5 mm . The stencil opening for chip resistors was 1 2 mm and the stencil opening for BGAs was 200- m diameter. All the components were daisy-chained and electrically monitored
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TABLE II VARIABLE AND LEVELS OF VARIABLES SELECTED FOR THIS STUDY
Fig. 3. Accelerated temperature profile for testing.
Fig. 4. Specimens reflowed at three different levels of reflow variables (A1: all reflow variables are at the lowest level, A2: all reflow variables are at the middle level, A3: all reflow variables are at the highest level).
throughout the accelerated temperature cycling test. The PWB was configured for singulation of each MLF and CTBGA, to facilitate immediate removal and destructive physical analysis of each failed component. Fig. 3 shows the temperature profile that was used for the accelerated thermal cycling test. Since the components were too small, the number of cycles to failure for temperature cycle 0–100 was predicted to be too long. So, due to a limited time frame, the most extreme temperature cycle was selected to fail the specimens faster. The maximum temperature was 125 C, and the minimum temperature was 55 C. Dwell times at hot and cold extremes are 15 and 10 min, respectively. Cooling and heating ramp rates were 10 C and 6 C, respectively, per minute.
Fig. 5. Number of grains (normalized with respect to number of grains at the base [close to PWA)] as function of distance from PWA bond pad.
reflow processes, but each selected specimen had all the reflow variables at a single level. Three such PWBs were chosen, one for each level. These are from runs 1–3 in Table I. The PWBs were correspondingly labeled A1, A2, and A3, as shown and discussed in Fig. 4. So A1 refers to the run 1, A 2 refers to run 2, and A3 refers to run 3 of experiment. The BGA solder joint cross sections were then analyzed under polarized light, and the number of grains was counted as a function of the distance from the PWB bond pad. As seen in specimens A1 and A2, the solder ball did not completely melt. The original solder ball contains only a few grains. It can be seen in Fig. 5 for these two specimens that the number of grains decreases as the distance from the pad increases. The number of grains is normalized with respect to the value close to the PWA bond pad. This shows that the solder ball did not completely melt during the reflow. The solder joint in these two cases responds anisotropically to thermomechanical loading. The solder ball in Specimen A3, however, has melted completely, and the grains have recrystallized and refined. Case A3, therefore, has statistically homogeneous properties and responds isotropically to thermomechanical loading. Furthermore, the finer grain size is expected to cause better static and fatigue strength in specimen A3 than in specimens A1 and A2.
IV. PRETEST MICROSTRUCTURAL ANALYSIS A few BGA specimens were selected to be cross-sectioned before the temperature cycling test, and after the print and reflow was completed to characterize the as-manufactured microstructure. All the selected specimens experienced complete print and
V. ACCELERATED THERMAL CYCLING TEST RESULTS Accelerated thermal cycling test was conducted and the cycles to failure were recorded. The test was conducted for 2300 cycles. Number of failures at the end of test was 513 out of 537
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Fig. 6. 3-P Weibull CDF plots for premature failures and main population of failures of CTBGAs. Fig. 7. 3-P Weibull probability density function for early and main population failures for CTBGA.
total specimens tested. The specimens are considered to have failed when the electrical resistance measured by the data acquisition system shows a sudden increase, “spike,” consistently through ten successive cycles. The results were analyzed for statistics and factor effects, using commercial software. Weibull distributions were first fit to the failure data using commercial software. Failure data for BGAs and MLFs were found to have two distinct Weibull slopes ( ) and were divided accordingly into two populations, premature failure subpopulation and main population of failures, using a competing failure modes analysis [22]. Both populations are shown in Fig. 6 for CTBGA components. The probability density functions of both populations are shown in Fig. 7. Competing failure mode analyses in the software used for this study is only available with two-parameter (2-P) Weibull distribution. However, 2-P Weibull, might not be the best fit for the physics of fatigue failures. Therefore three-parameter (3-P) Weibull analysis was then used to analyze both the premature and main failure populations. Comparison between the Weibull correlation coefficient shows that both the premature and main failure populations are better described with 3-P Weibull distribution than 2-P. The ), is 651 cycles and corresponding characteristic life ( Weibull slope ( ) is 1.26 for the main CTBGA population. The corresponding characteristic life for premature failures is 201 cycles and Weibull slope is 3.9. Fig. 8 shows the corresponding probability density functions (pdf’s) for both the premature failure population and the main population, for CTBGAs. The main population has a small overlap with the premature failure population. The premature failure population starts prior to zero cycles because it has a negative location parameter ( ). This implies that there was some cumulative damage introduced in the assembly and handling processes prior to the start of the thermal cycling test. Similar analysis was also carried out for the MLFs and ceramic chip resistors (CCRs) on the PWBs. This result shows that MLFs and CCRs are more than twice as durable as CTBGAs (3-P Weibull characteristic life for the
Fig. 8. 3-P Weibull CDF diagram for premature and main failure population for MLF components.
main population is 615 cycles for MLFs, 806 for CCRs and 349 for CTBGAs). There is no distinct premature failure population in CCRs, so competing failure mode analysis was not conducted for this component. Competing failure model analysis for MLFs, cumulative distribution function and probability distribution function for CCRs are provided in Figs. 9–11. Both MLFs and CTBGAs show a similar number of early failures (19 for MLFs and 22 for CTBGAs). The percentage of premature failures for both components is approximately 10% of the total number of components. The sample size for BGAs and MLFs is 216 and for CCR is 108.
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TABLE III SUMMARY OF THE WEIBULL PARAMETERS FOR CTBGAS
TABLE IV SUMMARY OF THE WEIBULL PARAMETERS FOR MLFS
TABLE V SUMMARY OF THE WEIBULL PARAMETERS FOR CCRS
Fig. 10. 3-P Weibull CDF diagram for CCR components.
Fig. 9. 3-P Weibull probability density functions for both premature and main population of failures for MLFs.
A summary of the Weibull parameters and correlation coefficients is provided in Tables III–V. VI. MICROSTRUCTURAL FAILURE ANALYSIS Most BGAs located on the PWBs that were assembled at the lowest levels of the reflow DOE variables (run A1 PWAs in Table I) experienced premature failures. Failure analysis of A1 PWAs showed that the cracking mostly occurred between the solder and PWA intermetallic. The part was assembled under the lowest levels of print and reflow variables (refer to Tables I and II). The failure analysis in Fig. 12 clearly shows that A1 PWA samples did not have enough time for the solder to melt and create sufficient interfacial intermetallic for a reliable joint. The intermetallic thickness varied along the surface, but generally was less than 1 m. VII. VOID ANALYSIS 2-D X-ray analysis was conducted for the failed BGA specimens for all DOE runs. An internal BGA module was used in the X-ray instrument to calculate void percentage for solder balls. One example of X-ray pictures and void calculation is shown
Fig. 11. 3-P Weibull probability density function for main population of failures for CCRs.
in Fig. 13. The void percentage was averaged over all the BGA components on the PWA in each specific DOE run. VIII. STATISTICAL ANALYSIS OF THE EXPERIMENT RESULTS A commercial DOE software was used to assess the factor effects based on the results of the experiment. Factor effects were quantified by estimating what Taguchi termed “signal-to-noise (S/N) ratios,” for all the factors at each level in the experiment [20]. The signal-to-noise ratio, as defined by Taguchi, is a function of the mean of the square of the response variable across the multiple replicates of each run in the DOE. The S/N ratio is based on the harmonic mean when the response variable has to be maximized (as in this study), and on the arithmetic mean,
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Fig. 12. Crack in intermetallic due to insufficient intermetallic formation in A1 PWA.
Fig. 13. X-ray analysis of voids.
when the response variable has to be minimized. In this study, a large S/N ratio implies a larger value for the response variable (high durability). The signal-to-noise ratio based on the harmonic mean of the square of the response variables, is estimated as follows [20]:
where , where are the response variables for replicates, for each DOE run, and is the square root of the harmonic mean of the outputs of each run. The S/N ratio for each variable at a given level is then obtained by averaging the S/N ratios for all the DOE runs with the same level of that variable. The main factor effects can then be estimated by examining the change in the S/N ratio for unit change in each factor. Factors that produce higher change in S/N ratio are the most significant factors in this study, and they affect the durability more than other factors.
IX. S/N RATIO ANALYSIS FOR DURABILITY AS A RESPONSE VARIABLE Commercial software was used in this study to calculate the S/N ratio for thermal cycling durability as the response variable. The results for the CTBGA component for all the factors are presented in Fig. 14. These figures show the S/N values for different levels of each variable. The S/N plots show that the response variable (durability) is a nonlinear function of at least two out of the five selected three-level factors, waiting time before print ( ), and peak temperature ( ). The S/N ratio plot for stencil thickness shows that as stencil thickness ( ) increases from 4 to 5 mil, the S/N ratio increases by 2.4. The graphs also show that the optimum level of waiting time ( ) and heating ramp rate ( ) and peak temperature ( ) can be found between the second and third levels of these variables. It also shows that durability increases monotonically and linearly with time-above-the-melting-point ( ). The change in the S/N ratio across the levels is highest for waiting time before printing ( ), implying that waiting time is the most significant factor. Sensitivity to waiting time depends on solder paste viscosity, particle size, and stencil finishing. The maximum S/N ratio, in this case, is valid only for the solder paste compound studied here. These results are provided for a limited range of the experimental variables, for the CTBGA only. In some cases, the optimum value of the variable might lie out of this range. In addition, these results are all process-dependent and are for ramp-to spike reflow profiles, and cannot be generalized to ramp-soak-spike profiles. The factor effects are also functions of component size and type. So the results of these experiments for MLF and CCR components may be slightly different and will be the subject of a future paper. It is clear, however, that reflow settings of A1 and A2 are producing a higher level of defects in BGAs than in MLFs and CCRs, although the total defect rate across all 18 DOE combinations are sim) for BGAs and MLFs. ilar ( RATIO ANALYSIS FOR VOID PERCENTAGE AS A RESPONSE VARIABLE X-ray analysis was conducted to study the effect of printing and reflow variables on the creation of voids. Analysis of the results using the same software shows a higher percentage of voids for higher stencil thickness. Higher volume of voids due X.
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Fig. 14. S/N ratio diagrams showing main factor effects for different process variables for CTBGA components.
to higher stencil thickness can be explained as follows. As the stencil thickness increases the volume of solder paste also increases. At the same time, the volume of flux increases. According to Chiu and his coworkers [23], this increase in flux volume should decrease the volume of voids. However, there is another competing factor that increases the void density and that is the pad area covered by solder paste. Since the stencil dimensions are smaller than the pad area, the area covered by solder paste at molten state is significantly different for different stencil thicknesses, and larger area contributes to higher void volume. This analysis also shows that void percentage increases as peak temperature ( ) and time above melt temperature ( ) increases. The study conducted by Liu, Huang, and Lee [24] showed that voiding is influenced by two competing factors, wetting and outgassing caused by flux content. As melting energy becomes larger (higher peak temperatures and longer time above melt in this study) the outgassing and wetting both increase. However, in his study, they showed that effect of wetting is stronger for the solder composite studied by Liu, thereby decreasing voids as the melting energy increased. However, this effect is subject to variations caused by solder composite as well as oxidation caused by extensive melting energy. In other words, for solders
with different composition, increase in wettability might not be as strong. In addition, as mentioned by Liu et al. [24], paste oxidation in higher temperatures and longer time above melt might cover the surface, thereby reducing the contact surface between the solder and copper pad, thus causing an increase in void percentage. A study conducted by Nurmi et al. [13] also showed an increase in voiding in higher peak temperatures and a longer time above melt. An increase in voiding in longer time above melt can also be attributed to excessive decomposition of flux entrapped within the molten joints. The relevant factor effects are shown in Fig. 15. XI. BGA SOLDER CLASSIFICATION BASED ON IPC STANDARD The durability results have been divided into two categories, the premature failures and main population of failures. The premature failures have been caused by defects such as incomplete reflow and nonwetting as observed in Fig. 12. According to IPC-A-610 [26], these two types of defects are unacceptable in all classes. Therefore, the early population of failures is not acceptable for any class of products. No defects were observed in main population of failures. The solder balls were inspected visually and using X-ray. The void percentage in all cases was less than 5%, thus accepted by all
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Fig. 15. Average effect of stencil thickness ( ), peak temperature (T ) and time above melting point (t ) on void percentage.
Fig. 16. Failure analysis of solder joint with highest durability.
three classes of the IPC standard in terms of defects. So, based on this classification, all the solders in the main population can be considered class 3. However, failure analysis of the solders in this group shows variability in solder strength thus causing variability in durability. The specimen shown in Fig. 16 was selected from one of the most durable specimens after the temperature cycling test. As seen, the joint between the solder ball and PWA is so strong that a crack has propagated from the component side. Fig. 17 shows one of the least durable solder balls in the main population of failures. This solder ball fails at the PWA side. A slight difference is observed in solder shape where the solder suffers slight misalignment in the cases where the solder ball failed at the PWA side. Although no conflict was observed visually with requirement for class 3 in terms of defects, since the characteristic life for the solder balls in the main population of failures is not very high ), and the longest durability in this group is 2024 ( cycles, the author concludes that the main population may not still be useful for class 3 due to low durability.
Fig. 17. One of the least durable failed specimens from the main population of failures.
Based on classification life requirements explained in IPC-A-610 (Section 1-4-1) [26], where class 2 is defined as products where continued performance and extended life is required, the author concludes that only a small portion of specimens satisfy this requirement. It is very difficult to visually classify the older joints into first and second class. The only distinction between higher durability and lower durability failed solders are slight misalignment observed in lower durability solders and crack propagation location after thermal cycling, which is at the PWA side for lower durability and at the component side for higher durability. To decide about the transition point, the result of cross sectioning was analyzed, and the point of transition was considered to be where the crack transits to the component side. By this definition, class 2 starts from cycle 1180. Components failed after 1180 had failed at the component side. So, class 1 is the group
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with durability up to 1180 cycles, and class 2 is the group with higher durability. XII. SUMMARY AND CONCLUSION A factorial experiment was designed and conducted to investigate the effect of printing and reflow variables on the quality and durability of Pb-free solder joints. Error-seeded specimens were assembled by varying selected process variables according to a L18 Taguchi DOE array. The response variables are thermal cycling durability and void area percentage. Prior to thermal cycling, microstructural analysis was conducted to compare microstructural features of solder balls produced under three different levels of reflow parameters. The results of the pretest microstructural analysis showed that solder joints produced using a lower to medium peak profile 1 and 2 suffered from insufficient melting, insufficient intermetallic formation, and insufficient wetting. Destructive failure analysis of a specimen reflowed under the first profile and subjected to thermal cycling also confirmed that intermetallic formation was insufficient in the first profile and resulted in premature failures. The Weibull distribution for BGA and MLF component showed two distinct populations, premature and main populations of failure. Premature failures, as expected, belonged mostly to the category of samples reflowed using the lowest levels of variables. These distinct populations were not observed in CCR failures. This shows the level of sensitivity of components to the reflow profile, and that smaller components are not as sensitive to variation of the profile. The reason could be attributed to the small thermal capacity of these components and rapid adjustment of temperature for these small components such as CCR. Statistical analysis of the response variable showed that waiting time, heating ramp, peak temperature, and cooling rate have nonlinear effects on thermal cycling durability. The optimum levels of heating ramp and waiting time can be found between second and third levels of these variables. Statistical analysis of voids shows that larger paste volume, longer time above melt, and higher peak temperature can cause higher void percentages. Higher percentage of voids did not result in lower durability. Another study conducted by the same author shows that the certain range of void sizes actually increase durability [25]. The results provided here are relevant for the type of the component and type of reflow profile considered. This study only presents the result for a few selected variables. Durability and void percentage in Pb-free solders might also be a function of solder and solder paste composition, flux content, surface finish and rheology, and many other factors that can be studied in future experiments. This study does not represent the optimum conditions to obtain void-free joints or minimum void percentage. Many other factors are needed to be investigated. ACKNOWLEDGMENT The authors would like to thank INdT for help with fabrication of error-seeded specimens and destructive failure analysis. They would also like to thank Dr. Okura (currently with Microsoft Corporation) for his valuable help and fruitful discussions at INdT.
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IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 31, NO. 1, JANUARY 2008
Leila J. Ladani received the M.S. degree (with honors) in heat and fluid mechanics from Isfahan University of Technology, Isfahan, Iran, and the M.S. and Ph.D. degrees in mechanical engineering from the University of Maryland, College Park. She is an Assistant Professor in the Mechanical and Aerospace Engineering Department, Utah State University, Logan. Her research is in the area of fatigue and durability of materials at the microscale level as well as reliability and quality of microelectronic devices. She has published several articles in these areas.
Abhijit Dasgupta is a Professor of mechanical engineering at the University of Maryland, College Park. He conducts his research on the mechanics of engineered, heterogeneous, active materials, with special emphasis on the micromechanics of constitutive and damage behavior. He applies his expertise to several multifunctional material systems, including electronic packaging material systems and “smart” composite material systems.
Idelcio Cardoso is a Research Engineer at the Instituto Nokia de Tecnologia (INDT), Manaus, Brazil.
Eduardo Monlevade is a Research Engineer at the Instituto Nokia de Tecnologia (INDT), Manaus, Brazil.
