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Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability
MR-11762; No of Pages 7 Microelectronics Reliability xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability M.Y. Tsai a,⁎, P.S. Huang a, C.H. Lin a, C.T. Wu b, S.C. Hu b a b
Department of Mechanical Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan, Taiwan Chemical Systems Research Division, Chung-Shan Inst. of Science and Technology, Tao-Yuan, Taiwan
a r t i c l e
i n f o
Article history: Received 27 May 2015 Received in revised form 29 July 2015 Accepted 19 August 2015 Available online xxxx Keywords: Aluminum nitride Copper film Thermal reliability Stress Crack
a b s t r a c t Direct-plated-copper (DPC) aluminum nitride (AlN) substrate with a high thermal conductivity can provide a good alternative to conventional aluminum oxide (Al2O3) substrate for better heat dissipation in the highpower module applications. However, the DPC AlN substrate suffers AlN crack initiating at the edge corner of Cu film during thermal cycling, due to the higher thermal expansion coefficient mismatch with copper material. This study is to resolve the AlN crack problem of DPC AlN substrate during thermal cycling and further to provide important parameters for mechanical design for ensuring good thermal reliability. Prior to the analysis, the outof-plane deformation measurement of a Cu-AlN bi-material plate subject to the solder reflow heating and cooling is conducted for evaluating the material property of the plated Cu film and residual stresses induced from the manufacturing and solder reflow process. The results show the hysteresis and Bauschinger-like behaviors for the Cu-AlN plate during the solder-reflow heating and cooling. It is also found from the validated finite element simulation that the Cu-film wedge angle, length, and thickness significantly affect the maximum 1st principal stress of AlN during thermal cyclic loading, and the predicted failure mode and location based on the maximum 1st principal stress is consistent with experimental observation. The other factors, such as single-side and doubleside Cu-film (sandwich-structure-alike) substrates, length difference of Cu films, and the nonlinear property of Cu film will be presented and discussed in detail as well. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Compared with conventional plastic-based printed circuit board (PCB), direct-plated-copper (DPC) and direct-bonded-copper (DBC) ceramics, with relatively high thermal conductivity, low coefficient of thermal expansion (CTE), excellent heat-resistance and chemicalresistance, are used for the high-power module applications such as LED lighting and IGBT packaging [1–4]. The DPC aluminum nitride (AlN) substrate with a high thermal conductivity (k = 170 W/mK) provides a good alternative to conventional aluminum oxide (Al2O3) substrate (with k = 24 W/mK) for better heat dissipation. However, in addition to its higher cost, AlN substrate (α = 4.3 ppm/°C) still suffers the higher CTE mismatch with copper material (α = 16.3 ppm/°C), compared with Al2O3 substrate (α = 6.9 ppm/°C). The high stresses in DPC AlN substrate induced by this CTE mismatch would lead to the AlN crack initiating at the edge corner of Cu film during thermal shock, as shown in Fig. 1 and thus result in a low reliability of the modules. The similar findings during thermal cycling were proposed by Dupont et al. [2] and Wei et al. [4], in which the former identified the Cu-film thickness effect and the latter offered the solution to it by ⁎ Corresponding author. E-mail address: [email protected] (M.Y. Tsai).
using stepped Cu-film structures. This study is going to take a close look at mechanics of DPC AlN under thermal loading and further to find out some important parameters to it in order to enhance the thermal reliability. 2. Methodologies 2.1. Experiment The specimens of Cu-AlN bi-material plate were fabricated by electrically plating Cu on the AlN substrate after sputtering a 0.1 μm-thick adhesion layer of titanium material on the substrate using a commercial equipment. Then, a full-field shadow moiré method [5,6] was used for measuring the out-of-plane displacements of the deformed specimens under thermal loads. The principle of the shadow moiré is that a light source illuminates the grating and specimen with an incident angle of α and a grating shadow appearing on the surface of the deformed specimen serves as the specimen grating created by projecting a beam of light through the reference grating. The moiré fringe pattern can be formed by superposing the specimen and reference gratings, and received and recorded by the camera with an observed angle of β. The fringes of the pattern represent the out-of-plane displacement (w) contours of the specimen surface. The curvature (K) of the specimen
http://dx.doi.org/10.1016/j.microrel.2015.08.010 0026-2714/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
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M.Y. Tsai et al. / Microelectronics Reliability xxx (2015) xxx–xxx
warpage and its mechanism for the bi-material plate under thermal loads, and further to calculate the related stress state in the local region of the bi-material plate. The 2-D axi-symmetry FEM model is applied with the material properties of Cu film and AlN including elastic moduli, Poisson's ratios and CTEs listed in Table 1, in which there is a nonlinear material property for Cu film. The finest mesh with an element size of 5 μm in the model is employed to the region with theoretical stress singularity (or un-limited stress) to pick up the stress raiser. 3. Results and discussion Fig. 1. Failure observation for DPC AlN substrate with Cu-film thickness of (a) 30 μm and (b) 60 μm, after thermal shock cyclic test in the temperature ranging from −40 °C to 125 °C, from optical microscope.
deformation is calculated, based on the assumption of small deformation, by using K¼
1 2w ¼ 2 ρ x
ð1Þ
where ρ is a radius of the curvature, and x is a pre-defined distance. The thermally-induced out-of-plane displacements of the Cu-AlN bimaterial plate were measured by the shadow moiré system with the sensitivity of 15 μm/fringe (at α = 45° and β = 37°) in this study. The thermal cyclic load was applied from room temperature to 245 °C to mimic the solder reflow condition. 2.2. Theoretical and numerical analyses For the theoretical analysis, by modifying Timoshenko's solution of thermal stresses and deformations for a bi-material strip [7,8], the solution to thermally-induced curvature (K) of a bi-material plate is described by: K¼
1 Δε ¼ " ρ t 1 1 þ nm3 ð1 þ mnÞ 2
3 nmð1 þ mÞ
#
ð2Þ
þ ð1 þ mÞ
2Þ where m ¼ tt 21 ; n ¼ EE21 =ð1−ν =ð1−ν1 Þ; Δε = (α2 − α1)(T − T0) T0 and T are stress-
free (initial) and final temperatures, respectively. ti, Ei, νi, and αi represent the thickness, elastic modulus, Poisson's ratio, and CTE in the longitudinal direction, respectively, for the bottom layer (i = 1) and top layer (i = 2) of the bi-material plate. Δεis a misfit strain between the top and bottom layers. And the detailed solutions of axial stresses for the top and bottom layers are also described in [8]. The FEM simulation using ANSYS code was also performed to understand the experimentally obtained
3.1. Thermal deformations A variation of typical warpages of a Cu-AlN bi-material plate (with tCu = 30 μm and tAlN = 370 μm) obtained from shadow moiré measurement is shown in Fig. 2 during the 2nd solder reflow process. The concave shape of the plate deformation at 25 °C gradually becomes flat at the heating process and then more convex as the temperature reaches to 245 °C. During the cooling process, the convex shape is changed to the concave in the reverse way. Note that one fringe represents an out-of-plane displacement of 15 μm. Those shadow moiré data can be converted to the curvature data by using the Eq. (1). The variation of curvatures for the bi-material plate in three cycles of solder-reflow heating and cooling is shown in Fig. 3 obtained from shadow moiré measurement. The three slopes (curvature change per unit temperature) observed in the linear region from moiré measurement are quite consistent with each other and also with those from Timoshenko's solution and FEM analysis. It is apparent that during the first-cycle heating, the curvature increases linearly with temperature increasing, but becomes nonlinear with a slight decrease beyond 100 °C due to the recrystallization of Cu film. In the contrast, during the first-cycle cooling process the curvature is kept in linearity shortly until cooling to 200 °C and then becomes nonlinear. The results also show the hysteresis and Bauschinger-like behaviors for the bi-material plate during the solder reflow heating and cooling after the first thermal cycling. The zerocurvature (or free-stress) temperature at the heating process shifts from about 40 °C at the first cycling to 105 °C after the first cycling, while that at cooling process always stays at 225 °C. Therefore, it is adequate that the free-stress temperature is assumed at 105 °C, rather than 40 °C in the stress analysis of the Cu-AlN bi-material specimen in thermal shock test in the temperature ranging from − 40 °C to 125 °C, since every specimen has to go through the solder reflow process at least one cycle. The detailed out-of-plane displacement distributions along the x-direction and axial stress σx (or σr) distributions along the thickness in the center region for the Cu-AlN bi-material specimen under ΔT = − 80 °C (cooling down from 105 °C to 25 °C) are shown in Fig. 4(a) and (b), respectively, from experiment, FEM simulation and
Fig. 2. Warpage (out-of-plane displacement) variation of a Cu-AlN bi-material plate during 2nd reflow process from shadow moiré (with 15 μm per fringe).
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
M.Y. Tsai et al. / Microelectronics Reliability xxx (2015) xxx–xxx
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Fig. 3. Curvature variation of the Cu-AlN bi-material plate during solder reflow heating and cooling cycling obtained from shadow moiré measurement, theoretical and FEM results.
Timoshenko's bi-material theory. The very consistency of all three displacement results indicates the validity for these three solutions. Thus, FEM and theory can provide the convincing results of the axial stress σx (or σr) distributions along the thickness. 3.2. Validation model and failure mode prediction For DPC AlN substrate (with 10 mm-long Cu film and 13 mm-long AlN) under ΔT = − 1 °C, out-of-plane displacement distributions along the Cu film and axial stress σr distributions along the thickness in the central region are shown in Fig. 5 (a) and (b), respectively, with various thickness of Cu film from FEM and theoretical analyses. Note that since the stresses and deformations from the elastic analysis can be normalized by the thermal load, the stress and deformation values under ΔT = − 1 °C are for representing those data in this study. It is shown that, unlike Fig. 4(a) and (b), both displacement and stress distributions from the FEM analysis are reasonably consistent with those from the theory with minor difference in the quantity between both. That is due to the Cu film shorter than AlN length for FEM results, compared with the same as AlN length for theoretical ones. Despite the minor difference, both results indicate that the thicker the Cu film, the higher the out-of-plane deformation (warpage) of the substrate and axial stress in AlN, but the lower the axial stress in Cu film. The stress contours in the corner region and detailed stress distributions along the Cu free edge across the thickness are shown in Fig. 6(a) and (b), respectively, for the DPC AlN substrate with 100 μm-thick Cu and 370 μm-thick AlN under ΔT = −1 °C from the FEM simulation. It is obvious that, unlike
Fig. 5. (a) Out-of-plane displacement distributions along the Cu film, and (b) axial stress σr distributions along the thickness in the central region, for DPC AlN substrate with various thickness of Cu film under ΔT = −1 °C, from FEM and theoretical analyses.
the central region (in which axial stress σr in AlN is predominant), there exist appreciable σy and τry, besides σr in AlN at the corner region. The maximum principle stress combined with those three stress components would govern the crack initiation of the AlN. Furthermore, the validated model is used to predict the failure mode of DPC AlN substrate under the thermal cyclic loads. The results of vectors of principal stresses from the FEM simulation and possible crack initiation on DPC AlN substrate are shown in Fig. 7(a) and (b) under thermal loads of ΔT = −165 °C and 165 °C, respectively. Note that the length and direction of the vector
Fig. 4. Comparison of (a) out-of-plane displacement distribution and (b) axial stress σx (or σr) distribution of Cu-AlN bi-material specimen, from experiment, FEM simulation and Timoshenko's bi-material theory.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
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Fig. 6. (a) Stress contours in the corner region and (b) detailed stress distributions along the Cu free edge across the thickness for the DPC AlN substrate with 100 μm-thick Cu and 370 μm-thick AlN, from FEM simulation under ΔT = − 1 °C.
represents the stress magnitude and direction, respectively. The results indicate that maximum tensile principal stress of AlN occurs at the edge corner of Cu film for cooling loads (ΔT = −165 °C), whereas maximum compressive principal stress is for heating loads (ΔT = 165 °C). That implies the crack initiation of AlN along the direction normal to maximum tensile principal stress would take place at cooling loads, rather than at heating loads, because of the tensile stress mostly dominating the failure of brittle materials such as ceramics. Therefore, the consistent failure mode and location with the experimental failure mode (shown in Fig. 1(b)) has been successfully predicted based on the maximum principal stress from FEM results (shown in Fig. 7(a)).
3.3. Parametric design and analysis 3.3.1. The effects of Cu-film thickness, length and wedge angle Since the crack initiation of AlN occurs at the edge corner of Cu film, the effect of Cu-film wedge angle (θ) on the maximum 1st principal stress of AlN is investigated for the DPC AlN substrate. The maximum 1st principal stresses (at the location P) of AlN with various wedge angles and thicknesses of Cu film under ΔT = − 1 °C were obtained from FEM simulation and are shown in Fig. 8. It is shown that for θ = 90° the maximum 1st principal stress decreases with the film thickness decreasing, similar to the observation by Dupont et al. [2]. Besides that, most importantly the maximum 1st principal stresses of AlN can be reduced by about 60% from θ = 90° to θ = 15° for 30 μm-thick film and by about 80% for 300 μm-thick film. In other words, the small wedge angle provides more reduction of the maximum 1st principal stress, in terms of both values and ratios, for the case with thick film than for that with thin film. Moreover, the Cu-film length effect is also taken into account
and the corresponding results from FEM simulation is shown in Fig. 9. It is found that the maximum 1st principal stresses for any thickness of Cu film slightly decrease with the film length increasing, dramatically drops in the region with the Cu-film-thickness order distance from the free end, and reaches the smallest value when the Cu-film length is up to 13 mm which is equivalent to the length of AlN and in the free end. 3.3.2. The effects of single and double-side Cu films In addition to the above-mentioned case with a single-side Cu film, the cases with double-side Cu films (like sandwich structures) have been studied and compared with the single-side case. The maximum 1st principal stresses of AlN for single and double-side DPC AlN substrates under ΔT = − 1 °C are shown in Fig. 10(a) and (b) for the cases with 30 μm-thick Cu film and with 200 μm-thick Cu film, respectively. Note that λ is defined as the length difference between the top and bottom Cu films for double-side case. The results show that the maximum 1st principal stresses of AlN for the double-side case are higher than those for the single-side case, especially more obvious in the thick-film case. And the dominant maximum 1st principal stress of AlN occurring at short-length Cu film (at Point 1), rather than at longlength Cu film (at Point 2), is getting higher and higher, and becomes saturated when the λ is increasing. Therefore, for the maximum 1st principal stresses of AlN, the double-side DPC AlN substrate with different Cu-film length (λ ≠ 0) are more severe than those with the same Cufilm length (λ = 0) and with the single-side. Moreover, the data of the Cu-film thickness effect was added into the results in Fig. 10 and was replotted by only taking into account the saturated value of the maximum 1st principal stress of AlN for a certain λ. The re-plotted results are shown in Fig. 11 for the maximum 1st principal stress of AlN against
Fig. 7. Vectors of principal stresses and possible crack initiation on DPC AlN substrate under thermal loads of (a) ΔT = −165 °C and (b) ΔT = 165 °C, from FEM simulation.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
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Fig. 8. Cu-film wedge angle (θ) effect on maximum 1st principal stress (at the location P) of AlN for DPC AlN substrate with various Cu-film thickness under ΔT = −1 °C.
the thickness of Cu film. It is shown that the maximum 1st principal stresses are very sensitive to whether the case is a single-side or double-side Cu film with the value of λ, especially for thick-film (300 μm-thick) case, but not for thin-film (30 μm-thick) case. For the double-side Cu film case, the short-length Cu film always has the relatively large and dominant maximum 1st principal stress on AlN at Point 1, compared with that at Point 2 for long-length Cu film which is close to single-side case. However, as the value of λ approaches to zero, the curves for both become identical. The question is why the maximum 1st principal stress of AlN in double-side DPC is larger than one in single-side. Mechanics of single-side and double-side DPC AlN substrates under thermal cooling load are illustrated in Fig. 12. It indicates that for single-side AlN substrate under thermal load the maximum 1st principal stress of AlN is induced by the CTE mismatch between Cu and AlN, while the one for double-side AlN is caused by the CTE mismatch plus mechanical constraint (equivalent to extra mechanical bending stress) from the bottom part of the structure with a bending downward. As a result, the maximum 1st principal stress of AlN for the double-side Cu is higher than the one for the single-side Cu, and has the highest value at the short-length Cu film. 3.3.3. The effect of nonlinear property of Cu film The nonlinear mechanical behavior of Cu film is also considered in the FEM simulation. This nonlinear property shown in Table 1 has a feature with yielding stress at about 214 MPa. The simulation results of the maximum 1st principal stress of AlN and maximum von Mises stress of
Fig. 10. Effects of single-side and double-side DPC AlN substrates (with Cu length difference of λ for double-side case) on maximum 1st principal stresses of AlN under ΔT = −1 °C, for θ = 90° (a)with 30 μm-thick Cu film, and (b) with 200 μm-thick Cu film.
Cu film against thermal load (ΔT) are shown in Fig. 13 by considering Cu-film linear and nonlinear property for a DPC AlN substrate with wedge angle = 90°. It is shown that nonlinear property of Cu-film significantly affects the maximum 1st principal stress of AlN starting from ΔT = −25 °C to − 75 °C (in which range the elastic modulus of Cu is lower than linear one). And beyond ΔT = −75 °C(in which the
Fig. 9. Cu-film length effect on maximum 1st principal stress (at the location P) on DPC AlN substrate with Cu film thicknesses of 30 μm, 50 μm, 100 μm, 200 μm and 300 μm under ΔT = −1 °C.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
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M.Y. Tsai et al. / Microelectronics Reliability xxx (2015) xxx–xxx Table 1 Material properties used in the finite element simulation (Note: Cu nonlinear properties is taken from [9]). Cu
AIN
Linear
Nonlinear
E (Gpa)
v
α (10
110
0.343
16.4
−6
/°C)
ε (10 1 4 10 20 40
−3
)
E (Gpa)
v
α (10−6/°C)
340
0.25
4.3
σ (MPa) 110 179 214 234 245
Fig. 11. Effects of thickness and λ of Cu film on maximum 1st principal stress of AlN for single-side and double-side DPC AlN substrates under ΔT = −1 °C, for θ = 90°. (Note: For double-side Cu substrate, the data was taken from the saturated value of the maximum 1st principal stress of AlN with a certain λ).
Cu film yields and its elastic modulus is close to zero), the maximum 1st principal stress of AlN reaches the plateaus value of about 490 MPa. This behavior is totally different from the linear results which show the linear increase of the stress with thermal loading. That implies that the nonlinear property of Cu-film can dramatically lower the maximum 1st principal stresses of AlN so that the linear stress analysis gives overprediction of this maximum stress. The maximum 1st principal stresses of AlN for various wedge angles (θ) (at the location P) for DPC AlN substrate under ΔT = −165 °C with various Cu-film thicknesses are plotted again in Fig. 14 by considering Cu-film linear and nonlinear properties. That indicates that, aside from the decrease of the maximum 1st principle stress of AlN in the nonlinear analysis, the Cu-film thickness and wedge angle still show some apparent effects on the maximum 1st principle stress of AlN in the Cu-film material nonlinear region. 4. Conclusions This study aims to resolve the crack problem of direct-plated-copper (DPC) aluminum nitride (AlN) substrate during thermal cycling and further to provide important parameters for mechanical design for ensuring good thermal reliability. The results of the out-of-plane deformation for a Cu-AlN bi-material plate subject to the solder reflow heating and cooling
Fig. 13. Maximum 1st principal stress of AlN and maximum von Mises stress of Cu film (at the location P) by considering Cu-film linear and nonlinear property for DPC AlN substrate with wedge angle = 90° under ΔT = 0 to −165 °C.
have been obtained and shown its hysteresis and Bauschinger-like behaviors. It was found from the validated finite element simulation that the Cu-film wedge angle, length, and thickness significantly affect the maximum 1st principal stress of AlN for the DPC AlN substrate during thermal cyclic loading, and the predicted failure mode and location based on the maximum 1st principal stress is consistent with experimental observation. Furthermore, this maximum 1st principal stress was found very sensitive to whether the DPC AlN substrate is with a single-side Cu film
Fig. 12. Mechanics of single-side and double-side DPC AlN substrates subject to thermal cooling load.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
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principal stress for the double-side Cu film case with a different Cu-film length (λ ≠ 0), which is more severe than those with the same Cu-film length (λ = 0) case and with the single-side Cu film case. This mechanics has been well interpreted in this study. In addition, it was found that the nonlinear property of Cu-film can dramatically lower the maximum 1st principal stresses of AlN which is over predicted based on the Cu-linear stress analysis. References
Fig. 14. Cu-film nonlinear property effect on maximum 1st principal stress of AlN for various wedge angle (θ) (at the location P) for DPC AlN substrate with various Cu-film thicknesses under ΔT = −165 °C.
or double-side Cu film with the value of Cu-film length difference (λ) especially for thick-film case, but not for thin-film case. The short-length Cu film always has the relatively large and dominant maximum 1st
[1] J. Schultz-Harder, advantages and new development of direct bonded copper substrates, Microelectron. Reliab. (43) (2003) 359–365. [2] L. Dupont, Z. Khatira, S. Lefebvre, S. Bontemps, Effects of metallization thickness of ceramic substrates on the reliability of power assemblies under high temperature cycling, Microelectron. Reliab. (46) (2006) 1766–1771. [3] H. Ru, V. Wei, T. Jiang, M. Chiu, Direct plated copper technology for high brightness LED packaging, Impact, 2011. [4] V. Wei, V. Cheng, D. Liu, C. Chang, H. Ru, Stress-reduced thick copper ceramic substrate for high power module applications, IMPACT, 2013. [5] D. Post, B. Han, P. Ifju, High sensitivity Moiré experimental analysis for mechanics and materials, Spring-Verlag, New York, 1993. [6] M.Y. Tsai, H.Y. Chang, M. Pecht, Warpage analysis of flip-chip PBGA packages subject to thermal loading, IEEE Trans. Device Mater. Reliab. 9 (3) (Sept. 2009) 419–424. [7] S. Timoshenko, Analysis of bi-metal thermostats, J. Opt. Soc. Am. 11 (1925) 233–255. [8] M.Y. Tsai, C.T. Wang, C.H. Hsu, The effect of epoxy molding compound on thermal/ residual deformations and stresses in IC packages during manufacturing process, IEEE Trans. Compon. Packag. Technol. 29 (3) (Sept. 2006) 625–635. [9] R. Iannuzzelli, Predicting plated-through-hole reliability in high temperature manufacturing process, the 41st Electronic Components and Technology Conference 1991, pp. 410–421.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability M.Y. Tsai a,⁎, P.S. Huang a, C.H. Lin a, C.T. Wu b, S.C. Hu b a b
Department of Mechanical Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan, Taiwan Chemical Systems Research Division, Chung-Shan Inst. of Science and Technology, Tao-Yuan, Taiwan
a r t i c l e
i n f o
Article history: Received 27 May 2015 Received in revised form 29 July 2015 Accepted 19 August 2015 Available online xxxx Keywords: Aluminum nitride Copper film Thermal reliability Stress Crack
a b s t r a c t Direct-plated-copper (DPC) aluminum nitride (AlN) substrate with a high thermal conductivity can provide a good alternative to conventional aluminum oxide (Al2O3) substrate for better heat dissipation in the highpower module applications. However, the DPC AlN substrate suffers AlN crack initiating at the edge corner of Cu film during thermal cycling, due to the higher thermal expansion coefficient mismatch with copper material. This study is to resolve the AlN crack problem of DPC AlN substrate during thermal cycling and further to provide important parameters for mechanical design for ensuring good thermal reliability. Prior to the analysis, the outof-plane deformation measurement of a Cu-AlN bi-material plate subject to the solder reflow heating and cooling is conducted for evaluating the material property of the plated Cu film and residual stresses induced from the manufacturing and solder reflow process. The results show the hysteresis and Bauschinger-like behaviors for the Cu-AlN plate during the solder-reflow heating and cooling. It is also found from the validated finite element simulation that the Cu-film wedge angle, length, and thickness significantly affect the maximum 1st principal stress of AlN during thermal cyclic loading, and the predicted failure mode and location based on the maximum 1st principal stress is consistent with experimental observation. The other factors, such as single-side and doubleside Cu-film (sandwich-structure-alike) substrates, length difference of Cu films, and the nonlinear property of Cu film will be presented and discussed in detail as well. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Compared with conventional plastic-based printed circuit board (PCB), direct-plated-copper (DPC) and direct-bonded-copper (DBC) ceramics, with relatively high thermal conductivity, low coefficient of thermal expansion (CTE), excellent heat-resistance and chemicalresistance, are used for the high-power module applications such as LED lighting and IGBT packaging [1–4]. The DPC aluminum nitride (AlN) substrate with a high thermal conductivity (k = 170 W/mK) provides a good alternative to conventional aluminum oxide (Al2O3) substrate (with k = 24 W/mK) for better heat dissipation. However, in addition to its higher cost, AlN substrate (α = 4.3 ppm/°C) still suffers the higher CTE mismatch with copper material (α = 16.3 ppm/°C), compared with Al2O3 substrate (α = 6.9 ppm/°C). The high stresses in DPC AlN substrate induced by this CTE mismatch would lead to the AlN crack initiating at the edge corner of Cu film during thermal shock, as shown in Fig. 1 and thus result in a low reliability of the modules. The similar findings during thermal cycling were proposed by Dupont et al. [2] and Wei et al. [4], in which the former identified the Cu-film thickness effect and the latter offered the solution to it by ⁎ Corresponding author. E-mail address: [email protected] (M.Y. Tsai).
using stepped Cu-film structures. This study is going to take a close look at mechanics of DPC AlN under thermal loading and further to find out some important parameters to it in order to enhance the thermal reliability. 2. Methodologies 2.1. Experiment The specimens of Cu-AlN bi-material plate were fabricated by electrically plating Cu on the AlN substrate after sputtering a 0.1 μm-thick adhesion layer of titanium material on the substrate using a commercial equipment. Then, a full-field shadow moiré method [5,6] was used for measuring the out-of-plane displacements of the deformed specimens under thermal loads. The principle of the shadow moiré is that a light source illuminates the grating and specimen with an incident angle of α and a grating shadow appearing on the surface of the deformed specimen serves as the specimen grating created by projecting a beam of light through the reference grating. The moiré fringe pattern can be formed by superposing the specimen and reference gratings, and received and recorded by the camera with an observed angle of β. The fringes of the pattern represent the out-of-plane displacement (w) contours of the specimen surface. The curvature (K) of the specimen
http://dx.doi.org/10.1016/j.microrel.2015.08.010 0026-2714/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: M.Y. Tsai, et al., Mechanical design and analysis of direct-plated-copper aluminum nitride substrates for enhancing thermal reliability, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.010
2
M.Y. Tsai et al. / Microelectronics Reliability xxx (2015) xxx–xxx
warpage and its mechanism for the bi-material plate under thermal loads, and further to calculate the related stress state in the local region of the bi-material plate. The 2-D axi-symmetry FEM model is applied with the material properties of Cu film and AlN including elastic moduli, Poisson's ratios and CTEs listed in Table 1, in which there is a nonlinear material property for Cu film. The finest mesh with an element size of 5 μm in the model is employed to the region with theoretical stress singularity (or un-limited stress) to pick up the stress raiser. 3. Results and discussion Fig. 1. Failure observation for DPC AlN substrate with Cu-film thickness of (a) 30 μm and (b) 60 μm, after thermal shock cyclic test in the temperature ranging from −40 °C to 125 °C, from optical microscope.
deformation is calculated, based on the assumption of small deformation, by using K¼
1 2w ¼ 2 ρ x
ð1Þ
where ρ is a radius of the curvature, and x is a pre-defined distance. The thermally-induced out-of-plane displacements of the Cu-AlN bimaterial plate were measured by the shadow moiré system with the sensitivity of 15 μm/fringe (at α = 45° and β = 37°) in this study. The thermal cyclic load was applied from room temperature to 245 °C to mimic the solder reflow condition. 2.2. Theoretical and numerical analyses For the theoretical analysis, by modifying Timoshenko's solution of thermal stresses and deformations for a bi-material strip [7,8], the solution to thermally-induced curvature (K) of a bi-material plate is described by: K¼
1 Δε ¼ " ρ t 1 1 þ nm3 ð1 þ mnÞ 2
3 nmð1 þ mÞ
#
ð2Þ
þ ð1 þ mÞ
2Þ where m ¼ tt 21 ; n ¼ EE21 =ð1−ν =ð1−ν1 Þ; Δε = (α2 − α1)(T − T0) T0 and T are stress-
free (initial) and final temperatures, respectively. ti, Ei, νi, and αi represent the thickness, elastic modulus, Poisson's ratio, and CTE in the longitudinal direction, respectively, for the bottom layer (i = 1) and top layer (i = 2) of the bi-material plate. Δεis a misfit strain between the top and bottom layers. And the detailed solutions of axial stresses for the top and bottom layers are also described in [8]. The FEM simulation using ANSYS code was also performed to understand the experimentally obtained
3.1. Thermal deformations A variation of typical warpages of a Cu-AlN bi-material plate (with tCu = 30 μm and tAlN = 370 μm) obtained from shadow moiré measurement is shown in Fig. 2 during the 2nd solder reflow process. The concave shape of the plate deformation at 25 °C gradually becomes flat at the heating process and then more convex as the temperature reaches to 245 °C. During the cooling process, the convex shape is changed to the concave in the reverse way. Note that one fringe represents an out-of-plane displacement of 15 μm. Those shadow moiré data can be converted to the curvature data by using the Eq. (1). The variation of curvatures for the bi-material plate in three cycles of solder-reflow heating and cooling is shown in Fig. 3 obtained from shadow moiré measurement. The three slopes (curvature change per unit temperature) observed in the linear region from moiré measurement are quite consistent with each other and also with those from Timoshenko's solution and FEM analysis. It is apparent that during the first-cycle heating, the curvature increases linearly with temperature increasing, but becomes nonlinear with a slight decrease beyond 100 °C due to the recrystallization of Cu film. In the contrast, during the first-cycle cooling process the curvature is kept in linearity shortly until cooling to 200 °C and then becomes nonlinear. The results also show the hysteresis and Bauschinger-like behaviors for the bi-material plate during the solder reflow heating and cooling after the first thermal cycling. The zerocurvature (or free-stress) temperature at the heating process shifts from about 40 °C at the first cycling to 105 °C after the first cycling, while that at cooling process always stays at 225 °C. Therefore, it is adequate that the free-stress temperature is assumed at 105 °C, rather than 40 °C in the stress analysis of the Cu-AlN bi-material specimen in thermal shock test in the temperature ranging from − 40 °C to 125 °C, since every specimen has to go through the solder reflow process at least one cycle. The detailed out-of-plane displacement distributions along the x-direction and axial stress σx (or σr) distributions along the thickness in the center region for the Cu-AlN bi-material specimen under ΔT = − 80 °C (cooling down from 105 °C to 25 °C) are shown in Fig. 4(a) and (b), respectively, from experiment, FEM simulation and
Fig. 2. Warpage (out-of-plane displacement) variation of a Cu-AlN bi-material plate during 2nd reflow process from shadow moiré (with 15 μm per fringe).
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Fig. 3. Curvature variation of the Cu-AlN bi-material plate during solder reflow heating and cooling cycling obtained from shadow moiré measurement, theoretical and FEM results.
Timoshenko's bi-material theory. The very consistency of all three displacement results indicates the validity for these three solutions. Thus, FEM and theory can provide the convincing results of the axial stress σx (or σr) distributions along the thickness. 3.2. Validation model and failure mode prediction For DPC AlN substrate (with 10 mm-long Cu film and 13 mm-long AlN) under ΔT = − 1 °C, out-of-plane displacement distributions along the Cu film and axial stress σr distributions along the thickness in the central region are shown in Fig. 5 (a) and (b), respectively, with various thickness of Cu film from FEM and theoretical analyses. Note that since the stresses and deformations from the elastic analysis can be normalized by the thermal load, the stress and deformation values under ΔT = − 1 °C are for representing those data in this study. It is shown that, unlike Fig. 4(a) and (b), both displacement and stress distributions from the FEM analysis are reasonably consistent with those from the theory with minor difference in the quantity between both. That is due to the Cu film shorter than AlN length for FEM results, compared with the same as AlN length for theoretical ones. Despite the minor difference, both results indicate that the thicker the Cu film, the higher the out-of-plane deformation (warpage) of the substrate and axial stress in AlN, but the lower the axial stress in Cu film. The stress contours in the corner region and detailed stress distributions along the Cu free edge across the thickness are shown in Fig. 6(a) and (b), respectively, for the DPC AlN substrate with 100 μm-thick Cu and 370 μm-thick AlN under ΔT = −1 °C from the FEM simulation. It is obvious that, unlike
Fig. 5. (a) Out-of-plane displacement distributions along the Cu film, and (b) axial stress σr distributions along the thickness in the central region, for DPC AlN substrate with various thickness of Cu film under ΔT = −1 °C, from FEM and theoretical analyses.
the central region (in which axial stress σr in AlN is predominant), there exist appreciable σy and τry, besides σr in AlN at the corner region. The maximum principle stress combined with those three stress components would govern the crack initiation of the AlN. Furthermore, the validated model is used to predict the failure mode of DPC AlN substrate under the thermal cyclic loads. The results of vectors of principal stresses from the FEM simulation and possible crack initiation on DPC AlN substrate are shown in Fig. 7(a) and (b) under thermal loads of ΔT = −165 °C and 165 °C, respectively. Note that the length and direction of the vector
Fig. 4. Comparison of (a) out-of-plane displacement distribution and (b) axial stress σx (or σr) distribution of Cu-AlN bi-material specimen, from experiment, FEM simulation and Timoshenko's bi-material theory.
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Fig. 6. (a) Stress contours in the corner region and (b) detailed stress distributions along the Cu free edge across the thickness for the DPC AlN substrate with 100 μm-thick Cu and 370 μm-thick AlN, from FEM simulation under ΔT = − 1 °C.
represents the stress magnitude and direction, respectively. The results indicate that maximum tensile principal stress of AlN occurs at the edge corner of Cu film for cooling loads (ΔT = −165 °C), whereas maximum compressive principal stress is for heating loads (ΔT = 165 °C). That implies the crack initiation of AlN along the direction normal to maximum tensile principal stress would take place at cooling loads, rather than at heating loads, because of the tensile stress mostly dominating the failure of brittle materials such as ceramics. Therefore, the consistent failure mode and location with the experimental failure mode (shown in Fig. 1(b)) has been successfully predicted based on the maximum principal stress from FEM results (shown in Fig. 7(a)).
3.3. Parametric design and analysis 3.3.1. The effects of Cu-film thickness, length and wedge angle Since the crack initiation of AlN occurs at the edge corner of Cu film, the effect of Cu-film wedge angle (θ) on the maximum 1st principal stress of AlN is investigated for the DPC AlN substrate. The maximum 1st principal stresses (at the location P) of AlN with various wedge angles and thicknesses of Cu film under ΔT = − 1 °C were obtained from FEM simulation and are shown in Fig. 8. It is shown that for θ = 90° the maximum 1st principal stress decreases with the film thickness decreasing, similar to the observation by Dupont et al. [2]. Besides that, most importantly the maximum 1st principal stresses of AlN can be reduced by about 60% from θ = 90° to θ = 15° for 30 μm-thick film and by about 80% for 300 μm-thick film. In other words, the small wedge angle provides more reduction of the maximum 1st principal stress, in terms of both values and ratios, for the case with thick film than for that with thin film. Moreover, the Cu-film length effect is also taken into account
and the corresponding results from FEM simulation is shown in Fig. 9. It is found that the maximum 1st principal stresses for any thickness of Cu film slightly decrease with the film length increasing, dramatically drops in the region with the Cu-film-thickness order distance from the free end, and reaches the smallest value when the Cu-film length is up to 13 mm which is equivalent to the length of AlN and in the free end. 3.3.2. The effects of single and double-side Cu films In addition to the above-mentioned case with a single-side Cu film, the cases with double-side Cu films (like sandwich structures) have been studied and compared with the single-side case. The maximum 1st principal stresses of AlN for single and double-side DPC AlN substrates under ΔT = − 1 °C are shown in Fig. 10(a) and (b) for the cases with 30 μm-thick Cu film and with 200 μm-thick Cu film, respectively. Note that λ is defined as the length difference between the top and bottom Cu films for double-side case. The results show that the maximum 1st principal stresses of AlN for the double-side case are higher than those for the single-side case, especially more obvious in the thick-film case. And the dominant maximum 1st principal stress of AlN occurring at short-length Cu film (at Point 1), rather than at longlength Cu film (at Point 2), is getting higher and higher, and becomes saturated when the λ is increasing. Therefore, for the maximum 1st principal stresses of AlN, the double-side DPC AlN substrate with different Cu-film length (λ ≠ 0) are more severe than those with the same Cufilm length (λ = 0) and with the single-side. Moreover, the data of the Cu-film thickness effect was added into the results in Fig. 10 and was replotted by only taking into account the saturated value of the maximum 1st principal stress of AlN for a certain λ. The re-plotted results are shown in Fig. 11 for the maximum 1st principal stress of AlN against
Fig. 7. Vectors of principal stresses and possible crack initiation on DPC AlN substrate under thermal loads of (a) ΔT = −165 °C and (b) ΔT = 165 °C, from FEM simulation.
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Fig. 8. Cu-film wedge angle (θ) effect on maximum 1st principal stress (at the location P) of AlN for DPC AlN substrate with various Cu-film thickness under ΔT = −1 °C.
the thickness of Cu film. It is shown that the maximum 1st principal stresses are very sensitive to whether the case is a single-side or double-side Cu film with the value of λ, especially for thick-film (300 μm-thick) case, but not for thin-film (30 μm-thick) case. For the double-side Cu film case, the short-length Cu film always has the relatively large and dominant maximum 1st principal stress on AlN at Point 1, compared with that at Point 2 for long-length Cu film which is close to single-side case. However, as the value of λ approaches to zero, the curves for both become identical. The question is why the maximum 1st principal stress of AlN in double-side DPC is larger than one in single-side. Mechanics of single-side and double-side DPC AlN substrates under thermal cooling load are illustrated in Fig. 12. It indicates that for single-side AlN substrate under thermal load the maximum 1st principal stress of AlN is induced by the CTE mismatch between Cu and AlN, while the one for double-side AlN is caused by the CTE mismatch plus mechanical constraint (equivalent to extra mechanical bending stress) from the bottom part of the structure with a bending downward. As a result, the maximum 1st principal stress of AlN for the double-side Cu is higher than the one for the single-side Cu, and has the highest value at the short-length Cu film. 3.3.3. The effect of nonlinear property of Cu film The nonlinear mechanical behavior of Cu film is also considered in the FEM simulation. This nonlinear property shown in Table 1 has a feature with yielding stress at about 214 MPa. The simulation results of the maximum 1st principal stress of AlN and maximum von Mises stress of
Fig. 10. Effects of single-side and double-side DPC AlN substrates (with Cu length difference of λ for double-side case) on maximum 1st principal stresses of AlN under ΔT = −1 °C, for θ = 90° (a)with 30 μm-thick Cu film, and (b) with 200 μm-thick Cu film.
Cu film against thermal load (ΔT) are shown in Fig. 13 by considering Cu-film linear and nonlinear property for a DPC AlN substrate with wedge angle = 90°. It is shown that nonlinear property of Cu-film significantly affects the maximum 1st principal stress of AlN starting from ΔT = −25 °C to − 75 °C (in which range the elastic modulus of Cu is lower than linear one). And beyond ΔT = −75 °C(in which the
Fig. 9. Cu-film length effect on maximum 1st principal stress (at the location P) on DPC AlN substrate with Cu film thicknesses of 30 μm, 50 μm, 100 μm, 200 μm and 300 μm under ΔT = −1 °C.
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M.Y. Tsai et al. / Microelectronics Reliability xxx (2015) xxx–xxx Table 1 Material properties used in the finite element simulation (Note: Cu nonlinear properties is taken from [9]). Cu
AIN
Linear
Nonlinear
E (Gpa)
v
α (10
110
0.343
16.4
−6
/°C)
ε (10 1 4 10 20 40
−3
)
E (Gpa)
v
α (10−6/°C)
340
0.25
4.3
σ (MPa) 110 179 214 234 245
Fig. 11. Effects of thickness and λ of Cu film on maximum 1st principal stress of AlN for single-side and double-side DPC AlN substrates under ΔT = −1 °C, for θ = 90°. (Note: For double-side Cu substrate, the data was taken from the saturated value of the maximum 1st principal stress of AlN with a certain λ).
Cu film yields and its elastic modulus is close to zero), the maximum 1st principal stress of AlN reaches the plateaus value of about 490 MPa. This behavior is totally different from the linear results which show the linear increase of the stress with thermal loading. That implies that the nonlinear property of Cu-film can dramatically lower the maximum 1st principal stresses of AlN so that the linear stress analysis gives overprediction of this maximum stress. The maximum 1st principal stresses of AlN for various wedge angles (θ) (at the location P) for DPC AlN substrate under ΔT = −165 °C with various Cu-film thicknesses are plotted again in Fig. 14 by considering Cu-film linear and nonlinear properties. That indicates that, aside from the decrease of the maximum 1st principle stress of AlN in the nonlinear analysis, the Cu-film thickness and wedge angle still show some apparent effects on the maximum 1st principle stress of AlN in the Cu-film material nonlinear region. 4. Conclusions This study aims to resolve the crack problem of direct-plated-copper (DPC) aluminum nitride (AlN) substrate during thermal cycling and further to provide important parameters for mechanical design for ensuring good thermal reliability. The results of the out-of-plane deformation for a Cu-AlN bi-material plate subject to the solder reflow heating and cooling
Fig. 13. Maximum 1st principal stress of AlN and maximum von Mises stress of Cu film (at the location P) by considering Cu-film linear and nonlinear property for DPC AlN substrate with wedge angle = 90° under ΔT = 0 to −165 °C.
have been obtained and shown its hysteresis and Bauschinger-like behaviors. It was found from the validated finite element simulation that the Cu-film wedge angle, length, and thickness significantly affect the maximum 1st principal stress of AlN for the DPC AlN substrate during thermal cyclic loading, and the predicted failure mode and location based on the maximum 1st principal stress is consistent with experimental observation. Furthermore, this maximum 1st principal stress was found very sensitive to whether the DPC AlN substrate is with a single-side Cu film
Fig. 12. Mechanics of single-side and double-side DPC AlN substrates subject to thermal cooling load.
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principal stress for the double-side Cu film case with a different Cu-film length (λ ≠ 0), which is more severe than those with the same Cu-film length (λ = 0) case and with the single-side Cu film case. This mechanics has been well interpreted in this study. In addition, it was found that the nonlinear property of Cu-film can dramatically lower the maximum 1st principal stresses of AlN which is over predicted based on the Cu-linear stress analysis. References
Fig. 14. Cu-film nonlinear property effect on maximum 1st principal stress of AlN for various wedge angle (θ) (at the location P) for DPC AlN substrate with various Cu-film thicknesses under ΔT = −165 °C.
or double-side Cu film with the value of Cu-film length difference (λ) especially for thick-film case, but not for thin-film case. The short-length Cu film always has the relatively large and dominant maximum 1st
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