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Degradation of moulding compounds during highly accelerated stress tests – A simple approach to study adhesion by performing button shear tests
Microelectronics Reliability 52 (2012) 1266–1271
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Degradation of moulding compounds during highly accelerated stress tests – A simple approach to study adhesion by performing button shear tests R. Pufall a,⇑, M. Goroll a, J. Mahler a, W. Kanert a, M. Bouazza b, O. Wittler b, R. Dudek c a
Infineon Technologies AG, Am Campeon 1-12, 85579 Neubiberg, Germany Fraunhofer IZM Berlin, Germany c Fraunhofer ENAS Chemnitz, Germany b
a r t i c l e
i n f o
Article history: Received 3 October 2011 Received in revised form 27 February 2012 Accepted 6 March 2012 Available online 3 May 2012
a b s t r a c t High temperature storage can degrade moulding compounds for chip encapsulation to such an extent that the adhesion to surfaces like copper (lead frames) or polyimide (chip coating) decreases drastically causing delamination. Also during normal operation of electronic components heat is generated locally (bond wire or chip surface) degrading the moulding compound and reducing the adhesion which in extreme cases can destroy the metallisation or the bond wires. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The adhesion between different materials on different interfaces is the base of high package reliability. The most important interfaces are moulding compound – chip surface and moulding compound – lead frame. The properties of these interfaces are influenced by various materials and process parameters for chip surface and bond pad conditioning. Standard tests as thermal cycling do not aim at quantifying adhesion but predict, in the case of adhesion loss, delamination. An evaluation method based on a suitable test vehicle which enables the study of decoupled influence parameters for adhesion is missing. To characterise the adhesion characteristics of these interfaces a shear-test method of moulding compound buttons on different base materials was developed. The moulding compound buttons (2 2 mm2) are applied on base material stripes with a dimension of 100 20 mm2 (e.g. from copper or silicon). The test setup and the typical test vehicles are shown in Fig. 1. The authors are aware that different setups for moulded buttons have already been proposed. Durix [1] used triangle shaped buttons for shear tests to observe stable interface crack propagation. Also cylindrical samples (tapered) are proposed to investigate adhesive strength [2]. The MMC (Mixed Mode Chisel) [3] method used for investigating the silicon-moulding compound interface allows determination of the adhesion of mode I (tension) and mode II (shear). A well prepared initial interface crack is necessary to obtain reproducible results. ⇑ Corresponding author. E-mail address: reinhard.pufall@infineon.com (R. Pufall). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.03.016
In our investigation it turned out that shearing 2 2 mm2 buttons no pre-crack is needed and also stable crack propagation is observed. The main advantage of the selected geometry is the observability of the crack by mean of a uniDAC (universal deformation analysis by correlation). The aim was to have a simple setup and to have the possibility to mould buttons on different surfaces also in order to study process influences. Adhesion of moulding compounds on polyimides was of major interest because for power electronics it is often used as a buffer material between chip and moulding compound allowing higher displacement without breaking the interface. Shear test results are expected to be highly dependent on the surface properties as shown in Fig. 2. Rough well wetted surfaces should have the best adhesion. Voids in the interface (surface– moulding compound) should lead to low adhesion. Performing shear tests with a constant shear speed (standard) of 250 lm/s force–displacement curve can be recorded. In Fig. 3 typical force–displacement plots are shown to investigate combinations of moulding button–copper lead frames or polyimide on silicon. Plotting the measured maximum shear force for different material combinations is shown in Fig. 4. Moulding buttons have been assembled on copper surfaces, copper surfaces with adhesion promoter and nickel surfaces. Apart from the nickel surface (all selected moulding compounds C, D, E, F and G do not show very high adhesion) the assumption that an adhesion promoter helps in any case is not true. For moulding compound B pure copper surface gives much better adhesion than with the adhesion promoter. Moulding compounds D and E show the expected adhesion benefit with an adhesion promoter.
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
1267
Moulding Compound Button Shear Force
Surface
Fig. 1. Experimental setup for enabling shear tests. The shear results are highly adhesion dependent.
(a)
(b)
Fig. 2. Surface topology influences the adhesion (a). (b) Different surfaces, rough (1), rough with interface voids (2) and smooth (3) are shown.
We have to conclude that tests have to be performed to find out the best combination of moulding compound and surface concerning adhesion.
Fig. 4. Shear results with different material combinations (Cu, Ni, Cu with adhesion promoter AP).
Shear tests have been enabled to measure highly reproducible force–displacement curves which can be used to calibrate simulation models using cohesive zone elements in order to describe the adhesion of the selected material combinations. Generally performing shear tests, two forces have to be taken in consideration:
2. Simulation and experiments First High Temperature Storage (HTS) experiments with temperatures up to 250 °C (normally 175 °C is not exceeded because of the decomposition of epoxy resins) on copper stripes with moulding compound MCA showed a degradation of the moulding compound adhesion represented by a reduction of the maximum shear forces.
300
Shearforce [N]
250
AC 121°C/100% r.h., 192h Shear Speed: 250µm/s Height: 100µm
Polyimide
1. Traction to separate two materials. 2. Shear to displace the moulding compound on the surface. Performing button shear tests always is a combination of the 2. It is a mixed mode depending on the shear height (Fig. 5). For the performed tests, shear heights of 100 lm and less were chosen to have Fs Ft. FEM simulations confirm this approach. In Fig. 6 a simple FEM model for the button shear test is built. The stress distribution along the shear path depends on the location and varies between tension and compression. The influence of the shear height is clearly shown in the simulation. The calculated principal stress S3 is shown in Fig. 7. The dotted line shows the shear path and in Fig. 8 the height dependency is
200 Cu
150 100 50 0 0
0.05
0.1
0.15
0.2
0.25
0.3
Displacement [mm] Fig. 3. Force–displacement curves for performed shear tests on copper and polyimide surfaces.
Fig. 5. Mode mixity (tension and shear).
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R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
2000
Shear height h
Principle stress S1 [MPa ]
1800 1600
h = 15 µm h = 500 µm h = 750 µm
1400 1200 1000 800 600 400 200 0 -200 -400 -600 -800
0.0
0.5
1.0
1.5
2.0
Path along y-direction [mm]
Fig. 6. Principal stress S1 (tension) showing the height dependency over the shear path.
Fig. 7. Principal stress S3 (shear) applying a constant force.
Fig. 8. Principal stress S3 along the shear path (dotted line in Fig. 7). The shear height is varied.
Fig. 9. Maximum shear force is speed dependent.
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
1269
Fig. 12. Button shear test with modelling of cohesive zone elements.
The red arrow in the uniDAC images for a load of 65 N and 87.5 N shows the position of the crack. Fig. 10. New multifunctional tester with uniDAC with fixed chisel and moving table (moulding compound button moved against the fixed chisel).
3. Cohesive zone elements
clearly visible and also the change from tensions to compression along the shear path. Following the idea that at a certain shear force an interface crack should occur, it was tried to stop the machine at maximum force. We never succeeded to stop the machine to observe the initiation of a crack. Slowing down the machine to 0.3 lm/s show the same slope as for the high speed but the typical down ramp is missing (Fig. 9). By reducing the shear speed it was assumed to be able to observe an interface crack initiation. In order to find the right moment for the crack initiation a shear tester with a uniDAC (universal deformation analysis by correlation) connected was used. The different principle (a fixed chisel and a moving table) is illustrated in Fig. 10. With a uniDAC [11] it is possible to observe the deformation of the moulding compound and the deformation of the copper lead frame. In Fig. 11 the crack initiation and propagation are shown. In this example the maximum force (87.5 N) is reached after 190 s. But already at 150 s the force is high enough to initiate the crack (50 N). The blue and the red curves start to separate because the crack allows the button and the lead frame to move more independently.
Standard simulation approaches assume a perfect adhesion (fracture toughness of the materials) to calculate stresses and strains at the interfaces. To describe the experimentally found behaviour a different approach is tried. Cohesive zones elements describe the damage mechanisms on the interface using a constitutive relation between the traction and opening displacement. In the literature a variety of cohesive zone formulations is found [4–8]. A simple approach is used here for the moulding buttons on different surfaces. For our case we used the (Xu and Needleman) [4] exponential cohesive zone formulation. For the button shear test two parameters of displacement U are used: Un is the maximum separation between button and lead frame, Ut is the maximum displacement of the button on the surface before it separates. This is illustrated in Fig. 12.
Fig. 11. Crack propagation visualised by uniDAC showing the strain in Y-direction (vertical).
Fig. 13. Modelling of cohesive zone elements. Assumption Un = Ut = 0.0369 mm yield to 15 MPa maximum shear stress.
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R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
Fig. 14. Ageing the samples at 250 °C in air shows a decrease of adhesion down to 0 after already 250 h.
Looking at force–displacement curves of aged samples, we would observe very small displacements (Fig. 13) just pushing against the Young’s modulus of the moulding compound (26 GPa). But in the experiment displacements of 0.03 mm are observed and can be used to determine the maximum shear stress. A more elaborated paper on cohesive zone simulation was given on the EUROSIME 2011 conference [9] considering the critical fracture energy Gc as failure criterion. 4. Results Selection of material combinations performing simple shear tests offers a quick method to find the most promising combinations. Not all epoxy moulding compounds behave in the same way especially after ageing. It is already shown (Fig. 4) that even combinations with adhesion promoters sometimes do not improve the adhesion. A not negligible factor seems to be the wettability of the metal surface by the moulding compound. Ageing samples in air at 250 °C and performing shear tests after 50 h up to 200 h, show a complete loss of adhesion after approximately 250 h. In Fig. 14 force–displacement curves are shown. It is clearly visible that ageing reduces the maximum shear force and the maximum displacement until rupture. The shear speed has been fixed at 250 lm/s and the shear height (chisel above the surface) was selected at 100 lm. In the upper part of Fig. 14a the decrease of adhesion is visible in the plot maximum shear force versus stress time. In Fig. 14b the
corresponding force displacement curves are shown for the aged samples. For a different moulding compound (MCB) the ageing behaviour is unexpectedly different. An increase of adhesion up to 110 N has been found after 210 h at 250 °C. In Fig. 15a the ageing behaviour concerning adhesion is shown for a MCB moulding compound. The adhesion values are initial around 80 N and decrease after 100 h of ageing to 60 N. Continuing ageing up to 210 h the value is nearly the same as the initial value of the MCA moulding compound. The force–displacement curves show (Fig. 15b) nearly the same slope for all experiments. Autoclave tests AC (121 °C, 100 rH%) up to 160 h show no decrease of adhesion for the MCA moulding compound. The MCB ‘‘high temperature’’ moulding compound loses 40% of adhesion after 160 h. Thermal shock TS (–55 °C to 150 °C, 2 min cycle time) did not show any adhesion change up to 500 cycles. In the paper [10] was shown that test acceleration factors up to 50 could be gained by performing TS instead of TC. The moulding button adhesion does not seem to be susceptible to temperature swings. It was found that moisture and high temperature storage [12] are the dominant factors influencing adhesion of moulding compounds on different surfaces. It should be possible to make degradation visible by using nondestructive methods as infra-red spectroscopy or fluorescence. Understanding the results and the degradation mechanisms will enable the formulation of an ageing model.
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
1271
Fig. 15. A MCB ‘‘high temperature’’ moulding compound shows a different behaviour. Ageing at 250 °C increases the adhesion to 110 N after 210 h.
5. Conclusions With a simple button shear test (optimised in size for adhesion investigations) is possible to determine adhesion of moulding compounds on different surfaces. For reliability considerations certain material combinations can be excluded or if an adhesion promoter is available this can be applied to fulfil the lifetime requirements. Calibrated cohesive zone elements and FEM simulation allow the prediction of delamination also for complex geometries (real products). The study of degradation of moulding compounds also looking at the loss of adhesion enables the package developer to select materials also for high cycle performance reducing the number of verification tests drastically. References [1] Durix L, Drebler M, Coutellier D, Wunderle B. Fracture mechanics investigation of delamination of epoxy/metal interface due to static and thermo-mechanical loading for automotive applications. In: 5th international conference on fracture of polymers, composites and adhesives, septembre, Les Diablerets, Switzerland; 2008. [2] Chiena Chi-Hui, Chena Thaiping, Hsiehc Chi-Chang. Adhesion features of bonded interfaces interpreted by Taguchi technique. In: SEM annual conference; 2006.
[3] Schlottig G, Xiao A, Pape H, Wunderle B, Ernst LJ. Interfacial Strength of Siliconto-Molding Compound, Changes with Thermal Residual Stress. in: Proc. 11th conf. thermal, mechanical and multiphysics simulation and experiments in microelectronics and microsystems, EuroSimE, Bordeaux, France; 2010. p. 41– 45. [4] Tvergaard V. Effect of fibre debonding in a Whisker-reinforced metal. Mater Sci Eng A 1990;125:203–13. [5] Tvergaard V, Hutchinson JW. The relation between crack growth resistance and fracture process parameters in elastic-plastic solids. J Mech Phys 1992;40:1377–97. [6] Xu XP, Needleman A. Void nucleation by inclusions debonding in a crystal matrix. Model Simul Mater Sci Eng 1993;1:111–32. [7] Barenblatt GI. The mathematical theory of equilibrium cracks in brittle fracture. Adv Appl Mech 1962;7:55–129. [8] Camacho GT, Ortiz M. Computational modelling of impact damage in brittle materials. Int J Solids Struct 1996;33:2899–938. [9] Dudek R, Pufall R, Seiler B, Michel B. Studies on the reliability of power packages based on strength and fracture criteria. In: Proceedings EuroSimE 2011, Linz. [10] Pufall R, Kanert W. Can temperature shock tests speed up development of reliable components for automotive applications. In: Proceedings EuroSimE 2008, Freiburg, Germany; April 2008. p. 129–132. [11] uniDAC Manual (V. 5.1), CWM GmbH, Chemnitz; February 2011. [12] Vreugd J de, Jansen KMB, Ernst LJ, Bohm C, Pufall R. High temperature storage influence on molding compound properties. In: Proceedings of the 11th international conference on thermal, mechanical and multiphysics simulation and experiments in micro-electronics and micro-systems, EuroSimE 2010, Bordeaux, France; April 2010. p. 26–28.
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Degradation of moulding compounds during highly accelerated stress tests – A simple approach to study adhesion by performing button shear tests R. Pufall a,⇑, M. Goroll a, J. Mahler a, W. Kanert a, M. Bouazza b, O. Wittler b, R. Dudek c a
Infineon Technologies AG, Am Campeon 1-12, 85579 Neubiberg, Germany Fraunhofer IZM Berlin, Germany c Fraunhofer ENAS Chemnitz, Germany b
a r t i c l e
i n f o
Article history: Received 3 October 2011 Received in revised form 27 February 2012 Accepted 6 March 2012 Available online 3 May 2012
a b s t r a c t High temperature storage can degrade moulding compounds for chip encapsulation to such an extent that the adhesion to surfaces like copper (lead frames) or polyimide (chip coating) decreases drastically causing delamination. Also during normal operation of electronic components heat is generated locally (bond wire or chip surface) degrading the moulding compound and reducing the adhesion which in extreme cases can destroy the metallisation or the bond wires. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The adhesion between different materials on different interfaces is the base of high package reliability. The most important interfaces are moulding compound – chip surface and moulding compound – lead frame. The properties of these interfaces are influenced by various materials and process parameters for chip surface and bond pad conditioning. Standard tests as thermal cycling do not aim at quantifying adhesion but predict, in the case of adhesion loss, delamination. An evaluation method based on a suitable test vehicle which enables the study of decoupled influence parameters for adhesion is missing. To characterise the adhesion characteristics of these interfaces a shear-test method of moulding compound buttons on different base materials was developed. The moulding compound buttons (2 2 mm2) are applied on base material stripes with a dimension of 100 20 mm2 (e.g. from copper or silicon). The test setup and the typical test vehicles are shown in Fig. 1. The authors are aware that different setups for moulded buttons have already been proposed. Durix [1] used triangle shaped buttons for shear tests to observe stable interface crack propagation. Also cylindrical samples (tapered) are proposed to investigate adhesive strength [2]. The MMC (Mixed Mode Chisel) [3] method used for investigating the silicon-moulding compound interface allows determination of the adhesion of mode I (tension) and mode II (shear). A well prepared initial interface crack is necessary to obtain reproducible results. ⇑ Corresponding author. E-mail address: reinhard.pufall@infineon.com (R. Pufall). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.03.016
In our investigation it turned out that shearing 2 2 mm2 buttons no pre-crack is needed and also stable crack propagation is observed. The main advantage of the selected geometry is the observability of the crack by mean of a uniDAC (universal deformation analysis by correlation). The aim was to have a simple setup and to have the possibility to mould buttons on different surfaces also in order to study process influences. Adhesion of moulding compounds on polyimides was of major interest because for power electronics it is often used as a buffer material between chip and moulding compound allowing higher displacement without breaking the interface. Shear test results are expected to be highly dependent on the surface properties as shown in Fig. 2. Rough well wetted surfaces should have the best adhesion. Voids in the interface (surface– moulding compound) should lead to low adhesion. Performing shear tests with a constant shear speed (standard) of 250 lm/s force–displacement curve can be recorded. In Fig. 3 typical force–displacement plots are shown to investigate combinations of moulding button–copper lead frames or polyimide on silicon. Plotting the measured maximum shear force for different material combinations is shown in Fig. 4. Moulding buttons have been assembled on copper surfaces, copper surfaces with adhesion promoter and nickel surfaces. Apart from the nickel surface (all selected moulding compounds C, D, E, F and G do not show very high adhesion) the assumption that an adhesion promoter helps in any case is not true. For moulding compound B pure copper surface gives much better adhesion than with the adhesion promoter. Moulding compounds D and E show the expected adhesion benefit with an adhesion promoter.
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
1267
Moulding Compound Button Shear Force
Surface
Fig. 1. Experimental setup for enabling shear tests. The shear results are highly adhesion dependent.
(a)
(b)
Fig. 2. Surface topology influences the adhesion (a). (b) Different surfaces, rough (1), rough with interface voids (2) and smooth (3) are shown.
We have to conclude that tests have to be performed to find out the best combination of moulding compound and surface concerning adhesion.
Fig. 4. Shear results with different material combinations (Cu, Ni, Cu with adhesion promoter AP).
Shear tests have been enabled to measure highly reproducible force–displacement curves which can be used to calibrate simulation models using cohesive zone elements in order to describe the adhesion of the selected material combinations. Generally performing shear tests, two forces have to be taken in consideration:
2. Simulation and experiments First High Temperature Storage (HTS) experiments with temperatures up to 250 °C (normally 175 °C is not exceeded because of the decomposition of epoxy resins) on copper stripes with moulding compound MCA showed a degradation of the moulding compound adhesion represented by a reduction of the maximum shear forces.
300
Shearforce [N]
250
AC 121°C/100% r.h., 192h Shear Speed: 250µm/s Height: 100µm
Polyimide
1. Traction to separate two materials. 2. Shear to displace the moulding compound on the surface. Performing button shear tests always is a combination of the 2. It is a mixed mode depending on the shear height (Fig. 5). For the performed tests, shear heights of 100 lm and less were chosen to have Fs Ft. FEM simulations confirm this approach. In Fig. 6 a simple FEM model for the button shear test is built. The stress distribution along the shear path depends on the location and varies between tension and compression. The influence of the shear height is clearly shown in the simulation. The calculated principal stress S3 is shown in Fig. 7. The dotted line shows the shear path and in Fig. 8 the height dependency is
200 Cu
150 100 50 0 0
0.05
0.1
0.15
0.2
0.25
0.3
Displacement [mm] Fig. 3. Force–displacement curves for performed shear tests on copper and polyimide surfaces.
Fig. 5. Mode mixity (tension and shear).
1268
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
2000
Shear height h
Principle stress S1 [MPa ]
1800 1600
h = 15 µm h = 500 µm h = 750 µm
1400 1200 1000 800 600 400 200 0 -200 -400 -600 -800
0.0
0.5
1.0
1.5
2.0
Path along y-direction [mm]
Fig. 6. Principal stress S1 (tension) showing the height dependency over the shear path.
Fig. 7. Principal stress S3 (shear) applying a constant force.
Fig. 8. Principal stress S3 along the shear path (dotted line in Fig. 7). The shear height is varied.
Fig. 9. Maximum shear force is speed dependent.
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
1269
Fig. 12. Button shear test with modelling of cohesive zone elements.
The red arrow in the uniDAC images for a load of 65 N and 87.5 N shows the position of the crack. Fig. 10. New multifunctional tester with uniDAC with fixed chisel and moving table (moulding compound button moved against the fixed chisel).
3. Cohesive zone elements
clearly visible and also the change from tensions to compression along the shear path. Following the idea that at a certain shear force an interface crack should occur, it was tried to stop the machine at maximum force. We never succeeded to stop the machine to observe the initiation of a crack. Slowing down the machine to 0.3 lm/s show the same slope as for the high speed but the typical down ramp is missing (Fig. 9). By reducing the shear speed it was assumed to be able to observe an interface crack initiation. In order to find the right moment for the crack initiation a shear tester with a uniDAC (universal deformation analysis by correlation) connected was used. The different principle (a fixed chisel and a moving table) is illustrated in Fig. 10. With a uniDAC [11] it is possible to observe the deformation of the moulding compound and the deformation of the copper lead frame. In Fig. 11 the crack initiation and propagation are shown. In this example the maximum force (87.5 N) is reached after 190 s. But already at 150 s the force is high enough to initiate the crack (50 N). The blue and the red curves start to separate because the crack allows the button and the lead frame to move more independently.
Standard simulation approaches assume a perfect adhesion (fracture toughness of the materials) to calculate stresses and strains at the interfaces. To describe the experimentally found behaviour a different approach is tried. Cohesive zones elements describe the damage mechanisms on the interface using a constitutive relation between the traction and opening displacement. In the literature a variety of cohesive zone formulations is found [4–8]. A simple approach is used here for the moulding buttons on different surfaces. For our case we used the (Xu and Needleman) [4] exponential cohesive zone formulation. For the button shear test two parameters of displacement U are used: Un is the maximum separation between button and lead frame, Ut is the maximum displacement of the button on the surface before it separates. This is illustrated in Fig. 12.
Fig. 11. Crack propagation visualised by uniDAC showing the strain in Y-direction (vertical).
Fig. 13. Modelling of cohesive zone elements. Assumption Un = Ut = 0.0369 mm yield to 15 MPa maximum shear stress.
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R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
Fig. 14. Ageing the samples at 250 °C in air shows a decrease of adhesion down to 0 after already 250 h.
Looking at force–displacement curves of aged samples, we would observe very small displacements (Fig. 13) just pushing against the Young’s modulus of the moulding compound (26 GPa). But in the experiment displacements of 0.03 mm are observed and can be used to determine the maximum shear stress. A more elaborated paper on cohesive zone simulation was given on the EUROSIME 2011 conference [9] considering the critical fracture energy Gc as failure criterion. 4. Results Selection of material combinations performing simple shear tests offers a quick method to find the most promising combinations. Not all epoxy moulding compounds behave in the same way especially after ageing. It is already shown (Fig. 4) that even combinations with adhesion promoters sometimes do not improve the adhesion. A not negligible factor seems to be the wettability of the metal surface by the moulding compound. Ageing samples in air at 250 °C and performing shear tests after 50 h up to 200 h, show a complete loss of adhesion after approximately 250 h. In Fig. 14 force–displacement curves are shown. It is clearly visible that ageing reduces the maximum shear force and the maximum displacement until rupture. The shear speed has been fixed at 250 lm/s and the shear height (chisel above the surface) was selected at 100 lm. In the upper part of Fig. 14a the decrease of adhesion is visible in the plot maximum shear force versus stress time. In Fig. 14b the
corresponding force displacement curves are shown for the aged samples. For a different moulding compound (MCB) the ageing behaviour is unexpectedly different. An increase of adhesion up to 110 N has been found after 210 h at 250 °C. In Fig. 15a the ageing behaviour concerning adhesion is shown for a MCB moulding compound. The adhesion values are initial around 80 N and decrease after 100 h of ageing to 60 N. Continuing ageing up to 210 h the value is nearly the same as the initial value of the MCA moulding compound. The force–displacement curves show (Fig. 15b) nearly the same slope for all experiments. Autoclave tests AC (121 °C, 100 rH%) up to 160 h show no decrease of adhesion for the MCA moulding compound. The MCB ‘‘high temperature’’ moulding compound loses 40% of adhesion after 160 h. Thermal shock TS (–55 °C to 150 °C, 2 min cycle time) did not show any adhesion change up to 500 cycles. In the paper [10] was shown that test acceleration factors up to 50 could be gained by performing TS instead of TC. The moulding button adhesion does not seem to be susceptible to temperature swings. It was found that moisture and high temperature storage [12] are the dominant factors influencing adhesion of moulding compounds on different surfaces. It should be possible to make degradation visible by using nondestructive methods as infra-red spectroscopy or fluorescence. Understanding the results and the degradation mechanisms will enable the formulation of an ageing model.
R. Pufall et al. / Microelectronics Reliability 52 (2012) 1266–1271
1271
Fig. 15. A MCB ‘‘high temperature’’ moulding compound shows a different behaviour. Ageing at 250 °C increases the adhesion to 110 N after 210 h.
5. Conclusions With a simple button shear test (optimised in size for adhesion investigations) is possible to determine adhesion of moulding compounds on different surfaces. For reliability considerations certain material combinations can be excluded or if an adhesion promoter is available this can be applied to fulfil the lifetime requirements. Calibrated cohesive zone elements and FEM simulation allow the prediction of delamination also for complex geometries (real products). The study of degradation of moulding compounds also looking at the loss of adhesion enables the package developer to select materials also for high cycle performance reducing the number of verification tests drastically. References [1] Durix L, Drebler M, Coutellier D, Wunderle B. Fracture mechanics investigation of delamination of epoxy/metal interface due to static and thermo-mechanical loading for automotive applications. In: 5th international conference on fracture of polymers, composites and adhesives, septembre, Les Diablerets, Switzerland; 2008. [2] Chiena Chi-Hui, Chena Thaiping, Hsiehc Chi-Chang. Adhesion features of bonded interfaces interpreted by Taguchi technique. In: SEM annual conference; 2006.
[3] Schlottig G, Xiao A, Pape H, Wunderle B, Ernst LJ. Interfacial Strength of Siliconto-Molding Compound, Changes with Thermal Residual Stress. in: Proc. 11th conf. thermal, mechanical and multiphysics simulation and experiments in microelectronics and microsystems, EuroSimE, Bordeaux, France; 2010. p. 41– 45. [4] Tvergaard V. Effect of fibre debonding in a Whisker-reinforced metal. Mater Sci Eng A 1990;125:203–13. [5] Tvergaard V, Hutchinson JW. The relation between crack growth resistance and fracture process parameters in elastic-plastic solids. J Mech Phys 1992;40:1377–97. [6] Xu XP, Needleman A. Void nucleation by inclusions debonding in a crystal matrix. Model Simul Mater Sci Eng 1993;1:111–32. [7] Barenblatt GI. The mathematical theory of equilibrium cracks in brittle fracture. Adv Appl Mech 1962;7:55–129. [8] Camacho GT, Ortiz M. Computational modelling of impact damage in brittle materials. Int J Solids Struct 1996;33:2899–938. [9] Dudek R, Pufall R, Seiler B, Michel B. Studies on the reliability of power packages based on strength and fracture criteria. In: Proceedings EuroSimE 2011, Linz. [10] Pufall R, Kanert W. Can temperature shock tests speed up development of reliable components for automotive applications. In: Proceedings EuroSimE 2008, Freiburg, Germany; April 2008. p. 129–132. [11] uniDAC Manual (V. 5.1), CWM GmbH, Chemnitz; February 2011. [12] Vreugd J de, Jansen KMB, Ernst LJ, Bohm C, Pufall R. High temperature storage influence on molding compound properties. In: Proceedings of the 11th international conference on thermal, mechanical and multiphysics simulation and experiments in micro-electronics and micro-systems, EuroSimE 2010, Bordeaux, France; April 2010. p. 26–28.