Adhesive

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1 Robert Bosch GmbH, Automotive Electronics, Tübingerstr. ... method and the testing methodology can be used for ... molding compound (EMC) interface [5].
Characterization and Simulation of LTCC/Adhesive and Alloy 42/Adhesive Interface Strength for Automotive Applications B. Öztürk1,2, P. Lou1, P. Gromala1, C. Silber1, K.M.B. Jansen2, L.J. Ernst3,4 Robert Bosch GmbH, Automotive Electronics, Tübingerstr. 123, 72762, Reutlingen, Germany 2 Delft University of Technology, IO, Landbergstraat15, 2628 CE Delft, The Netherlands 3 Emeritus Professor of Delft University of Technology, The Netherlands 4 Ernst Consultant, Schoonhoven, The Netherlands Amperelaan 8, 2871 ZC, Schoonhoven, The Netherlands Phone: +49-(0)7121-35-1472, e-mail: [email protected] 1

Abstract Thermoset-based adhesives are used as thermal and electrical interfaces. In automotive applications, they are required to have excellent adhesion since delamination may precipitate other electrical, thermal or mechanical failure mechanisms. A vast amount of literature is available on the investigation of molding compounds and various material interfaces. However, only very few studies focus on delamination of adhesive interfaces. The reason is that apparently it was not possible to initiate an interface crack in a delamination sample. In various attempts, random cracking in the adhesive was obtained instead. Yet interface cracks are found in real products and really form a reliability issue. But so far the absence of adequate interface strength data makes it hardly possible to design for reliability of products with adhesive interfaces. The present paper solves the above problem. We succeeded to get an interface delamination between the adhesive and two different materials (e.g. Low temperature cofired ceramic (LTCC) and alloy 42). The specimens are made by identical fabrications processes as during the fabrication of the electronic control unit under study. The interface to be investigated is preconditioned for delamination initiation, by adding a single step to the fabrication process, thus enabling the investigation of different interfaces that have the same processing conditions as the real product. The presented specimen preparation method and the testing methodology can be used for determination of critical adhesion properties of different interfaces (including brittle materials like LTCC) in electronic control units. Specimens are investigated by delamination experiments near Mode-I loading conditions at room temperature. The obtained interface data is interpreted via image processing and finite element modeling of the J-integral method. In particular, cohesive zone modeling is used to validate the critical energy release rates for different interfaces. Introduction As the complexity of microelectronic packages increases, the use of inherently dissimilar materials is a necessity in order to meet the demanding

978-1-4799-9950-7/15/$31.00 ©2015 IEEE

requirements of existing and new applications. This necessity brings its own set of challenges. As reported by Zhang et al. [1], 65 % of total failures in microelectronics industry are thermo-mechanical related and they mostly originate from product/process design phase. One of the biggest contributors of these failures is the delamination phenomenon which is observed between the interfaces of two or contact point of three dissimilar materials. Since delamination can trigger other electrical, thermal or mechanical failures, it is very important for the reliability engineer to assess the performance of an interface under specified conditions. Over the years, intensive research and considerable progress has been made in applying fracture mechanics concepts to microelectronics [2]. The turning point in delamination testing in microelectronics was led by Xiao et al. [3] who used test specimens from the production line. These specimens were tested within a delamination testing device which is originally developed by Reeder et al. [4]. As it would be addressed by different authors later, the critical values for fracture parameters are closely associated with processing and fabrication conditions. This is the main reason why many customized interface fracture setups are developed by different groups in order to meet the size and geometry requirements of package-based sample testing. Xiao et al. [3] investigated the adhesion between silver filled die-attach (DA)/Copper (Cu) and the Cu/Epoxy molding compound (EMC) interface [5]. Double cantilever beam (DCB), mixed-mode bending (MMB) and three point bending (3PB) setups were separately used [6]. In order to find the critical interface parameters, the J-integral method was implemented in FE calculations as demonstrated by He et al. [7] and Lee et al. [8]. With DCB and MMB setups, unstable crack growth could be tracked. Schlottig et al. [9] presented a new method to fabricate specimens as in production conditions which could be used to test the silicon (Si) and EMC interfaces with a mixed-mode chisel (MMC) setup [10], which was developed by the same author. Ma et al. [11] investigated the critical interface properties of Cu/DA adhesive. They used production-like samples which consisted of three layers: lead-frame, DA and EMC. He constructed a four point bending (4PB) setup, which was initially

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suggested by Charalambides [12]. Ma has observed stable delamination propagation between room temperature and 150°C. He compared the critical energy release rate, , values obtained from analytical calculations [12] and numerical simulations and concluded that both approaches yield very similar . Maus et al. [13] introduced a new test setup called micro mixed-mode tester (µMMT), which was a miniature version of MMB [78], for testing the interface between the DA and Cu. The samples were cut directly from low profile quaf flat package (LQFP), which consisted of a Cu lead frame, a DA adhesive and EMC. Sadeghinia et al. [14] tested the same Cu/EMC with a MMB setup which was installed in a pressure chamber. They discussed the effects of high temperature and high humidity on the calculated fracture toughness. In order to avoid the difficult specimen handling issues of MMB, Wunderle et al. [15] introduced an advanced mixed-mode bending setup (AMB). This allowed relatively easier specimen handling and used independent displacement controlled actuators to load the specimen, which enabled a wider range of mode-mixity calculations. The contributions described in the above paragraph formed the starting point for the current research. It is aimed to develop a methodology for taking delamination samples from relatively large control units (ECUs) which are used in automotive electronics. Attention is given to describe the delamination between the LTCC and heat conductive adhesive. The specimens are made by identical fabrication processes as used in the manufacturing of the ECU. The interface to be investigated is preconditioned for delamination initiation by adding a single step for pre-crack introduction to the fabrication process, thus enabling the investigation of different interfaces that have the same processing conditions as the real product (e.g. Alloy 42/Adhesive). Prepared specimens are tested using the AMB setup [15]. The presented specimen preparation method and the testing methodology can be used for determination of critical adhesion properties of different interfaces (including brittle materials like LTCC) in electronic units. 2. Theory 2.1. Describing the Fracture via Critical Energy Release Rate There are two main approaches used when describing fracture: the stress-based approach and the energybased approach. In 1913, Inglis [16] calculated and suggested a way to determine stresses around an elliptic hole in an elastic plate. When Griffith [17] began his work, he observed the mathematical difficulties in applying Inglis’ solution at the limit of a perfectly sharp crack, where stresses approach infinity at the crack tip [18]. Due to the interpretation difficulties of the stress-based approach, Griffith suggested an energy-balance approach which was

applicable for isotropic materials that obeyed Hooke’s law at all stresses. Griffith’s energy approach stated that energy added to an object in order to fracture an infinitesimally small area is equal to the energy that is required to create this area. He defined the energy release rate, , as the energy dissipated during fracture per unit of newly created fracture surface area. According to this definition, when an object is subjected to the threshold energy per area, it fractures: Eq. 1 To assess the crack growth, a criterion can be written by the energy balance of the material. When the crack opens, newly created area can be described as a function of newly created crack length , and sample thickness , that is : 1

1

Eq. 2

where is the change in external work, is the change in the stored elastic energy of the material, is the change in energy dissipated by crack growth. , the change in energy dissipation caused by other , the change in the kinetic mechanisms, and energy, are assumed to be zero. Eq. 2 states that the energy release rate is the absolute change in potential energy per unit of crack area. In order to assess if a crack will propagate under given circumstances, the energy release rate is compared to the threshold value which is the (Eq. 1). 2.2. Mode Mixity in a Homogenous Media and at the Interface Crack growth can occur by three different separation modes (see Figure 1). During crack propagation, Mode I, Mode II and Mode III may co-exist together. The energy release rate defined in section 2.2 changes for different mixes of fracture modes. In order to design for reliability, both the pure crack propagation modes and the mixed mode loading conditions should be investigated.

y x z Mode I

Mode II

Mode III

Figure 1: Three loading modes of fracture. The following explanations are adapted from Hutchinson et al. [19]. Although is a material property, it is not unique. For an isotropic, homogenous depends on temperature ( ), moisture ( ) material, and the type of stress state in front of the crack tip ( ), which is the ratio of shear stress to tensile stress. This stress ratio is called the mode mixity and is given by:

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lim

Eq. 3



elastic mismatch in bimaterial problems and can be defined by:

The terms in Eq. 2 are illustrated in Figure 2:

μ1 μ2 μ1 μ2

Figure 2: Crack tip coordinates and stresses at the crack tip of an isotropic, homogenous material. where µ,E and ν denote the shear modulus, Young’s modulus and the Poisson’s ratio, respectively. Accordingly, the criterion for fracture in Eq. 1 becomes: Eq. 3 , , , , When two isotropic elastic materials are joined as in Figure 3, calculation of the mode mixity must be modified. Solutions on bi-material interface problems are presented by several authors [94-99].

Figure 3: A crack along a bi-material interface and basic material properties. The asymptotic stress field for an interface crack can be expressed in the following form: Eq. 4 , √2 In Eq. 4, and are the local polar coordinates for a coordinate system located on the crack front in planes perpendicular to the crack front. The oscillatory behavior for small arises from the terms . is the complex stress intensity factor [19,26,27] which has is a complex function and real and imaginary parts. represents the osciallatory index which is given by: 1 1 Eq. 5 2 1 where represents the second Dundurs’ parameter [28]. Dundurs’ parameters are used to quantify the

2

1

μ2

1 2

1 1

μ1 μ2

1

1

μ1

1

1

Eq. 6

2 1

1 1

Eq. 7

2

1

where μ is the shear modulus, 3 4 for plane strain or 3 / 1 for plane stress ( is Poisson’s ratio). Subscripts 1 and 2 are associated with the different materials on either side of the interface. Eq. 6 is a measure of the mismatch in the elastic stiffness normal to the plane of the bimaterial interface. Eq. 7 provides the mismatch in the in-plane bulk modulus [19]. When 0 at the interface, the solution of the Eq. 4 is simplified into: Eq. 8 √2 22 12 Eq. 3. 1 where at the crack tip. and are stress intensity factors for mode I and mode II cracking, respectively. is defined as: Eq. 9 cos ln sin ln Eq. 3. 2 When there is no material mismatch, , , and are zero and the stress state reduces to the homogenous crack case. Accordingly, mode-mixity is specified by Eq. 3. However, for an interface crack, Dundurs’ parameters are never zero. As a result, stress components very close to the crack tip are oscillatory in nature. This is why the mode mixity according to Eq. 3 can be misleading when interface cracks are investigated. Alternative methods exist to define the mode-mixity for interfaces which can be implemented with the help of FEA. Ryoji et al. [29,30] suggested the calculation of G and the mode mixity at a bi-material interface as: ∗ Eq. 10 2 1 4 . 64 Eq. 3. 3 2

Eq. 11

Eq. 3. 4 and are the differences in displacement where components and is the distance from the crack tip. ∗ is the reduced modulus which is defined as ∗ 2 1 2⁄ 1 is the reference length which is 2 ). suggested between one nanometer [31] and one micrometer [25]. Schlottig [32] mentions other suggestions as geometry measures for the reference length, such as the length of the crack and the layer size. In order to calculate the unknowns in Eq. 10 and

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Eq. 11, nodal displacement should be calculated as in virtual crack closure technique (VCCT) [33,34]. 2.3. Numerical Methods for Interface Fracture In order to determine , fracture experiments must be performed. Analytical solutions are available for all testing methods. These methods previously had the drawback that they did not account for residual stresses. Although this issue was later addressed by different authors [35-37] who included the residual stresses analytically in the calculations of , FE methods can provide more precise solutions as long as the requirements of the correct approximation are satisfied. In order to obtain the correct approximations for , the analysis has to consider various aspects of a FEA, which include correct description of the material properties, mesh dependency and solution related issues (e.g. time stepping and correct selection of penalty stiffness, when applicable). , crack There is no standard method to calculate initiation and propagation in FEA. There are several methods available, including J-integral [38,39], VCCT [33,34], cohesive zone modeling (CZM) [40], global energy approach [41] and crack surface displacement extrapolation method (CSDEM) [28,29]. In this study, delamination tests are conducted and simulated only at room temperature where the viscoelastic effects of the are negligible due to the glassy microstructure of the adhesive. It is therefore appropriate to use the J-integral method to calculate . The values obtained from J-integral method are used to validate experimental findings of delamination tests by simulating the experiments where CZM is incorporated. The constitutive response of the cohesive elements is defined by a bi-linear traction-separation law. 3. Delamination Experiments and FE-modeling 3.1. Specimen preparation A control unit has been chosen to study the adhesion behavior of the adhesive. The 2D scheme of the control unit with the relevant dimensions is given in Figure 4. There are two variations of the control unit with different exposed pads (ePADs) and thicknesses. These are Al2O3 ePAD (0.64 mm) and Alloy 42 ePAD (0.4 mm). EMC Adhesive

Electronic components LTCC

ePAD

Figure 4: 2D scheme of the control unit which used to obtain the delamination samples.

Schlottig [32] and Xiao [41] discussed thoroughly the requirements of a good delamination sample for Si/EMC and Cu/EMC interfaces, respectively. Two requirements which are of utmost importance are a sharp pre-crack and minimal damage of the adherents during specimen preparation steps (e.g. possible flaws at silicon surfaces). These requirements apply to LTCC/Adhesive interface as well. If these conditions are not satisfied, random cracking of the adherents may occur. In addition, specimen preparation conditions must be as close as possible to the real processing steps of the actual components so that the effect of processing steps on interface properties can be captured. Accordingly, the test specimens are prepared by means of identical production steps. Although the main focus is given to the specimens taken directly from the production line, a parallel lab specimen approach is carried out in order to assess the difference between the obtained datasets. The specimen preparation in lab conditions is found to be very demanding in terms of material handling and correct processing. Although considerable amount of effort is spent to produce specimens in lab conditions where no automated machines are available (e.g. a dispensing machine for adhesive application, a holder for LTCC application etc.), sample preparation by hand yielded neither samples nor results that are reproducible. The specimen preparation approach consists of producing the package, as in real application with all the materials except the electronic components on and at the backside of the LTCC. Before the processing steps, the backside of the LTCC is treated with a commercial mold release agent (MRA) ACMOscoat 82-6007 in order to prepare the necessary conditions to form a sharp pre-crack. Curing of the MRA is performed as suggested in the data sheet of the compound. The adhesive is applied to the ePAD by a screen-printing process. The LTCC is then placed onto the ePAD/Adhesive structure under vacuum. The formed LTCC/Adhesive/ePAD structure is placed into an oven and curing of the adhesive is done according to the product specifications. Once the curing of the adhesive is complete, this structure is cut via different methods. Acceptable cutting results are obtained via water-jet cutting and other methods (including laser, dicing saw and diamond blade) fail to produce samples. This three-layer specimen fabrication route is shown to yield specimens in a reproducible manner. 3.2. Testing method In the course of this research, the AMB [15] test setup, which is a variation on the MMB setup [3,4], is used. The delicate MMB setup makes it difficult to handle the samples. At the same time, the load arrangements (i.e. loading beams, levers hinges and wires as introduced in [3]) might cause reproducibility issues. The AMB setup in Figure 5 uses similar specimens and

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3.3. Calculation of Gc for Alloy 42 / Adhesive interface Figure 7 shows the evolution of the reaction force as d1 is applied at a constant rate of 0.5 µm/sec. 1.5 Elastic loading Overshooting due to the pre-crack shape

Sample #12

1.2

Force / N

allows for independent displacement control by two actuators, which increases the possibilities of varying the mode-mixity. The two possible loads due to the two different displacement actuators are measured by sensors. Although the AMB setup is initially designed for two layer specimens (Cu/EMC), the open architecture allows testing of three layer specimens as well. At the same time, the setup allows the measurement of different lengths since the X position of the Y-fixation can be varied easily.

Sample #13-1

0.9

Sample #13-2

0.6 0.3

Nearly steady crack growth

0 0

100

200

300

400

500

Displacement / µm

Figure 7: Force vs. displacement curves of Alloy 42/Adhesive interface.

Figure 5: Schematic of the AMB setup (taken from [15]). Figure 6 illustrates how the delamination sample cut from the control unit is clamped into the AMB setup with the constraints and displacement points. Displacement d1 alone is responsible for Mode I opening (e.g. DCB testing) and displacement d2 alone is responsible for Mode II opening (end-notch fixture (ENF) testing). When these displacements are applied at the same time, mode-mixity can be varied. In this research, displacement d2 is not used. The obtained results are therefore very close to pure Mode I opening. The sample is fixed in X and Y directions and a separate Y fixation is used in order to prevent possible sliding and rotation. The pre-cracks are shown as black dashed lines. If Alloy 42/Adhesive interface is to be investigated, the upper surface of the Alloy 42 is modified such that MRA forms a pre-crack between adhesive and the LTCC (Loc1 inFigure 6). When the pre-crack is desired to form between the LTCC and the adhesive, the LTCC backside is modified with MRA (Loc2 in Figure 6). The crack is tracked with the help of a camera which captures an image every second. The obtained pictures are used to interpret the test results. XY fix

Y fix LTCC

d1

y

Loc2 x

Loc1

Adhesive Alloy 42 or Al2O3 ePAD d2

Figure 6: Clamping of the delamination sample to the AMB setup.

The pre-crack is opened as the test progresses which is an elastic response. Since the pre-crack shape is not sharp enough, as soon as the pre-crack propagates, an overshooting in the load reaction can be captured which is approximately 0.35 N for both specimens. The force decreases as a result of the growing crack, and causes an unstable crack growth. Since the crack length increases continuously, the stiffness of the interface decreases continuously as well. This is illustrated by Sample #13. Once the crack grows to approximately 300 µm, the displacement d1 is removed (Sample #13-1) and then applied again (Sample #13-2). It is seen that the force reaction curve of the #13-2 follows that of #13-1 in a steady manner. A nearly stable delamination is achieved. In principle, the critical interface data should be extracted in the region where the crack growth is stable. However, it is not always possible to obtain this behavior in a delamination test. A full parametric FE-model is generated such that mesh, clamp locations, load application and pre-crack locations (i.e. all geometric considerations that vary from test to test) can be implemented and changed with ease. Experimental data in Figure 7 is used to extract values at different crack lengths by implementing the J-integral algorithm in FEA. In J-integral simulations, two dimensional, 4-node structural solid elements with a plane stress assumption are used (see Figure 8). Linear elastic material properties are implemented for all materials. Mesh analysis and the are investigated. It is effect of the clamping area on observed that the mesh is fine enough and decreasing the element size (i.e. increasing the element number) by a factor of two or four does not have a significant (see Figure 9 A). influence on the calculated Besides, since the exact contact area between the XY fix and the sample is not known, the effect of the

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contact area is investigated to determine if over constraining of the model has a profound effect on the calculated . Figure 9 B shows that the change in is negligible as the contact area is decreased to half or a quarter of its original size. The data points labeled as “used” in Figure 9 A and B represent the element number and contact area implemented in the following FE-investigations. Before the calculation of the Jintegral, the cooling process of the structure is simulated in order to account for the residual stresses due to CTE mismatch of the components during cooling.

of geometric entities can be found in mm. In order to determine the pre-crack location as precisely as possible, the image contrast is changed such that precrack (or crack position) can be located in an accurate manner. Backside resistors of the LTCC can also be seen in Figure 10 which should not be interpreted as an existing crack. Force and the corresponding time values are taken from multiple points in the experimental results and used in ANSYS to establish the critical energy release rate. This is presented along with the calculated values in Figure 11. It is seen that the calculated values are found within a range between 0.015 N/mm and 0.030 N/mm. XY fix

Y fix LTCC

d1

y

Adhesive Alloy 42 ePAD

Loc1

x

174.31

A resistor

78.13 117.19 579.95

Figure 8: Geometry of 2D FEM model and the crack tip mesh.

Pre-crack location

718.18

A resistor

Assessment of the geometric entity locations

Figure 10: Determination of the relevant dimensions in Matlab for sample #12. The numbers represent the number of pixels.

1.2 Used 2 times finer 4 times finer 0

50000

100000

Number of elements

1 0.8 0.6 0.4 0.2

B 1.200 1.150 1.100 1.050 1.000 0.950 0.900 0.850 0.800

0

Used Half Quarter

0

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Time / sec

600

800

B

0.035 0.030 0

0.2

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Gc / N/mm

Relative change in Gc

Sample #12 J-integral points #12 Sample #13-1 J integral point #13-1 Sample #13-2 J integral points #13-2

A

1.4

Force / N

Relative change in Gc

A 1.200 1.150 1.100 1.050 1.000 0.950 0.900 0.850 0.800

0.8

Contact area of the clamp / mm2

Figure 9: A) Mesh dependency of the calculated values via J-integral and B) the effect of the used calculations. clamping area on

0.025 0.020 0.015

Sample #12

0.010

Sample #13-1

0.005

The XY-fix, Y-fix and the contact location of the d1 actuator to the sample are evaluated from the obtained images using Matlab. One example is seen for sample #12 in Figure 10. The image pixel number and the image sizes are correlated such that the actual positions

Sample #13-2

0.000

0

2

4

6

8

Pre-crack location/ mm

10

Figure 11: Chosen points for J-integral calculations and calculated values.

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A separate FEA is also conducted to calculate nodal displacements as in the VCCT approach [33,34] in order to calculate from CSDEM [28,29] and the corresponding mode mixity (according to Eq. 10 and Eq. 11) with the reference length one micron meter [25]. Results are presented in Figure 12. It is seen that is smaller than one radian, the mode mixity thus very close to the pure Mode I opening expected by the experimental procedure. For all the interfaces and specimens investigated, similar mode-mixity values are found, which are very close to pure Mode I opening. values from Figure 11 and Figure 12 are When the compared, it is also seen that the J-integral and CSDEM calculations yield very similar values.

1.40

Exp. Sample #12 Sim. Used mesh Sim. 2 times finer mesh Sim. 4 times finer mesh

Force / N

1.20 1.00 0.80 0.60 0.40 0.20 0.00 0

200

1.4

Force / N

0.035 3.93 mm

Gc/N/mm

0.025 2.74 mm

0.020 0.010

Experiment #12 Simulation Input #12 Simulation #12 Gc=0.029 Simulation #12 Gc=0.024

1.0 0.8 0.6 0.2 0.0

Sample #13-1 CSDEM (lref=1µm)

0.005

0

0.000 0

0.5

1

1.5

600

0.4

2.09 mm

0.015

400

Figure 13: Mesh study of the CZM.

1.2

0.030

Time / s

100

1.4

2

Force / N

400

1.0 0.8 0.6 0.4 0.2 0.0 0

100

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Displacement / µm

1.4

400

Experiment #13_2 Simulation points #13_2 Simulation #13_2 Gc=0.029 Simulation #13_2 Gc=0.024

1.2

Force / N

Before the validation of , CZM is investigated in terms of its mesh density. Figure 13 illustrates that a two or four times finer mesh does not have a visible effect on the predictions. It is concluded that the chosen mesh is dense enough to obtain accurate results. values from J-integral are used for The obtained validation in CZM. In CZM simulations, two dimensional, 4-node structural solid elements with the plane stress assumption are used. In addition, TARGE169 and CONTA175 2D elements are used for the target and contact elements, respectively (i.e. where the cohesive zone elements (CZE) are located). Residual stresses are also considered by the initial simulation step which accounts for cooling down from curing temperature to room temperature. Simulations are run with two different values, 0.029 N/mm and 0.024 N/mm, selected from Figure 11, and results are presented in Figure 14. No modification to the contact stiffness of the interface is made and it is calculated by has a great default. It is seen that the choice of influence on the response of the CZEs. The critical energy release rate of 0.024N/mm is observed to give the best fit to the experimental findings.

300

Experiment #13_1 Simulation points #13_1 Simulation #13_1 Gc=0.029 Simulation #13_1 Gc=0.024

1.2

Mode mixty/rad and the mode mixities at different Figure 12: pre-crack locations according to CSDEM for Sample #13-1.

200

Displacement / µm

1.0 0.8 0.6 0.4 0.2 0.0 0

200

400

Displacement / µm

600

Figure 14: CZM of Alloy 42/Adhesive interface with two different values. 3.4. Gc for LTCC / Adhesive interface Samples are prepared with the same procedure. Three representative results are shown in Figure 15. It is observed that, although the crack kinks into the LTCC, it enters and leaves the adhesive interface and bulk LTCC multiple times.

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At the same time, SEM investigation proves that the crack kinking only happens at the sides of the LTCC and the backside of the LTCC is almost without any residual material. This is illustrated in Figure 16. It is seen that a small amount of residual adhesive is left during delamination tests, see 1st region. However, most of the interface is free of residual adhesive, see 2nd region.

Sample #21 Sample #22 Sample #33

0.035 0.030

Gc / N/mm

0.025 0.020 0.015 0.010 0.005

10 8 Force / N

0.000

Sample #21

0

Sample #22

6

2 a)

0 0

b)

100 200 300 400 Displacement / µm

4

6

8

10

12

Pre-crack location/ mm

14

16

Figure 17: Calculated values via J-integral at points where the propagating crack is between LTCC and adhesive.

Sample #33

4

2

500

a) Sample #21 b) Sample #21

Figure 15: Force vs. displacement curves of LTCC/Adhesive interface and the illustration of the crack kinking during the test.

values in Figure 17 are used as Calculated (three estimations of the critical energy release rate values are used: 0.015 N/mm, 0.020 N/mm, 0.025 N/mm) to simulate the interface delamination behavior of the LTCC/adhesive interface. The first thing to realize in Figure 18 is the fact that the normal stiffness of the cohesive zone elements are calculated accurate enough such that the slopes of the force vs. displacement graphs between experiments and simulations are fitting together. Thus, no modification to the normal stiffness of the cohesive zone elements are necessary. When the effect of the critical energy release rate on the delamination response of the simulation is checked (see sample #21 in Figure 18), it is found that the critical energy release rate of 0.025 N/mm provides a good prediction of the delamination behavior observed in the experiments.

Figure 16: Fracture surface of the LTCC backside after the delamination test. Sample #22 is used for illustration. The J-integral calculations are done at the points where the crack is spotted between adhesive and LTCC. Results are presented Figure 17. A discrepancy still exists in the obtained data.

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7.0

Experiment #21 Simulation #21 Gc=0.015 Simulation #21 Gc=0.020 Simulation #21 Gc=0.025

6.0 5.0

Force (N)

interpreted by J-integral and CSDEM in order to find the for Alloy 42/Adhesive and LTCC/adhesive interfaces. It is observed that both methods yield similar values. The calculated values are used as input parameters for CZM, which are implemented in FEA to validate the experimental and numerical (e.g. ) findings. By correct selection of the simulation parameters, good agreement between experimental and simulation work is achieved. The experimental findings is implemented for both are reproduced and interfaces. Various issues with respect to the sample preparation procedures and the interpretation of the obtained experimental and numerical results are discussed.

4.0 3.0 2.0 1.0 0.0 0

10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

100

200

300

Displacement (µm)

400

The described specimen preparation method, the extraction and the implementation steps enable the application of fracture mechanics to product simulations in order to assess package reliability.

Force (N)

Experiment #22 Simulation #22 Gc=0.025

References

0

3.0

200

300

Experiment #33

2.5

Force (N)

100

Displacement (µm)

Simulation #33 Gc=0.025

2.0 1.5 1.0 0.5 0.0 0

100

200

Displacement (µm)

300

Figure 18: Cohesive zone modeling of LTCC/Adhesive interface with three different values which are obtained from J-integral calculations (for sample #21) and the quantitative comparison between the experimental results presented in Figure 15 and simulation results. 4. Conclusions In this paper, a method to obtain delamination specimens from relatively large ECUs is presented. The specimens are produced in conditions close to the actual production with the addition of a single step preconditioning. This step initiates the delamination at the interface of interest. This approach enables the investigation of different interfaces that have the same processing conditions as the real product. A series of nearly pure Mode I fracture experiments are conducted with the AMB test setup. Experimental data is

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