Simulation of SnAgCu solder balls including the local microstructure

Transcription

1 Simulation of SnAgCu solder balls including the local microstructure student T.M. Assman supervisor M. Erinç internship to report number MT07.25

2 Contents 1 Introduction 3 2 Experimental analysis Manufacturing defects in ball grid arrays Surface preparation for OIM Orientation imaging microscopy Simulations D mesh D disc D solder ball The slice model from a 3D BGA package Conclusions 28 5 Recommendations 28 References 29 Appendix 30 TargetSystem manual Void data

3 1 Introduction Computers, embedded systems and handheld devices have micro-processors to execute certain functions. These chips are soldered on a printed circuit board (PCB) with solder balls. The chip is electrically connected to the board, which uses conductive pathways to link it to other components. The solder balls are arranged in a ball grid array (BGA), shown in Figure 1. Up to 2002 solders were still made of tin and lead (SnPb). Lead is toxic, has a high surface tension and coarsens under thermal loading. Because of these disadvantages most solders are now lead-free and made of tin, silver and copper (Sn-4%Ag-0.5%Cu or SAC). Figure 1: Ball grid array packages and chips on a printed circuit board. Electrical signals are exchanged between the chip and the board. A part of the electrical energy is turned into thermal energy due to electric resistance. This heat causes the temperature of the chip and the board to rise. The chip has a different coefficient of thermal expansion (CTE) than the board, generating forces in different directions on the top and bottom of the ball. Repeated operations of the package causes cyclic mechanical strains on the solder joints, generated by cyclic thermal loading. Thermo-mechanical fatigue loading causes fatigue cracks to initiate and leads ultimately to overall failure, illustrated in Figure 2. Figure 2: Solder fatigue in BGA packages due to thermal mismatch. 3

4 The current miniaturization trend in micro-electronics leads to smaller components. As things get smaller, continuum material models are not valid anymore because microstructure and local imperfections have a greater influence on overall mechanical properties. Therefore the microstructure and defects are investigated in this study. A statistical study on manufacturing defects (i.e. voids, Figure 3a) is given in Section 2.1. Orientation imaging microscopy (OIM) is employed to make crystallographic orientation maps of solder balls, described in Section 2.3. This microstructural information (Figure 3b) is converted to a Marc Mentat mesh (Section 3.1) and analyzed with the finite element method (Section 3.2). In Section 3.3, the cross-section is expanded to a solder ball. Other layers (i.e. molding compound, substrate, solder mask, PCB and the different pads) are added to the mesh as explained in Section 3.4. (a) Voids. (b) Granular structure (polarized light). Figure 3: Cross-sections of 760 µm diameter Sn-4%Ag-0.5%Cu solder balls. 4

5 2 Experimental analysis In the Multi-Scale Laboratorium, there are three types of grinding machines available. Sample preparation can be done at the TargetSystem (full automatic), the TegraSystem (semi automatic) or one of the manual grinders. Their different advantages and purposes are discussed in this section. 2.1 Manufacturing defects in ball grid arrays The manual grinder (Figure 4) gives the user full control. However, it is hand-work. It requires some experience to equally apply pressure such that an equal amount of material is removed on every side. Pushing too hard will result in missing the target. Different grinding papers can be placed. The only requirement for the sample is that the user should be able to hold and apply pressure to it without hurting himself on the grinding paper. Sample holders for too small samples can easily be made by molding. Figure 4: A manual grinder Figure 5: The cross-section of a BGA-256 package For the void analysis samples with solder balls mounted on PCB are needed. To get enough data six samples were prepared, of which one is shown in Figure 5. First, all the samples were grinded on the TegraSystem, then one by one finished on the manual grinder. This turned out to be the fastest way, with still enough precision and surface quality. 5

6 Voids are studied with a computer controlled optical microscope, equipped with a digital camera. Figure 6 shows examples of defects in 760 µm diameter BGA solder balls. Voids degrade the mechanical strength of the solder balls. (a) The biggest void of 354 µm diameter. (b) Under-flow. (c) Under-flow on left side. (d) Void on board-side. Figure 6: Blowholes in BGA solder balls. In such a 2D analysis, one can only underestimate the size of a void, since the cross-section will never be exactly at the largest diameter of a void. Voids can also be deeper into the ball and invisible at the current cross section. Sometimes the voids are not spherical, and a diameter is hard to determine, i.e. Figure 6b. On several occasions there were tiny voids, also difficult to see and to measure. 6

7 The relevant information about a void is the void position, diameter and its side being board or chip-side, illustrated in Figure 7. A total of 286 solder balls have been inspected. Figure 7: The void parameters. The collected data (also given in Appendix B) was analyzed in Matlab using structure arrays with the following fields: ball(# of sample, # of row, # of ball).n = total number of voids.r = void/ball volume ratio.void(# of void).d = diameter of void.x = x distance to closest edge.y = y distance to closest edge.s = side (1=chip 0=board) 7

8 The data is statistically analyzed and the distributions are plotted in Figure 8. (a) Side distribution. (b) Diameter distribution. (c) Volume distribution. Figure 8: Void analysis results. Most of the voids (84%) are on the chip side. Balls with big voids (>100µm) are rare (<1%). The void diameters can be divided into three almost equal proportions: 0-25 µm, µm and µm. In 43% of the solder balls examined, there were no voids visible. If voids are present, most of the time (47%) the total solder ball volume consist for 1% of void. 8

9 2.2 Surface preparation for OIM The TargetSystem is shown in Figure 9. Removal is first controlled by a quick continuous electronic measuring system. Near the grinding target, the sample holder also needs to move between the turntable and a laser measurement station. During the process it asks for finer grinding papers, so at the end the best surface quality is reached at the target depth. Figure 9: The TargetSystem. Once the system is set, interruption is not possible on occasions which might be beneficial. The special sample holders (Figure 10) designed to fit in the machine, give limitations to the usable specimen dimensions (32x20x6 mm). The machine estimates the removal rate of the paper. Since the removal rate decreases due to wear, this can result in over-grinding. One has to establish the grinding sequence for a certain type of material by trial and error, before starting with the real sample, if the target depth is very important. During the current study, the TargetSystem was newly acquired. A user s manual is written within the current study, which is also enclosed in Appendix A. 9

10 Figure 10a shows the solder balls on the PCB glued with epoxy resin to the special sample holder of the grinder. When the sample is put in the SEM, the sample surface charges because it is not conductive enough. To overcome charging the bottom of the sample holder is cut off, as shown in Figure 10b and 10c. The sample was still charging. In order to increase conductivity copper powder was mixed with the epoxy glue, and instead of reflowed balls on a PCB non-reflowed solder balls were placed without the PCB shown in Figure 10d. The sample was still charging. A copper plate was inserted in Figure 10e, but since it could not be fixed well it gave problems with grinding. Even though SEM images could be acquired with silver paint (Figure 10f), it was still not possible to make an OIM scan. Due to the large dimensions of the sample holder compared to the vacuum-chamber, the prolonged e-beam was shifting the tilted sample down. (a) With bottom. (b) Without bottom. (c) Specimen. (d) Copper powder. (e) Copper strip. (f) Silver paint. Figure 10: Different attempts to make the sample holder conductive enough to work with SEM. 10

11 A different kind of specimen was made on the TegraSystem which is shown in Figure 11. So far it was necessary to mold the sample in a special sample holder to mount it in the TargetSystem. The weight of this sample holder is mainly responsible for the sample movement during the scan. In order to make smaller specimens, a sample holder (Figure 12) and a distinct plexiglass sample mold are made (Figure 13). Grinding fluid is automatically added as the grinding procedure is selected. Compared to the TargetSystem, grinding-time is given by the user. Figure 11: The TegraSystem. Figure 12: The sample holder for grinding Figure 13: The plexiglass mold. 11

12 Non-reflowed solder balls (Figure 14a) are molded in epoxy resin mixed with copper powder. The samples still needed to be finished with silverpaint (Figure 14b at the bottom) for conductivity in the SEM. The optical microscope was used to check whether the largest diameter was reached and an OIM scan was possible without significant charging or sample movement. The disadvantages here were that the epoxy needed to dry in the mold for one day. Also the design of the plexiglass mold has to be improved to take the samples out intact. (a) SAC balls. (b) Copper powder mixed with epoxy resin. Figure 14: A 1mm thick conductive specimen containing solder balls. 12

13 2.3 Orientation imaging microscopy A crystal is a solid in which the atoms, molecules, or ions are packed in a regularly ordered repeating pattern having a single orientation. Crystalline structures occur in all classes of materials. Almost all metals exist in a polycrystalline state consisting of several crystals or grains. These structures can have a profound effect on the properties of the materials. A scanning electron microscope extended with a orientation imaging microscopy module makes a crystallographic orientation map of the metal surface. This setup is shown in Figure 15. Figure 15: Schematic HR-SEM with OIM. In the SEM electrons are emitted from a cathode and accelerated towards an anode. These electrons result in a beam ranging from 10 2 to 10 5 electron Volt and a spot size from 1 nm to 5 nm. The electron interacts with the atomic lattice planes of the crystalline structures. A part of the electron energy is absorbed by the interaction volume of the sample causing it to charge. The rest is backscattered out of the sample. In OIM, electron backscatter diffraction (EBSD) is used to detect the grains. Since the angle of the specimen is 70 with the horizontal, most of the reflected electrons are collected by a phosphor screen causing it to fluoresce. The fluorescent light is then detected by a CCD camera to produce a diffraction image. Each diffraction pattern will show several intersecting lines termed Kikuchi bands, these correspond to each of the lattice diffracting planes and can be indexed individually by the Miller indices of the diffracting plane which formed it. This is done with database material id s by software using a certain step size defined by the user. 13

14 Figure 16: HR-SEM with OIM module. Surface quality determines the accuracy of an OIM scan. In SnAgCu, there are hard intermetallic compounds in the soft Sn matrix, which are very difficult to level by grinding. These IMC makes hills on the surface, blocking detection. These measuring errors are removed by a software cleaning procedure with the following parameters: 1) A minimal neighbor confidence index of 0.8 out of 1 (80% sure). 2) Cropping the ball. 3) Grain dilation with a tolerance angle of 5 and a minimum grain size of ) A single grain tolerance angle of 10. 5) Again cropping the ball. First all spots with a low confidence number are removed. The cropping between the steps is necessary to avoid that the crystallographic orientations close to the edge of the ball are being scaled out of the solder ball. If the orientation of two grains differs less then the grain tolerance, they are combined and seen as one single grain. Figure 17 shows the result. (a) Raw image. (b) After cleaning. (c) Pole figure. Figure 17: OIM scan of an Sn-4%Ag-0.5%Cu solder ball. 14

15 3 Simulations 3.1 2D mesh A txt-file with the OIM measurements is imported in Matlab by an external Matlab script. Then it is exported to a proc-file which is read by Msc. Marc Mentat 2005 to make a finite element mesh. First grain boundary lines are drawn. Extra elements are placed at triple points shown in Figure 18. A round frame with a radius of 760 µm is made around the mesh defining the edge. The grain boundaries are divided into blocks, starting and ending at a triple point or the frame. Then, grains are meshed with these blocks shown in Figure 20. The blocks and triple points are converted into special interface elements for further use. Finally local crystallographic orientations are assigned to the mesh. Figure 18: Grain boundaries connected at a triple point Figure 19: OIM scan. Figure 20: The Msc. Marc Mentat mesh. Figure 21 shows OIM scans, where in Table 1 the total number of grains with the average grain diameter is given. Some cross-sections are single crystal, where others show up to 15 grains. For solder balls with more then seven grains, the average grain diameter is in the range of µm. In most scans, there are multiple grains with (almost) the same orientation. 15

16 Figure 21: The OIM scans of 16 Sn-4%Ag-0.5%Cu solder balls. Table 1: The number of grains and average grain diameter in µm for the OIM scans of Figure 21. a b c d e f g h grains diameter i j k l m n o p grains diameter

17 To save calculation time, it is necessary to reduce the number of elements. The triple points and the grains can be meshed with triangular elements with three nodes (tria3 or ) or square elements with four nodes (quad4 or ). In the first row of Figure 22 the element density can be compared for all the combinations. The area of the meshes marked by a circle are shown magnified in the second row of Figure 22. For example, Figure 22a shows the mesh created using the OIM scan shown in Figure 19, with in some places many tiny elements. A triple point of type quad is highlighted in Figure 22e below it. Because this quad has one side which is not connected to a grain boundary, automeshing forms many small elements around the triple point. It is seen in Table 2 that triple points of the type tria3 and elements of the type quad4 give the least number of elements. (a) Triple= Elem=. (b) Triple= Elem=. (c) Triple= Elem=. (d) Triple= Elem=. (e) Triple= Elem=. (f) Triple= Elem=. (g) Triple= Elem=. (h) Triple= Elem=. Figure 22: Reducing the number of elements. Table 2: Influence of element types on the number of elements. Figure Triple point Matrix Total number of element type element type elements 22a quad4 tria b tria3 tria c quad4 quad d tria3 quad

18 In the output of the OIM scan, grain boundaries are exported as a collection of vectors. Apart from the element type used, very short grain boundary lines also cause small elements to form. To overcome this problem, a threshold is defined as the minimum length of a grain boundary line vector. If a grain boundary line is smaller than the threshold, it is removed. The effect of threshold (T) is presented in Figure 23. The arrow in Figure 23a marks a small extension. With a higher threshold value it shrinks and finally disappears in Figure 23l. The total number of elements (N) is given in the brackets and plotted in Figure 24. From a threshold of 0.04, the number of elements significantly reduces. Between values 0.05 and 0.06 there is not much improvement. A threshold of 0.07 seems a good compromise. (a) T=0.00 N=564 (b) T=0.01 N=564 (c) T=0.02 N=564 (d) T=0.03 N=557 (e) T=0.04 N=487 (f) T=0.05 N=360 (g) T=0.06 N=335 (h) T=0.07 N=275 (i) T=0.08 N=214 (j) T=0.09 N=167 (k) T=0.10 N=140 (l) T=0.11 N=133 Figure 23: Influence of the threshold (T) on the number of elements (N) generated in the mesh of ball

19 Figure 24: The influence of the threshold (T) on the number of elements (N) D disc The Marc Mentat mesh is expanded to a 3D disc with a thickness of 0.1 mm for finite element analysis of the equivalent Von Mises stress (σ vm ). The boundary conditions are set as follows: Fix the Z direction in the whole bottom, fix at the bottom one node in the X and Y directions and another node in Y. Figure 27 shows the orientation of the ball with respect to the axes. The package is heated from 0 C to 100 C. The material properties are given in Table 3. Table 3: Material properties used in the simulations Material E(GPa) ν α(ppm/k) MC [1] Substrate [2] Soldermask [2] PCB [1] Cu Ni Ni3Sn4 [6] SnAgCu [3]

20 Figure 25 shows the stress distribution in the created meshes using different threshold values, heated to 100 C. The disappearance of the extension is marked by an arrow for threshold values of 0.00 and 0.11 and no high stresses develop at this region. In Figure 23, the middle grain is more twisted at a lower threshold, and simpler at a higher one. At the right bottom corner the stresses are more uniformly distributed with higher threshold values. The reason for the divergence of the grain boundary stress zones here, can be found in the fact that the grains are relatively small at this part of the slice. Figure 25: Influence of threshold [ ] on the σ vm (MPa) distribution at 100 C. 20

21 Figure 26 shows the stress distributions after the temperature is increased to 100 C. Figure 26: The influence of temperature [0 C C] on the σ vm (MPa) of ball

22 Figure 27 shows information of solder balls 1, 7, 9 and 10. The first row contains the OIM scans, the second row the meshes and the third the equivalent Von Mises stresses (σ vm ). In ball 9 a small grain is showed, being automeshed as one single triangle and resulting in local high Von Mises stresses. A small grain of ball 10 is not meshed at all. It can be speculated that ball 9 will have the shortest fatigue life compared to the other specimens, because it shows the highest stress concentrations, especially at the triple points. Figure 27: The OIM scans, 3D FE meshes and σ vm (MPa) distribution at 100 C for the solder balls indicated. 22

23 3.3 3D solder ball There are a couple of ways to make a ball from the disc. One is to make OIM scans at different sections of a solder ball. In theory, these scans can be connected to each other, and for an infinite number of sections, the real microstructure can be simulated. In practice, this is an expensive method. (a) Duplicate. (b) Scale. (c) Sweep. Figure 28: Expanding the disc to a solder ball. The method for creating the ball is illustrated in Figure 28. The dimensions are given in Section 3.4. A 3D disc is generated by expanding the 2D mesh out-of-plane. Then it is scaled to the right dimensions and duplicated four times, shifting it up by the height of the disc every time as shown in Figure 28a from side-view. With a selection box the nodes of each separate cutting pane are selected, (for example the gray line) and scaled to the right dimensions, illustrated in Figure 28b. By sweeping, the slices will attach to each other and the ball is finished (Figure 28c and 29). Figure 29: The mesh of a 760 µm diameter solder ball with several grains. 23

24 For visualization, the ball is cut into sections in Figure 30. For the horizontal planes, the width of the ball is divided into sections of 0.33/7 = mm. In the same way, vertical sections have a thickness of 0.23/7 = mm. This method assumes that there are vertical columnar grains, where in reality there might be horizontal grain boundaries. High stresses occur at the grain boundaries, therefore it is logical to expect fatigue damage localization here on thermal loading. Figure 30: Horizontal (top) and vertical (bottom) cutting planes. 24

25 3.4 The slice model from a 3D BGA package To be able to construct a complete BGA package, a slice shown in Figure 31 was modeled by adding the surrounding layers to the mesh. The dimensions of the middle cross-section of the slice are given in Figure 32. Figure 31: The slice of the total BGA package. Figure 32: Dimensions of the model in mm s (not to scale). 25

26 The boundary conditions were applied the same way as for the disc. The bottom nodes were fixed in the Z-direction, one node was fixed in the X and Y directions and another node was fixed in the Y direction. The ball is attached to the rest of the mesh by contact surfaces. The package is heated from 0 C to 100 C homogeneously. Figure 33: The slice model. 26

27 The results are presented in Figure 34. High stresses occur not only at the grain boundaries due to the thermal anisotropy of tin, but also at the pads due to a CTE mismatch between the different layers. Figure 34: A cross-section of the package showing σ vm (MPa) at 100 C. In continuation to this study, the defects shown in Section 2.1 can be added to the model. The contribution of the defects to the failure process becomes important under fatigue loading, which is out of the scope of this study. 27

28 4 Conclusions The experimental part of this study consists of inspecting BGA packages with Sn-4%Ag-0.5%Cu solder balls for manufacturing defects and local crystallographic orientations. In the numerical section, solder balls are modeled using the experimental data. Different meshing techniques and their effect on the overall mechanical response to thermal loading are discussed in detail. A user s manual for the TargetSystem is written, which is currently in use in the Multi-Scale Lab. In an arbitrary BGA package half of the solder balls contain voids, visible by an optical microscope, % 5 being more than % 10 of the solder ball diameter. The use of tria3 elements for triple points and elements of type quad4 for matrix elements resulted in the least amount of elements. Defining the minimal grain boundary length by a threshold of 0.07 reduced the number of elements without changing the mechanical response excessively. High stress concentrations occur at the grain boundaries, especially at the triple points on thermal loading. One slice of the BGA package was modeled for further analysis. Here highest stresses occurred at the bump/pad interfaces. 5 Recommendations Horizontal grains could be scanned, meshed and compared to the vertical model. The inspection of multiple cross-sections per solder ball improves the void counting accuracy. In the 3D slice model, periodic boundary conditions would be more realistic. 28

29 References [1] Jansen, M., Creep calculations BGA with Amkor model. Philips CFT Internal Report, 2003 [2] Lai, Y., Wang, T.H., Lee, C., Thermal-Mechanical Coupling Analysis for Coupled Power and Thermal Cycling Reliability of Chip-scale Packages. EuroSimE, Taiwan, 2005 [3] Erinc, M., Schreurs, P.J.G., Geers, M.G.D., Integrated numerical-experimental analysis of interfacial fatigue fracture in SnAgCu solder joints. International Journal of Solids and Structures, in press, Online available, 2007 [4] Telang, A.U., Bieler, T.R., Crimp, M.A., Grain boundary sliding on near-7, 14, and 22 special boundaries during thermomechanical cycling in surface-mount lead-free solder joint specimens. Materials Science and Engineering A, 2006 [5] Telang, A.U., Bieler, T.R., Lucas, J.P., Subramanian, K.N., Lehman, L.P., Xing, Y., Cotts, E.J., Grain-boundary character and grain growth in bulk tin and bulk lead-free solder alloys. Journal of Electronic Materials, [6] Jang, G.Y., Lee, J.W., Duh, J.G., The nanoindentation characteristics of Cu 6 Sn 5, Cu 3 Sn, and Ni 3 Sn 4 intermetallic compounds in the solder bump. Jounal of Electronics Materials,

30 Appendix A TargetSystem manual Illustrated in 40 steps Tim Assman 1 The TargetSystem. 2 Open the air and water handles. 3 Push the power button. 4 The instruments: - TargetGrip - SampleChair - Screwdrivers - Tweezers - Glue - Stickers 5 Glue sample to SampleChair. If the sample is longer and sticks out, this will later result in an error. 6 Get TargetGrip out of the box. Put SampleChair in TargetGrip, pay attention to two pins at the bottom. 7 Fix it at three points. Fixed is fixed. The sample side of the SampleChair should be in the direction of the TargetGrip top screw. Don t scratch the coarse surface, it s reference. 8 Move TargetZ to top. 9 Put TargetGrip in TargetZ.

31 10 Mount the TargetGrip to the TargetZ. 11 Loosen the top tilt locking screw. 12 Move TargetZ to target. 13 Adjust magnification and light. 14 Adjust the tilt adjustment screw on the TargetGrip and Y- position of the microscope. 15 Adjust the X-position of the sample and focus in the Z-direction. 16 Look at the screen and find your spot. Align the TargetZ with your sample: Play with the tilt adjustment screw, until you manage to move in the local Y-direction of sample, by only changing the Y-position of the TargetZ. Example: Not aligned. Example: Aligned.

32 17 If you are satisfied, fasten the top tilt locking screw. 18 Put the center of the cross on the point you want to cut, approaching from one side. Then push zero. 19 Move TargetZ to reference. 20 Adjust the X-position untill you find the reference edge of the TargetGrip, again approaching from one side. Then push enter. 21 Unmount the TargetGrip, be carefull not to mix it up with the tilting screw! 22 Put a sticker on top of the sample chair, and fill it with epoxy, so your sample stays fixed during the grinding process. 23 The F-functions on the TargetDoser correspond to the text at the bottom of the screen. Esc goes to the root, you can browse with the arrows, and select with enter. 24 Choose Struers methods then target method, then F1: use template, then F4: send to TargetMaster 25 The TargetMaster. Be carefull not to close the safety glass when the water valve is standing up! 26 Move the sample mover with the yellow arrows to the middle. Open the safety glass with the orange button. 27 Insert the TargetGrip into the sample mover. 28 Fasten the screw and close the glass again. Move the sample mover to the left. Open the safety glass.

33 29 Insert the grinding paper, and close the safety glass again. Push on the valve button. 30 Open the water tap slowly. 31 Push on it again to stop it if it s ok. 1 Grinding paper 3 m 2 Sat 3 Dac 4 Nap 32 Push the green start button. 33 The screen shows different phases, going from coarse to soft paper. Behind the phase is the amount of material that needs to be removed. 34 You need to fix the papers and cloths to a disc, and then change them between the phases. 35 At the left side is the washing station (water, alcohol and air), at the right side the measurement is done. 36 You need to put three droplets of diamond powder on Sat/Dac/Nap, by pushing the orange diamond button. 37 If you write down the removal rates for the next time, the machine does not need to guess and learn. It s faster. 38 When you re done, lift the tube, to be sure all the water flows away from the TargetMaster. 39 Put the end of the tube in a bucket, and then empty the water tank. 40 And last: Clean up the workspace. Turn the TargetSystem off. Close the water and air valve.

34 Appendix B The quantities measured with the optical microscope for void analysis in Matlab (Section 2.1) Sample Row Ball Void Dia Xdist Ydist Chip

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