Enhanced Polymer Passivation Layer for Wafer Level Chip Scale ...

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Figure 2.9 Cross section of package structure of Fujitsu’s Super CSP [49] FormFactor developed the so-called MOST WLCSP with a compliant bump structure based on their microspring technology. Microsrpings created by extending wire bonding technology are formed as a joint for the electrical circuit between the silicon chip and the FR-4 substrate. This spring can easily deform under any mechanical stresses [47]. Several examples of Microspring technology are shown in Figure 2.10. Fraunhofer IZM/Technical University of Berlin developed a wafer level package using a double ball. The technological structure of this double-ball CSP is a pad redistributed die with a solder ball array. A stress compensation layer embeds the first solder balls before the second solder balls are stencil printed or placed on top of embedded balls. Figure 2.11 shows a schematic process flow of the redistribution and bumping technology for this WL-CSP. The first screen-printed solder balls can be covered in a CTE matched "underfill" and mechanically polished to expose the solder balls, and then solder balls can be placed at these sites. This produces a solder ball stack that is ~2x the height of a normal solder ball, while the half 26
"underfilled" structure distributes the stress on the solder columns and protects the redistribution layer from stress concentrations. Such structures have shown ~2x the cycles to first fail in thermal cycling [50]. (a) (b) (c) Figure 2.10 Examples of Microspring technology (a). Probe Cards (b). Sockets Interposers (c). Die Size Spring Grid Array (DSSGA) Modules National Semiconductor enlarged the passivation layer opening at the solder die interface to improve the solder joint reliability [51], and a WLCSP technology evaluated by Keser in Motorola SPS demonstrated that board level solder joint reliability of the WLCSP can be improved through wafer thinning [52]. 27
Page 1 and 2: Enhanced Polymer Passivation Layer
Page 3 and 4: SolderBrace coatings were low tempe
Page 5 and 6: Table of Contents Abstract ........
Page 7 and 8: 4.4 Failure Analysis ..............
Page 9 and 10: List of Figures Figure 1.1 Trends i
Page 11 and 12: Figure 3.17 SAC305 Reflow profile .
Page 13 and 14: CHAPTER 1 INTRODUCTION In the era o
Page 15 and 16: This effect changes the package to
Page 17 and 18: 1.4 Solder Joint Fatigue Figure 1.2
Page 19 and 20: leading to the solder joint failure
Page 21 and 22: umped wafers. The coating is stenci
Page 23 and 24: 11 Bumped Wafer Print over ball Pre
Page 25 and 26: 2.1 Chip Scale Package Technology C
Page 27 and 28: Mountable with conventional assembl
Page 29 and 30: Figure 2.3 Cross section of a typic
Page 31 and 32: Table 2.1 Comparison between tradit
Page 33 and 34: In-Situ Bumped Wafers Placed Prefor
Page 35 and 36: of solder joint failure, and they o
Page 37: (a) (b) Figure 2.8(a). Metalized ph
Page 41 and 42: have generally been the modificatio
Page 43 and 44: are “low temperature” wafer lev
Page 45 and 46: 2.3.4 Optimized SolderBrace Materia
Page 47 and 48: CHAPTER 3 WLCSP DIE FABRCIATION In
Page 49 and 50: Table 3.1 Test Die used for Reliabi
Page 51 and 52: Figure 3.4 Fabrication Process Flow
Page 53 and 54: spin coating. A layer of light sens
Page 55 and 56: layer, is investigated as a potenti
Page 57 and 58: 10. Final cleaning: After the passi
Page 59 and 60: Cyclopentanone, a colorless liquid
Page 61 and 62: used to check the basic spin and ph
Page 63 and 64: Figure 3.7 SolderBrace film thickne
Page 65 and 66: 6. Post-UV exposure bake: This step
Page 67 and 68: 8. Pattern characterization: The su
Page 69 and 70: offers the optimum flux release cha
Page 71 and 72: fatigue resistance, lower cost SAC
Page 73 and 74: Figure 3.12 Solder Ball Placement M
Page 75 and 76: of solder balls to the holes on the
Page 77 and 78: of thermal shock. If the temperatur
Page 79 and 80: Figure 3.17 SAC305 Reflow profile -
Page 81 and 82: foaming and high performance cleane
Page 83 and 84: - Polish clean: used the optimized
Page 85 and 86: Figure 3.22 SolderBrace printed waf
Page 87 and 88: Voids Figure 3.25 SolderBrace Mater
Page 89 and 90:
cutting speed was set at a low valu
Page 91 and 92:
Figure 4.2 Process Flow of Board As
Page 93 and 94:
pitch, and bump diameter), and subs
Page 95 and 96:
4.2.4 X-Ray Inspection Figure 4.5 R
Page 97 and 98:
Figure 4.6 Air to air thermal cycli
Page 99 and 100:
Figure 4.7 Thermal Cycling Setup 87
Page 101 and 102:
4.4 Failure Analysis The main purpo
Page 103 and 104:
Figure 4.10 Pad cratering failure o
Page 105 and 106:
CHAPTER 5 FINITE ELEMENT ANALYSIS I
Page 107 and 108:
5.1.1 Modeling Approaches Due to th
Page 109 and 110:
are shown in Figure 5.2. However, t
Page 111 and 112:
5.1.2 Material Properties Many pack
Page 113 and 114:
strain rate sensitivity. The evolut
Page 115 and 116:
Morris et a1 [96] used a double pow
Page 117 and 118:
focus on the time-dependent effects
Page 119 and 120:
5.2 Modeling Procedure In this proj
Page 121 and 122:
Figure 5.6 The diagonal symmetry mo
Page 123 and 124:
5.2.3 Meshing Table 5.2 Anand const
Page 125 and 126:
direction, but that the surface is
Page 127 and 128:
PREP7 tref,398 ! set zero strain te
Page 129 and 130:
5.2.7 Thermal Fatigue Life Predicti
Page 131 and 132:
CHAPTER 6 CONCLUSIONS Wafer level c
Page 133 and 134:
References 1. Future Trends in Elec
Page 135 and 136:
23. Electronic Packaging and Interc
Page 137 and 138:
48. Ultra Low Stress and Low Temper
Page 139 and 140:
70. Factors for Successful Wafer-Le
Page 141 and 142:
92. Microstructural Dependendence o
Page 143:
111. Nonlinear Analysis of Full Mat
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