Electromigration-induced failure in SnAg3.5 solder joints has been investigated under the current densities 1 × 104 A/cm2 and 5 × 103 A/cm2 at 150℃. When stressed at 1 × 104 A/cm2, failure may occur on both anode/chip and cathode/chip sides. Joule heating became very serious under 1 × 104 A/cm2 current stressing, causing a temperature increase of 54.5 ℃ in the solder and a thermal gradient of 365 ℃/cm across the solder joint. Thermal migration due to the built thermal gradient and volume expansion due to the IMCs formation may account for the void formation on the anode/chip side under the stressing of 1 × 104 A/cm2. Three-dimensional simulation on current density distribution supported the
contention that current crowding effect was considered to be responsible for the failure on the cathode/chip side. This is because for the solder joints stressed under 5 × 103 A/cm2,
migration of Al atoms in the Al trace may contribute to the UBM failure in the cathode/chip side.
- 31 -
References
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- 33 -
Figure Captions
Fig. 1 (a) A schematic diagram of a flip chip package and a FR4 board.
(b) Cross-sectional schematic diagram of a solder joint in IBM’s C-4 structure.
Fig. 2 A schematic diagram depicting the wire-bonding packages and flip-chip packages.
Fig. 3 A schematic diagram depicting various flip chip technologies.
Fig. 4 (a) A schematic diagram of a bumped die. (b) SEM image of a bumped die.
Fig. 5 A sketch of electromigration effect in an Al line.
Fig. 6 Electromigration in Al stripe of different lengths [16].
Fig. 7 (a) Schematic diagrams of cross-sectional view showing the structure of a SnAg joint.
(b) Schematic diagrams of flip chip solder joints under current stressing.
The directions of electron flow are indicated by arrows in the figure.
Fig. 8 (a)-(d) Schematic diagrams of wafer bumping process.
Fig. 9 Schematic diagrams of flip chip process.
Fig. 10 The schematic diagram of the flip chip sample was placed on the hot plate during current stressing.
Fig. 11 (a) Schematic diagram of Bump 1, in which 97% of the solder remains.
(b) Schematic of experimental setup during temperature measurement of the flip chip package
(c) Schematic of experimental setup during temperature measurement for flip chip package with a Cu heat spreader attached to chip side.
- 34 -
Fig. 12 The treatment process of the plane-view sample.
Fig. 13 Schematic diagram of the simplified flip chip solder joint.
Fig. 14 The meshed 3-D Finite element model of the flip chip solder joint.
Fig. 15 (a) Cross-sectional SEM image of an as-prepared SnAg solder joint.
(b) Cross-sectional SEM image of a SnAg joint aged at 150°C for 20 hours.
Fig. 16 Plane-view SEM images showing the morphology of contact openings on the chip side for (a) an as-prepared SnAg solder joint; (b) a SnAg joint aged at 150°C for 20 hours. Ternary compounds of Ni, Cu, and Sn were found after the aging.
Fig. 17 Cross-sectional SEM image after the current stressing of 1 × 104 A/cm2 at 150°C for 22 hours, (a) for Bump 2, (b) for Bump 1. Voids formed on both the cathode/chip and anode/chip sides.
Fig. 18 Elemental EDS mappings of Ti, Cr, Cu, and Ni for the bump pairs in Figure 5. Ni and Cu atoms accumulated on the anode/chip side.
Fig. 19 Plane-view SEM images depicting the morphology of the contact openings after the stressing of 1 × 104 A/cm2 at 150°C for 22 h. (a) on the anode/chip side, (b) on the cathode/chip side. IMCs formed on both sides.
Fig. 20 Cross-sectional SEM images for the joints after current stressing of 5 × 103 A/cm2 at 150°C for 218 h (a) for bump 2, in which electrons drifted downward. (b) for the enlarged image on the cathode/chip side.
Fig. 21 Plane-view SEM images showing the morphology of the contact openings after
- 35 -
stressing by 5 × 103 A/cm2 at 150 °C for 218 h. (a) on the anode/chip side, (b) on the cathode/chip side. Failure occurred at the cathode/chip side.
Fig. 22 (a) Temperature distribution in the SnAg joint when stressed at 1 × 104 A/cm2 at 70
°C. The temperature increase due to the current stressing was 54.5 °C; (b) the corresponding temperature profile along the dashed line in (a), showing a thermal gradient of 365°C /cm across the joint.
Fig. 23 (a) The 3-D current density distribution in the solder joint with the Ti/Cr-Cu/Cu thin film UBM. (b) The current density distribution at the Z-axis cross-section in (a).
The white dotted lines show the five cross-sections examined in this study.
Fig. 24 The 3-D current density distribution at the different cross-sections (a) cross-section Y1, which is located inside the UBM; (b) cross-section Y2, which is the IMC layer;
(c) cross-section Y3, which is the top layer of the solder connected to the IMC formed between the UBM and the solder; (d) cross-section Y4, which has the largest diameter in the joints; (e) cross-section Y5, which has a smaller diameter due to solder mask process; (f) cross-section Y6, which is situated at the bottom of the solder joint.
Fig. 25 (a) Plane view SEM image of the morphology on the contact opening of the chip/cathode side of the solder joint after the current stressing by 0.567 A at 150 ℃ for 20hrs. (b) The simulated current density distribution in the top layer of the SnPb solder. The IMC/UBM dissolved much faster near the entrance of the Al trace into the solder joint, where it had much higher current density.
Fig. 26 The crowding ratios inside the solder joint for the five UBM structures at different cross-sections.
- 36 -
Fig. 1 (a) A schematic diagram of a flip chip package and a FR4 board.
(b) Cross-sectional schematic diagram of a solder joint in IBM’s C-4 structure.
- 37 -
Fig. 2 A schematic diagram depicting the wire-bonding packages and flip-chip packages.
- 38 -
Fig. 3 A schematic diagram depicting various flip chip technologies.
- 39 - (a)
(b)
Fig. 4 (a) A schematic diagram of a bumped die. (b) SEM image of a bumped die.
- 40 -
Fig. 5 A sketch of electromigration effect in an Al line.
e
-Mass flow
- 41 -
Fig. 6 Electromigration in Al stripe of different lengths .
- 42 - (b)
Fig. 7 (a) Schematic diagrams of cross-sectional view showing the structure of a SnAg joint.
(b) Schematic diagrams of flip chip solder joints under current stressing.
The directions of electron flow are indicated by arrows in the figure.
SnAg
(a)
SnAg
- 43 -
Fig. 8(a) Schematic diagrams of wafer bumping process.
IC Passivation
Pad
Spin coating
Photo mask Expose
- 44 -
Fig. 8(b) Schematic diagrams of wafer bumping process.
Dielectric 1
Rerouting metal Via 1
Rerouting metal Dielectric 1
Expose
Dielectric 1
Dielectric 1 Rerouting Mask
PR.
- 45 -
Fig. 8(c) Schematic diagrams of wafer bumping process.
Mask Via 2
Rerouting
Expose
PR.
UBM
Rerouting
- 46 -
Fig. 8(b) Schematic diagrams of wafer bumping process.
Fig. 8(d) Schematic diagrams of wafer bumping process.
Rerouting
Solder paste
Stencil Squeegee
UBM
Solder bump UBM
Dielectric
- 47 -
Fig. 9 Schematic diagrams of flip chip process.
Substrate Flip bumped die
Substrate Squeegee
Metal stencil
Solder paste Solder paste
- 48 -
Fig. 10 The schematic diagram of the flip chip sample which was placed on the hot plate during current stressing.
- 49 -
Fig. 11 (a) Schematic diagram of Bump 1, in which 97% of the solder remains.
(b) Schematic of experimental setup during temperature measurement of the flip chip package
board
Hot plate chip
underfill e -IR dectector
bump
(b)
(a) e
Al trace
Cu line
- 50 -
Fig. 12 The treatment process of the plane-view sample.
- 51 -
Fig. 13 Schematic diagram of the simplified flip chip solder joint.
- 52 -
Fig. 14. The meshed 3-D Finite element model of the flip chip solder joint.
- 53 -
Fig. 15 (a) Cross-sectional SEM image of an as-prepared SnAg solder joint.
(b) Cross-sectional SEM image of a SnAg joint aged at 150°C for 20 hours.
- 54 -
Fig. 16 Plane-view SEM images showing the morphology of contact openings on the chip side for (a) an as-prepared SnAg solder bump; (b) a SnAg bump aged at 150°C for
20 hours. Ternary compounds of Ni, Cu, and Sn were found after the aging.
(a)
- 55 -
(a)
(b)
Fig. 17 Cross-sectional SEM image after the current stressing of 1 × 104 A/cm2 at 150°C for 22 hours, (a) for Bump 2, (b) for Bump 1. Voids formed on both the cathode/chip and anode/chip sides.
- 56 -
Fig. 18 Elemental EDS mappings of Ti, Cr, Cu, and Ni for the bump pairs in Figure 5. Ni and Cu atoms accumulated on the anode/chip side.
Ti
Ni Cu Cr
e e
- 57 -
Fig. 19 Plane-view SEM images depicting the morphology of the contact openings after the stressing of 1 × 104 A/cm2 at 150°C for 22 h. (a) on the anode/chip side, (b) on the cathode/chip side. IMCs formed on both sides.
- 58 -
(a)
(b)
Fig. 20 Cross-sectional SEM images for the joints after current stressing of 5 × 103 A/cm2 at 150°C for 218 h (a) for bump 2, in which electrons drifted downward. (b) for
the enlarged image on the cathode/chip side.
- 59 -
(a)
(b)
Fig. 21 Plane-view SEM images showing the morphology of the contact openings after stressing by 5 × 103 A/cm2 at 150 °C for 218 h. (a) on the anode/chip side, (b) on the cathode/chip side. Failure occurred at the cathode/chip side.
- 60 -
(a)
(b)
Fig. 22 (a) Temperature distribution in the SnAg joint when stressed at 1 × 104 A/cm2 at 70
°C. The temperature increase due to the current stressing was 54.5 °C; (b) the
corresponding temperature profile along the dashed line in (a), showing a thermal gradient of 365°C /cm across the joint.
- 61 -
(a) (a) (a)
Fig. 23 (a) The 3-D current density distribution in the solder joint with the Ti/Cr-Cu/Cu thin film UBM. (b) The current density distribution at the Z-axis cross-section in (a).
The white dotted lines show the five cross-sections examined in this study.
- 62 -
Y1: UBM layer
Y2: IMC layer Current density (A/cm
2) Curren t dens it y ( A/c m
2)
Position (µm)
(a)
(b) Position (µm)
Y1: UBM layer
Y2: IMC layer Current density (A/cm
2) Curren t dens it y ( A/c m
2)
Y1: UBM layer
Y2: IMC layer Current density (A/cm
2) Curren t dens it y ( A/c m
2)
Position (µm)
(a)
(b) Position (µm)
Fig. 24 The 3-D current density distribution at the different cross-sections (a) cross-section Y1, which is located inside the UBM; (b) cross-section Y2, which is the IMC layer;
- 63 -
Fig. 24 The 3-D current density distribution at the different cross-sections (c) cross-section Y3, which is the top layer of the solder connected to the IMC formed between the UBM and the solder; (d) cross-section Y4, which has the largest diameter in the joints;
- 64 -
Fig. 24 The 3-D current density distribution at the different cross-sections UBM and the solder; (e) cross-section Y5, which has a smaller diameter due to solder mask process; (f) cross-section Y6, which is situated at the bottom of the solder joint.
- 65 -
(a)
(b) (a)
(b)
Fig. 25. (a) Plane view SEM image of the morphology on the contact opening of the chip/cathode side of the solder joint after the current stressing by 0.567 A at 150 ℃ for 20hrs. (b) The simulated current density distribution in the top layer of the SnPb solder. The IMC/UBM dissolved much faster near the entrance of the Al trace into the solder joint, where it had much higher current density.
- 66 -
Position
Cr o w din g ra tio
Fig. 26 The crowding ratios inside the solder joint for the five UBM structures at different cross-sections.
- 67 -
List of Tables
Table 1. The melting point and composition of binary solders.
Table 2. Common IMCs of Sn.
Table 3. Wafer bumping process.
Table 4. Flip chip process.
Table 5. The resistivities of the materials used in the simulation models.
Table 6. The maximum current densities and the crowding ratios at different cross-sections for the five models.
- 68 -
Table 1. The melting point and composition of binary solders.
systems Liquidus temp. ℃ Solidus temp. ℃ Composition (wt%)
Pure tin 232 232 100%Sn
Sn-Ag 221 221 3.5%Ag
Sn-Sb 240 234
Sn-Pb-Ag 189 177 36%Pb；2%Ag
Sn-Pb 183 183 37%Pb Sn-Cu 227 227 0.7%Cu
Sn-Au 217 217 10%
Sn-Zn 198.5 198.5 0.9%
Sn-Bi 139 139 57%
Sn-In 120 120 51%
- 69 - Table 2. Common IMCs of Sn.
Impurity elements IMCs
Al -- Sb SbSn
Cu Cu6Sn5, Cu3Sn
Au AuSn4, AuSn2,AuSn
Fe FeSn, FeSn2
Ni Ni3Sn2, Ni3Sn4, Ni3Sn, NiSn3
Ag Ag3Sn
Zn --
- 70 - Table 3. Wafer bumping process
Item Process Step
1 Wafer incoming 2 UBM Pre-Clean 3 UBM Sputter 4 Positive PR Coating 5 Positive PR exposure 6 Positive PR development 7 UBM etching
8 PR Removal
9 UBM etch inspection
10 Solder deposition by printing method 11 Solder paste reflow
12 Saw the wafer into chips
- 71 - Table 4. Flip chip process
Item Process Step
1 Subatrate cleaning
2 Print solder paste on the substrate 3 Flip chip on the BT substrate 4 Substrate reflow
5 Substrate baking 6 Underfill
7 Underfill curing 8 Package cutting
- 72 -
Table 5. The resistivities of the materials used in the simulation models.
Materials Resistivity (µΩ-cm)
Al 4.3
Ti 43.1
Ni(V) 63.2
Cu6Sn5 IMC 17.5
Sn63Pb37 14.6
Ni3Sn4 IMC 28.5
Ni 6.8 Cu 1.7 Cr 12.9
- 73 -
Table 6. The maximum current densities and the crowding ratios at different cross-sections for the five models.
- 74 -
Publication list
1. T. L. Shao, K.C. Lin, and Chih Chen, Electromigration Studies of Sn95/Sb5 Flip Chip Solder Bumps, Journal of Electronic Materials, 32, 11, 1278 (2003).
NSC-90-2216-E-009-042, SCI
2. T. L. Shao, Y. H. Chen, S. H. Chiu and Chih Chen, Electromigration Failure Mechanisms for SnAg3.5 Solder Bumps on Ti/Cr-Cu/Cu and Ni(P)/Au Metallization Pads, Journal of Applied Physics, Vol. 96, 8, 4518 (2004).
NSC-90-2216-E-009-042, SCI.
3. T. L. Shao, S. H. Chiu, Chih Chen, D.J. Yao, and C.Y. Hsu, Thermal Gradient in Solder Joints under Electrical Current Stressing, Journal of Electronic Materials, 33 (11): 1350-1354, 2004.
NSC-90-2216-E-009-042, SCI.
4. T. L. Shao, T. S. Chen, Y. M. Huang, and Chih Chen, Cross Interactions on
Interfacial Reactions of Solder Bumps and Metallization Layers during Reflow, Journal of Materials Research, Vol. 19, No. 12, p.3654 (2004).
NSC-90-2216-E-009-042, SCI.
5. T. L. Shao, Shih-Wei Liang, T.C. Lin, and Chih Chen, “3-D Simulation on Current Density Distribution in Flip Chip Solder Joint under Electrical Current
Stressing”, to be published in the September, 2005 issue J. Appl. Phys.
NSC 92-2216-E-009-008. SCI.
6. Y. C. Hsu, T. L. Shao, C. J. Yang, and Chih Chen, Electromigration study in SnAg3.8Cu0.7 Solder Joints on Ti/Cr-Cu/Cu Under Bump Metallization, Journal of Electronic Materials, 32, 11, 1222 (2003).
NSC-90-2216-E-009-042, SCI
7. C. M. Lu, T. L. Shao, C. J. Yang, and Chih Chen, Microstructure Evolution
- 75 -
during Electromigration in Eutectic SnPb Solder Bumps, Journal of Materials Research, Vol. 19, No. 8, 2394 (2004).
NSC-90-2216-E-009-042, SCI.
8. Y. H. Chen,T. L. Shao, P.C. Liu, Chih Chen, and T.. Chou, ”Microstructure Evolution during Electromigration in Eutectic SnPb Solder Bumps “, to be published in the September 2005 issue of J. Mater. Res..
9. S. H. Chiu, T. L. Shao, Chih Chen, D. J. Yao, and C. Y. Hsu, Infrared
Microscopy of Hot Spots Induced by Joule Heating in Flip-chip SnAg Solder Joints under Accelerated Electromigration, accepted by Appl. Phys. Lett. With major revisions 08/01/05.
NSC 92-2216-E-009-008. SCI.