| 摘要(英) |
The effect of Ni and Cu additive on electromigration(EM) in Sn solder joints and lines have been studied. Both EM test samples were stressed under the current density of 104 A/cm2. The electromigration behaviors in Sn0.7Ni solder joints were investigated at 155℃, 180℃, 200℃, respectively. We found that the Cu additives were transported to the anode side and formed Cu-Sn intermetallic compound. For Ni doped Sn solder joint, a thin layer of intermetallic compound (IMC) formed at the interface which was identified to be the Ni-Cu-Sn ternary compound phase. Under EM test, we found that for the pure Sn solder, the interfacial Cu-Sn compound layer at the cathode side will dissolve into Sn. However, for the Sn0.7Ni solder joint, remarkablely, we found that the Ni-Cu-Sn ternary compound layer is more stable than the Cu-Sn compound layer under EM test. Another intriguing finding is that a line of voids (Kirkendall voids) occurred at the interface between IMC and Cu pad. From the FIB examination, the Kirkendall voids, which at the anode side is more serious than that at the cathode side. Depending on the direction of electron flow, two cracking failure modes were observed. One is on the anode side, the cracking was occur at the IMC / Cu interface, the other is on the cathode side, the cracking was occur at the IMC / solder. Comparing the activation energy between Sn0.7Ni alloy (Q= 0.85 eV/ atom ) and Sn3Ni alloy (Q= 1.13 eV/ atom ), we know that, under the EM current stressing, the Sn3Ni has the highest activation energy than Sn0.7Ni, therefore the Cu consumption rate of Sn3Ni alloy is the slowest. The reason for Sn3Ni alloy has the slowest Cu dissolution rate than Sn0.7Ni than pure Sn (Q= 0.68 eV/ atom, reference data from Ellen Ge). The reason for the lower Cu consumption rate is mainly due to the formation of (Cu1-yNiy)6Sn5 can induced a lower Cu consumption rate[33]. The first reason is that (Cu1-yNiy)6Sn5 has a lower growth rate and is an effective diffusion barrier under the EM stressing. Otherwise, the (Cu1-yNiy)6Sn5 also is much stable under the EM stressing, as aforementioned. The second reason is that, from the growth kinetic of data for the IMC, as seen the Fig 5.2~5.5, we found that the thickness of IMC of Sn3Ni alloy is much smaller than the thickness of IMC of Sn0.7Ni alloy. Considering the mass balance of Cu, the amount of Cu consumed is equal to the amount of Cu dissolved into the solder, and incorporated with the IMC compounds. The formation IMC thickness of Sn3Ni alloy is less than the IMC thickness of Sn0.7Ni alloy, therefore, the Cu consumption rate in the Sn3Ni alloys is the slowest than in Sn0.7Ni and even in pure Sn under the EM current stressing.
Using solder lines structure, we have studied the electromigration phenomenon on Sn (Cu) alloys and Sn (Ni) alloys lines, which are Sn, Sn0.7Cu, Sn3Cu, Sn0.7Ni, Sn3Ni. Two EM test temperature were at the room temperature and 60℃. The samples were stressed under the current density of 104 A/cm2. Mass accumulation near the anode and void nucleation near the cathode were observed during the current stressing. The electromigration rates of the above alloys were determined by knowing the extrusion where at the anode side. Our results show that the magnitude were Sn0.7Cu＞Sn＞Sn3Cu＞Sn0.7Ni＞Sn3Ni. We known that Sn0.7Ni alloy and Sn3Ni alloy shows much higher electromigration resistance than Sn (Cu) alloys. Furthermore, the grain boundary diffusion is known to be the main kinetic path of atomic transport in EM. From the Fig 5.12, we know that the Sn (Ni) alloy has a larger grain size than Sn (Cu) alloy, compare from the Table III, we know that the grain size of Sn (Cu) alloys just 3~10μm, however, the grain size of Sn (Ni) alloy is almost 20μm, it’s means that the Sn (Ni) alloy has the less grain boundary density and the electromigration’s diffusion path is decrease. Therefore, the Sn (Ni) alloys can retard the electromigration than the Sn(Cu) alloys. However, a more detail study is needed to verify this suggestion. Otherwise, we can estimate the accumulation region, the atomic flux driven by the electromigration can be calculated. Then the effective charge number and DZ* value of the different solder alloy lines were obtained. The DZ* values agree with the literature data. |
| 論文目次 |
Table of Contents Pages
Chinese Abstract……………………………………………… i
Abstract…………………………………………………………… iii
List of Figures…………………………………………………… viii
List of Tables…………………………………………………… xi
CHAPTER 1. Introduction…………………………………………………… 1
CHAPTER 2. Literature Review…………………………………………… 3
2.1 Electromigration……………………………………………………… 3
2.1.1 Introduction of electromigration…………………………… 3
2.1.2 Driving force of electromigration……………………… 4
2.1.3 Measurement of electromigration……………………… 7
2.1.4 Mean-time-to-failure(MTTF) of electromigration………… 9
2.2 Electromigration in SnPb solder stripes, lines and joints… 10
2.2.1 Electromigration in SnPb solder joints………………… 10
2.2.2 Electromigration in SnPb solder stripes………………... 15
2.3 Electromigration failure on flip chip solder joints due to rapid dissolution of copper…………………………………………………… 16
CHAPTER 3. Experiments…………………………………………………… 17
3.1 Pb-free solders preparation……………………………………… 18
3.2 Samples preparation…………………………………………. 19
3.2.1 Solder joints structure preparation…………………… 19
3.2.2 Solder lines structure preparation……………………… 21
CHAPTER 4. Results………………………………………………………… 23
4.1 Electromigration in Sn0.7Ni solder joints…………………… 23
4.2 Electromigration in the solder lines………………………… 32
CHAPTER 5. Discussions…………………………………………………… 42
5.1 Electromigration in Sn0.7Ni solder joints…………………… 42
5.1.1. Cracking mode at different interface………………………… 42
5.1.2. Enhanced growth of IMC……………………………………… 43
5.1.3. The activation energy of Sn0.7Ni and Sn3Ni alloys… 48
5.2 Electromigration in solder lines………………………… 54
5.2.1. The measurement of DZ* value ………………………………… 54
5.2.2. Alloying effect…………………………………………………… 55
CHAPTER 6. Conclusion…………………………………………………… 60
References…………………………………………………………………… 63
List of Figures Pages
Fig.2.1 A sketch of the cross-sectional view of morphology changes after electromigration effect. Figure is taken from reference [7]. 4
Fig.2.2 A schematic picture of the applied force of electromigration… 5
Fig.2.3 A sketch of the diffusion of the shaded Al atom to a neighboring vacancy. (a) before diffusion and (b) halfway during diffusion. Figure is taken from reference [7 ]…………………………………………………… 6
Fig.2.4 The isothermal isotope method for measure DZ* and D. Figure is taken from reference [7]…………………………………………………………………… 8
Fig.2.5 The SEM images of the cross-sectioned solder ball (a) before current stressing, (b) after 70 hrs current stressing, (c) after 324 hrs current stressing. Figure is taken from reference [3]………………………… 11
Fig.2.6 Optical microscopic image of the segregation in eutectic Sn-Pb solder bump. Figure is taken from reference [8]………………………………… 12
Fig.2.7 A Schematic diagram of void propagation along the cathode interface. Figure is taken from reference [9] ………………………………………… 13
Fig.2.8 The SEM images of the void formation and propagation after applying current (a) 38 hrs, (b) 40 hrs, and (c) 43 hrs. Figure is taken from reference [9]………………………………………………………………………… 14
Fig.2.9 (a) A schematic picture of the eutectic Sn-Pb solder thin strip sample. (b) The scanning electron microscopic image of an eutectic Sn-Pb solder stressed by a direct electrical current density of 105 A/cm2 at room temperature for 19 days. Figure is taken from reference [6]…………………………………………………………………………………………… 15
Fig.2.10 Fig 2.10 Microstructure evolution in the No.2 solder, we can find the Cu dissolution after 45minutes. Figure is taken from reference [11 ] 16
Fig.3.1 A flowchart of experiment paragraph…………………… 17
Fig.3.2 A schematic diagram of solder joint preparation………… 20
Fig.3.3 A schematic SEM image of a finished solder joint………… 20
Fig.3.4 A schematic diagram of solder joints preparation……………… 22
Fig.3.5 A schematic SEM image of a finished solder line…………… 22
Fig.4.1 A schematic SEM image of Sn0.7Ni solder at the (a)cathode and (b)anode interface after stressing time of 10hours, 20hours, 30hours, 40hours at temperature of 155℃……………………………………………………………… 25
Fig.4.2 A schematic FIB diagram of Sn0.7Ni solder at the (a)cathode interface and (b)anode interface, respectively……………………………………… 26
Fig.4.3 A schematic SEM images of Sn0.7Ni solder on the (a) cathode interface and (b) anode interface on the temperature 180℃ after stressing time of 10hours, 20hours, and 30hours, respectively……………………………… 28
Fig.4.4 A schematic of Pure Sn SEM image at the (a) cathode interface and (b) anode interface after stressing time of 1hours, 2hours, 3hours, 4hours, respectively.[Reference from Ellen Ge]……………………………………… 30
Fig.4.5 A schematic of Sn0.7Ni solder SEM image at the (a) cathode interface and (b) anode interface after stressing time of 1hours, 2hours, 3hours, 4hours, respectively…………………………………………………………… 31
Fig.4.6 The evolution SEM image of Pure Sn, Sn0.7Cu, and Sn3Cu alloys after stressing time of 689hours.…………………………………………………… 35
Fig.4.7 A schematic diagram of Sn0.7Cu which length of 450 µm, was stressed by the current density of 9 x10 A/cm2 for 297 hours.…………………… 36
Fig.4.8 A schematic diagram of Sn0.7Ni, which length of 450 µm, was stressed by the current density of 9 x10 A/cm2 for 297 hours.…………………… 37
Fig.4.9 A schematic diagram of Sn3Cu solder line which was after stressing time of 209 hours ……………………………………………………………… 38
Fig.4.10 A schematic diagram of Sn3Ni solder line which was after stressing time of 209 hours……………………………………………………………… 39
Fig.4.11 The evolution SEM image of Sn0.7Ni alloys line after stressing time of 62hours……………………………………………………………………… 40
Fig.4.12 The enlarge SEM image of the anode interface of Sn0.7Ni alloys after stressing time of 43hours and 62hours……………………………… 41
Fig.5.1 A schematic diagram of the mechanism of cracking mode at different interface……………………………………………………………………………… 45
Fig.5.2 Plot the IMC thickness vs. current stressing time at the anode interface of the Sn0.7Ni alloy.…………………………………………………… 46
Fig.5.3 Plot the IMC thickness vs. current stressing time at the cathode interface of the Sn0.7Ni alloy.…………………………………………………… 46
Fig.5.4 Plot the IMC thickness vs. current stressing time at the anode interface of the Sn3Ni alloy.…………………………………………………… 47
Fig.5.5 Plot the IMC thickness vs. current stressing time at the cathode interface of the Sn3Ni alloy.…………………………………………………… 47
Fig.5.6 Plot of the squares of the Cu consumed thickness versus time on the annealing case for Sn0.7Ni alloy.………………………………………… 51
Fig.5.7 ln D vs. 1 / kT for the Cu dissolution to determine the activation energy for annealing case for Sn0.7Ni alloy. ……………………………………51
Fig.5.8 Plot the thickness of the Cu consumed versus time on the EM current stressing case for Sn0.7Ni.…………………………………………………… 52
Fig.5.9 ln D vs. 1 / kT for the Cu dissolution to determine the activation energy for EM current stressing case for Sn0.7Ni alloy.…………………… 52
Fig.5.10 Plot the thickness of the Cu consumed versus time on the EM current stressing case for Sn3Ni.…………………………………………………… 53
Fig.5.11 ln D vs. 1 / kT for the Cu dissolution to determine the activation energy for EM current stressing case for Sn3Ni alloy. ………………………53
Fig.5.12 The microstructure of Sn (Ni) alloy. ……………………………… 59
List of Tables Pages
Table.I Comparison of DZ* values between the different alloys. 58
Table.II Comparison of DZ* values with the literature data 58
Table.III Property of Sn (Cu) alloy [14] 59 |
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