Browsing by Author "Shao, TL"

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3-D simulation on current density distribution in flip-cbip solder joints with thick CuUBM under current stressing
(2005)
Liang, SW
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Shao, TL
;
Chen, C
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材料科學與工程學系
;
Department of Materials Science and Engineering
In flip-chip solder joints, Cu has been used as a under-bump metallization (UBM) for its excellent wettability with solders. In addition, electromigration has become an crucial reliability concerns for fine-pitch flip chip solder joints. In this paper, 3-D finite element method was employed to simulate the current density distribution for the eutectic SnPb solder joints with 5 mu m, 10 mu m, and 20 mu m thick Cu UBM. It was found that the thicker the UBM is, the lower the maximum current density inside the solder. The maximum current density decreased from 4.37 X. 10(4) A/cm(2) to 7.54 x 10(3) A/cm(2) when the thickness of the UBM changed from 5 gm to 20 mu m. Thicker Cu UBM can, effectively relieve the current crowding effect inside the solder.
Cross interactions on interfacial compound formation of solder bumps and metallization layers during reflow
(2004-12-01)
Shao, TL
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Chen, TS
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Huang, YM
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Chen, C
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材料科學與工程學系
While the dimension of solder bumps keeps shrinking to meet higher performance requirements, the formation of interfacial compounds may be affected more profoundly by the other side of metallization layer due to a smaller bump height. In this study, cross interactions on the formation of intermetallic compounds (IMCs) were investigated in eutectic SnPb, SnAg3.5, SnAg3.8Cu0.7, and SnSb5 solders jointed to Cu/Cr-Cu/Ti on the chip side and Au/Ni metallization on the substrate side. It is found that the Cu atoms on the chip side diffused to the substrate side to form (Cu-x,Ni1-x)(6)Sn-5 or (Ni-y,Cu1-y)(3)Sn-4 for the four solders during the reflow for joining flip chip packages. For the SnPb solder, Au atoms were observed on the chip side after the reflow, yet few Ni atoms were detected on the chip side. In addition, for SnAg3.5 and SnSn5 solders, the Ni atoms on the substrate side migrated to the chip side during the reflow to change binary Cu6Sn5 into ternary (Cu-x,Ni1-x)(6)Sn-5 IMCs, in which the Ni weighed approximately 21%. Furthermore, it is intriguing that no Ni atoms were detected on the chip side of the SnAg3.8Cu0.7 joint. The possible driving forces responsible for the diffusion of Au, Ni, and Cu atoms are discussed in this paper.
Electromigration failure mechanism of Sn96.5Ag3.5 flip-chip solder bumps
(2004)
Shao, TL
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Chen, IH
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Chen, C
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材料科學與工程學系
;
Department of Materials Science and Engineering
The electromigration-induced failure of SnAg3.5 flip chip solder bumps was investigated at the current density of I x 10(4) A/cm(2) at 150 degreesC, and they failed after 22 hours of current stressing. Failure was found to be on anode/chip side, and large (Cu,Ni)(6)Sn-5 intermetallic compounds (IMCs) were observed on the interface of the UBM and the solder bump. A novel electromigration failure mechanism is proposed. Nickel atoms were migrated by electron flow from substrate side to chip side to form (Cu,Ni)(6)Sn-5 IMCs. The volume expansion due to the IMC formation may induce local high stress around the UBM and fail the contact of the anode/chip side.
Electromigration failure mechanisms for SnAg3.5 solder bumps on Ti/Cr-Cu/Cu and Ni(P)/Au metallization pads
(2004-10-15)
Shao, TL
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Chen, YH
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Chiu, SH
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Chen, C
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材料科學與工程學系
The electromigration behavior of SnAg3.5 solder bumps is investigated under the current densities of 1x10(4) A/cm(2) and 5x10(3) A/cm(2) at 150degreesC. Different failure modes were observed for the two stressing conditions. When stressed at 1x10(4) A/cm(2), damage occurred in both the anode/chip side and the cathode/chip side. However, failure happened only in the cathode/chip side under the stressing of 5x10(3)A/cm(2). A three-dimensional simulation of the current-density distribution was performed to provide a better understanding of the current-crowding behavior in the solder bump. The current-crowding effect was found to account for the failure in the cathode/chip side. In addition, both the temperature increase and the thermal gradients were measured during the two stressing conditions. The measured temperature increase due to Joule heating was as high as 54.5degreesC, and the thermal gradient reached 365degreesC/cm when stressed by 1x10(4)A/cm(2). This induced thermal gradient may cause atoms to migrate from the chip side to the substrate side, contributing to the failure in the anode/chip side. Moreover, the formation of intermetallic compounds in the anode/chip side may also be responsible for the failure in the anode/chip side. (C) 2004 American Institute of Physics.
Electromigration induced failure in SnAg3.8Cu0.7 solder joints for flip chip technology
(2002)
Hsu, YC
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Shao, TL
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Chen, C
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材料科學與工程學系
;
Department of Materials Science and Engineering
Electromigration of SnAg3.8Cu0.7 Solder was investigated in flip chip solder bump. An under-bump metallization (UBM) of Cr/Cr-Cu/Cu tri-layer was deposited on the chip side and electroless Cu/Ni/Au pad was deposited on the BT board side. Electromigration damage was observed under the current density of 2 x 10(4) A/cm(2) at 100 degreesC. Voids were found at cathode side and crack was observed at solder/thin film UBM interface after current stressing, and the bump failed after 168-hour stressing. Copper atoms were found to move in the direction of electron flow. Intermetallic compounds of Cu-Sn an Ni-Cu-Sn were also observed to spread into the solder bump due to current stressing.
Electromigration studies of flip chip Sn95/Sb5 solder bumps on Cr/Cr-Cu/Cu under-bump metallization
(2003-11-01)
Shao, TL
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Lin, KC
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Chen, C
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材料科學與工程學系
;
Department of Materials Science and Engineering
The electromigration-induced failure of Sn95/Sb5 flip chip solder bumps was investigated. The failure of the joints was found at the cathode/chip side after current stressing with a density of 1 X 10(4) A/cm(2) at 150degreesC for 13 sec. The growth of intermetallic compounds (IMCs) was observed at the anode side after current stressing. Voids were found near the current crowding area in the cathode/chip side, and the (Cu,Ni)(6)Sn-5 IMC at the cathode/chip end was transformed into the Sn phase. The failure mechanism for Sn95/Sb5 flip chip solder joint is proposed in this paper.
Electromigration study in SnAg3.8Cu0.7 solder joints on Ti/Cr-Cu/Cu under-bump metallization
(2003-11-01)
Hsu, YC
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Shao, TL
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Yang, CJ
;
Chen, C
;
材料科學與工程學系
This paper investigates the electromigration-induced failures of SnAg3.8Cu0.7 flip-chip solder joints. An under-bump metallization (UBM) of a Ti/Cr-Cu/Cu trilayer was deposited on the chip side, and a Cu/Ni(P)/Au pad was deposited on the BT board side. Electromigration damages were observed in the bumps under a current density of 2 x 10(4) A/cm(2) and 1 x 10(4) A/cm(2) at 100 degreesC and 150 degreesC. The failures were found to be at the cathode/chip side, and the current crowding effect played an important role in the failures. Copper atoms were found to move in the direction of the electron flow to form intermetallic compounds (IMCs) at the interface of solder and pad metallization as a result of current stressing.
Infrared microscopy of hot spots induced by Joule heating in flip-chip SnAg solder joints under accelerated electromigration
(2006-01-09)
Chiu, SH
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Shao, TL
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Chen, C
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Yao, DJ
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Hsu, CY
Joule heating effect in solder joints was investigated using thermal infrared microscopy and modeling in this study. With the increase of applied current, the temperature increased rapidly due to Joule heating. Furthermore, modeling results indicated that a hot spot existed in the solder near the entrance point of the Al trace, and it became more pronounced as the applied current increased. The temperature difference between the hot spot and the solder was as large as 9.4 degrees C when the solder joint was powered by 0.8 A. This hot spot may play an important role in the initial void formation during electromigration.
Microstructural evolution during electromigration in eutectic SnAg solder bumps
(2005-09-01)
Chen, YH
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Shao, TL
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Liu, PC
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Chen, C
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Chou, T
Microstructural changes induced, by electromigration were studied in eutectic SnAg solder bumps jointed to under-bump metallization (UBM) of Ti/Cr-Cu/Cu and pad metallization of Cu/Ni/Au. Intermetallic compounds (IMCs) and phase transformations were observed during a current stress of 1 X 104 A/cm(2) at 150 degrees C. On the cathode/ substrate side, some of the (Cu-y,Ni1-y)(6)Sn-5 transformed into (Ni-x,Cu1-x)(3)Sn-4 due to depletion of Cu atoms caused by the electron flow. It is found that both the cathode/ chip and anode/chip ends could be failure sites. On the cathode/chip side, the UBM dissolved after current stressing for 22 h, and failure may occur due to depletion of solder. On the anode/chip side, a large amount of (Cu-y,Ni1-y)(6)Sn-5 or (Ni-x,Cu1-x)(3)Sn-4 IMCs grew at the low-current-density area due to the migration of Ni and Cu atoms from the substrate side, which may be responsible for the electromigration failure at this end.
Microstructure evolution during electromigration in eutectic SnPb solder bumps
(2004-08-01)
Lu, CM
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Shao, TL
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Yang, CJ
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Chen, C
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材料科學與工程學系
A technique has been developed to facilitate analysis of the microstructural evolution of solder bumps after current stressing. Eutectic SnPb solders were connected to under-bump metallization (UBM) of Ti/Cr-Cu/Cu and pad metallization of Cu/Ni/Au. It was found that the Cu6Sn5 compounds on the cathode/chip side dissolved after the current stressing by 5 x 10(3) A/cm(2) at 150 degreesC for 218 h. However, on the anode/chip side, they were transformed into (NixCu1-x)(3)Sn-4 in the center region of the UBM, and they were converted into (Cu-y,Ni1-y)(6)Sn-5 on the periphery of the UBM. For both cathode/substrate and anode/substrate ends, (Cu-y,Ni1-y)(6)Sn-5 compounds were transformed into (Ni-x,Cu1-x)(3)Sn-4. In addition, the bumps failed at cathode/chip end due to serious damage of the UBM and the Al pad. A failure mechanism induced by electromigration is proposed in this paper.
Relieving the current crowding effect in flip-chip solder joints during current stressing
(2006-01-01)
Liang, SW
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Shao, TL
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Chen, C
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Yeh, ECC
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Tu, KN
Three-dimensional simulations for relieving the current crowding effect in solder joints under current stressing were carried out using the finite element method. Three possible approaches were examined in this study, including varying the size of the passivation opening, increasing the thickness of Cu underbump metallization (UBM), and adopting or inserting a thin highly resistive UBM layer. It was found that the current crowding effect in the solder bump could be successfully relieved with the thick Cu UBM or with the highly resistive UBM. Compared to the solder joint with Al/Ni(V)/Cu UBM, for instance, the maximum current density in a solder bump decreased dramatically by a factor of fifteen, say from 1.11 x 10(5) A/cm(2) to 7.54 x 10(3) A/cm(2) when a 20-pm-thick Cu UBM was used. It could be lowered by a factor of seven, say to 1.55 x 10(4) A/cm(2), when a 0.7-mu m UBM of 14770 mu Omega cm was adopted. It is worth noting that although a resistive UBM layer was used, the penalty on overall resistance increase was negligible because the total resistance was dominated by the Al trace instead of the solder bump. Thermal simulation showed that the average temperature increase due to Joule heating effect was only 2.8 degrees C when the solder joints with UBM of 14770 mu Omega cm were applied by 0.2 A.
Thermal gradient in solder joints under electrical-current stressing
(2004-11-01)
Shao, TL
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Chiu, SH
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Chen, C
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Yao, DJ
;
Hsu, CY
The thermal gradient and temperature increase in SnAg3.5 solder joints under electrical-current stressing have been investigated by thermal infrared microscopy. Both positive and negative thermal gradients were observed under different stressing conditions. The magnitude of the thermal gradient increases with the applied current. The measured thermal gradients reached 365degreesC/cm as powered by 0.59 A, yet no obvious thermal gradient was observed when the joints were powered less than 0.25 A. The temperature increase caused by joule heating was as high as 54.5degreesC when powered by 0.59 A, yet only 3.7degreesC when stressed by 0.19 A. The location of heat generation and path of heat dissipation are believed to play crucial roles in the thermal gradient. When the major heat source is the Al trace, the thermal gradient in the solder bumps is positive; but it may become negative because the heat generated in the solder itself is more prominent.