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Title: Reliability studies of flip-chip and ball-grid-array solder joints under current stressing
Other Titles: Dao zhuang jing pian he qiu shan zhen lie han dian zai dian liu zai he xia de ke kao xing fen xi
Authors: Wu, Boyi (鄔博義)
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Flip chip technology.
Ball grid array technology.
Solder and soldering -- Reliability.
Electric currents.
Notes: xx, 141 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2007.
Includes bibliographical references (leaves 129-137)
CityU Call Number: TK7870.17 .W83 2007
Type: thesis
Abstract: With the development trends of electronics towards miniaturization, hash working condition and Pb-free application, the electrical and mechanical integrity of solder joints are of great importance. This thesis presents a study of the electrical, thermal, and mechanical behaviour of flip-chip (FC) and ball-grid-array (BGA) solder joints. An electrical-thermal-mechanical analysis of Sn3.5Ag1Cu (SAC) FC half bumps has been performed with an average current density of 3.67 × 10 8 A m -2 in the bumps at 75 °C. Current crowding occurs at the Al-to-solder entrance corner, where the maximum current density is 5.76 × 10 9 A m -2. The aluminum trace is a major heating source responsible for a peak temperature of 138 °C and a mean thermal gradient of 1.3 × 10 4 K m -1. Stress concentration locates around the Al-to-solder interface, with a maximum stress of 138.0 MPa and a stress gradient of 1.67 × 10 13 Pa m -1 in the Ni + (Ni,Cu)3Sn4. At the failure site, the driving force on nickel for electromigration (EM), thermomigration (TM) and stressmigration (SM) is 2.4 × 10 -16 N, 1.2 × 10 -18 N and 1.8 × 10 -16 N, respectively. The microstructural evolution has been examined, and intermetallic compound (IMC) dissolution, void formation, hillock accumulation, whisker growth and crack propagation are identified as major failure modes. The drift velocity of tin for voiding is 0.13 Å s -1; and the growth rate of Cu6Sn5 whiskers is 0.12 Å s -1. The failure mechanism is clarified to be a serious atomic migration due to EM and SM, rather than TM. The degradation modes and mechanisms of SAC FC full bumps under 1.83 × 10 8 A m -2 have been investigated by thermal modelling, temperature mapping, microstructure examining and strength testing. At 20 °C, the maximum temperature is 211.0 °C, with a thermal gradient of 7.7 × 10 4 K m -1 and 4.6 × 10 4 K m -1 in corner and centre solder joint, respectively. After 288 h, the degradation modes are void formation, crack propagation at the cathode Al-to-solder interfaces, and IMC growth at the solder-to-pad. The Cu6Sn5 is about 15.2 μm thick, to which diffusion has a contribution of 4.4 μm; and EM and SM contribute to the else. The solder joints degrade by roughly 50% in strength, with a fracture mode of Al-to-solder separation. At -5 °C, the microstructures and mechanical behaviour do not change with the time, which suggests that migration and diffusion do not occur significantly. The degradation mechanism is proposed as: substantial increase in temperature → accelerated EM, SM and diffusion → serious void formation, crack propagation and interfacial reaction → strength weakening. The effect of a low-density current on the reliability of Sn37Pb (SP) and SAC BGA solder joints on Au/Ni/Cu pads has been studied. For the solder joints between FR4 substrates under 9.0 × 10 2 A cm -2 at 125 °C, the maximum temperature reaches 160 °C; and the maximum current density, thermal gradient and stress gradient in Y direction is 4.1 × 10 7 A m -2, 1.6 × 10 3 K m -1 and 1.1 × 10 11 Pa m-1, respectively. The driving force on nickel for EM, TM and SM in the SP is 4.2 × 10 -18 N, 1.3 × 10 -19 N and 1.2 × 10 -18 N, respectively. Hence, no significant migration-induced failure occurs. However, a microstructural degradation in the form of extensive phase coarsening and slight interfacial reaction is found. The IMC growth at the solder-to-Ni interface exhibits a sub-parabolic manner. After 600 h, the solder joints fail at the solder-to-Ni interface, with a moderate decrease in strength, as a result of thermal stresses (maximum 14.3 MPa in the SP), and mechanical stresses. For the solder joints between flexible substrates with narrow and thin traces, the heat dissipation is poor, and the effect of Joule heating is prominent. Under 6.0 × 10 2 A cm -2 at 20 °C, a liquid-state-reaction mechanism is proposed for the failure with intensive pad dissolution, IMC growth, and the presence of dendritic IMC in the solder joints. The effect of a low-density current of 9.0 × 10 2 A cm -2 on the phase coarsening and IMC growth in BGA solder joints between OSP/Cu pads has been quantitatively investigated through current stressing at -5 °C and 125 °C, and aging at 125°C. For these three cases, the maximum temperature is identified to be about 29 °C, 125 °C and 160 °C; and correspondingly, the coarsening of Pb-rich phases and Ag-rich phases are identified to be slow, moderate and significant. The dominant controlling kinetics changes from grain boundary diffusion towards volume diffusion with the increase in temperature. The IMC growth at the solder-to-Cu interfaces depends substantially on the temperature, as does the coarsening process. It is believed that copper is the dominant diffusion species at low temperature; whilst tin becomes the main diffusing species at high temperature. The degradation mechanism here is proposed to be: moderate increase in temperature → enhanced diffusion → accelerated phase coarsening and interfacial reaction.
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