Showing papers by "Zhong Chen published in 2003"

Fracture toughness of Cu-Sn intermetallic thin films

Abstract: Intermetallic compounds (IMCs) are formed as a result of interaction between solder and metallization to form joints in electronic packaging. These joints provide mechanical and electrical contacts between components. The knowledge of fracture strength of the IMCs will facilitate predicting the overall joint property, as it is more disposed to failure at the joint compared to the solder because of its brittle characteristics. The salient feature of this paper is the measurement of the fracture toughness and the critical energy-release rate of Cu3Sn and Cu6Sn5 intermetallic thin films, which is the result of the interaction between Sn from the solder and Cu from the metallization. To achieve the objective, a controlled buckling test was used. A buckling test in the current work refers to one that displays large transverse displacement caused by axial compressive loading on a slender beam. The stress and strain along the beam can be easily calculated by the applied displacement. Fracture-toughness values of Cu3Sn and Cu6Sn5 are 2.85 MPa √m ± 0.17 MPa √m and 2.36 MPa √m ± 0.15 MPa √m, respectively. Corresponding critical energy-release rate values are 65.5 J/m2 ± 8.0 J/m2 and 55.9 J/m2 ± 7.3 J/m2, respectively. The values obtained were much higher than the ones measured in bulk intermetallic samples but correlated well with those values obtained from conventional fracture-toughness specimens when fracture was confined within the intermetallic layers. Hence, the controlled buckling test is a promising fast and effective way to elucidate mechanical properties of thin films.

Effect of plating parameters on the intrinsic stress in electroless nickel plating

Abstract: With the increasing application of electroless nickel (EN) as a bumping or under-bumping metalization material, the microelectronic packaging engineers are required to have a deeper understanding and control of intrinsic stress in EN plating. The purpose of this work is to investigate the effect of some of the most important operating parameters on the formation of intrinsic stress during plating. The analysis of variances method was applied to obtain both single effects and compound effects of selected factors. Results indicate that high pH value and aged solution affect the intrinsic stress significantly, while surface roughness does not have much influence on the intrinsic stress for the range investigated. Aging at 190 °C for 170 h changes neither the Ni–P structure nor the intrinsic stress to a significant degree. Further investigation indicated that the intrinsic stress was probably related to the Ni–P microstructure as a result of the deposition rate. It was found that the higher the deposition rate, the higher the intrinsic stress.

Theoretical study on 19F magnetic shielding constants of some metal fluorides

Abstract: A study is presented of 19F NMR magnetic shielding constants of MF2 (M = Zn, Cd), β-PbF2, MF3 (M = Al, Ga, In), AMF3 (A = K, Rb, Cs, Ba; M = Mg, Ca, Li, Zn, Cd), BaMgF4, BaZnF4 and Ba2ZnF6 using the DFT/GIAO method. The aug-cc-pVTZ basis set was used for the fluorine atom under investigation and LanL2DZ for the remaining fluorine atoms. The 3–21G(2d) basis set was used for the aluminum atom and CRENBL for the other metal atoms. When appropriate cluster models were employed, the theoretical results obtained from the B3LYP/GIAO calculations are in good agreement with experimental measurements and may be better than those obtained from empirical calculations. The correlation coefficient and the slope of the fitting line between our theoretical predictions and the experimental observations are close to unity. Copyright © 2003 John Wiley & Sons, Ltd.

Prediction of the energy dissipation rate in ductile crack propagation

Abstract: In this paper, energy dissipation rate D vs. Δa curves in ductile fracture are predicted using a ‘conversion’ between loads, load-point displacements and crack lengths predicted by NLEFM and those found in real ELPL propagation. The NLEFM/ELPL link was recently discovered for the DCB testpiece, and we believe it applies to other cracked geometries. The predictions for D agree with experimental results. The model permits a crack tip toughness R(Δa) which rises from Jc and saturates out when (if) steady state propagation is reached after a transient stage in which all tunnelling, crack tip necking and shear lip formation is established. JR is always greater than the crack tip R(Δa) and continues to rise even after R(Δa) levels off. The analysis is capable of predicting the usual D vs. Δa curves in the literature which have high initial values and fall monotonically to a plateau at large Δa. It also predicts that D curves for CCT testpieces should be higher than those for SENB/CT, as found in practice. The possibility that D curves at some intermediate Δa may dip to a minimum below the levelled-off value at large Δa is predicted and confirmed by experiment. Recently reported D curves that have smaller initial D than the D-values after extensive propagation can also be predicted. The testpiece geometry and crack tip R(Δa) conditions required to produce these different-shaped D vs. Δa curves are established and confirmed by comparison with experiment. The energy dissipation rate D vs. Δa is not a transferable property as it depends on geometry. The material characteristic R(Δa) may be the ‘transferable property’ for scaling problems in ELPL fracture. How it can be deduced from D vs. Δa curves (and by implication, JR vs. Δa curves) is established.

Fracture toughness of Cu-Sn intennetallic compounds in electronic packages

Abstract: Intermetallic compounds (IMC) in a solder joint play an important role in the joint reliability for electronic packages. Experimental observation shows there is a clear trend of intermetallic fracture when the layer thickness increases. However there has been so far not much work carried out on the direct measurement of IMC fracture toughness in the thinfilm form. The main reason is that the thickness of these intermetallic compounds at solder joints is only of the order of micra, which makes the direct measurement very difficult. Conventional shear test or butt pull test on a solder joint could not reveal the fracture strength of intermetallics because the fracture path is often very complicated involving more than one fracture mechanism. The current work employed a new testing approach of directly measuring the fracture strain and fracture toughness of around one micron thick Sn-Cu intermetallic compounds by controlled buckling test. Single phase Cu& or Cu’Sn was prepared by co-sputtering Cu and Sn of the right atomic ratio followed by appropriate annealing. The substrates used were polymeric thin plates. During experiment the thin film intermetallics fractured by channeling cracks at the maximum strain site. By monitoring the formation of channeling, critical strain at which the cracking initiates can be calculated by large displacement beam theory. Based on the critical fracture strain, the mode I fracture toughness of the intermetallic compounds can be calculated. It was found that the toughness of the two.types of thin film intermetallics between Cu and Sn is around 55-65 Jim'. This new approach is also applicable to the fracture toughness measurement of other types of brittle thin films.