J. Liang

Effects of strain rates and biaxial stress conditions on plastic yielding and flow stress of solder alloys

The rate-dependent mechanical properties of Sn3.8Ag0.7Cu (SAC387) Pb-free alloy and Sn-Pb eutectic alloy were investigated in this study under pure shearing and biaxial stress conditions with thin-walled specimens using a servo-controlled tension-torsion material testing system. The pure shearing tests were conducted at strain rates between 6.7 × 10−7 and 1.3 × 10−1/sec. In addition, axial tensile stresses were superimposed onto the shearing samples to examine the effects of biaxial stress conditions on the yielding and on post-yielding plastic flow of the solder alloys. Strain hardening is observed for the Pb-free alloy at all the tested strain rates, while strain softening happens with the Sn-Pb eutectic solder at low strain rates. Special tests were also conducted for sudden strain-rates changes and stress relaxation for the purpose to develop a viscoplastic model to simulate time-dependent multiaxial deformation and to assess damage and fatigue life of general solder interconnections.

Effects of thermal history on the intermetallic growth and mechanical strength of Pb‐free and Sn‐Pb BGA solder balls

Purpose – To investigate effects of the thermal history on intermetallic thickness and morphology and on the resulting shear strength of the ball attachment for a variety of BGA components.Design/methodology/approach – In this study, a variety of BGA components with balls made of Pb‐free Sn‐Ag‐Cu (SAC) 305, Sn‐Pb eutectic and high‐temperature 90Pb‐10Sn alloys, were subjected to different thermal histories, including up to ten reflow cycles, and aged at 125°C from 24 to 336u2009h. The intermetallic thickness and morphology after these thermal events were then examined under optical and scanning electronic microscopes. Ball shearing tests were conducted to investigate effects of the thermal history and intermetallic thickness and morphology on shearing strength of these solder balls.Findings – The results show that effects directly from intermetallic layers may or may not be detectable; and the shear strength of solder balls is largely dependent on the solder alloy and its microstructure. Shear strength increas...

Effects of multiple BGA rework on strength of solder joints

Purpose – The purpose of this paper is to investigate effects of the multiple rework of ball grid array (BGA) components on mechanical strength of BGA balls, as well as any possible intermetallic (IMC) embrittlement, and obtain data correlated with possible estimation on the maximum permitted limits of BGA rework.Design/methodology/approach – In this paper, mechanical strength of BGA components assemblies with multiple numbers of rework operations was evaluated. Mechanical evaluation was conducted using BGA ball shear tests and four‐point bending tests of BGA assemblies. Test samples were prepared under the following conditions: virgin, one, two, three and five BGA reworks. Failure mechanism was evaluated using cross‐section and SEM analysis.Findings – The results show that both ball shearing tests and four‐point bending tests indicates that strength of BGA solder ball itself was not reduced significantly after repair/rework operation from one to five cycles. The IMC structure layer after rework is a mixt...

Tin Whisker Nucleation and Growth on Sn-Pb Eutectic Coating Layer Inside Plated Through Holes With Press-Fit Pins

It has been long believed that residual stress is the root cause for tin whisker formation on pure tin-plated component leads. However, tin whisker formation could be observed on the surface of other tin-based alloys under certain conditions. In this study, the whisker formation was reported on a coating layer of Sn-Pb eutectic hot air solder leveling (HASL), which was under compression stress conditions due to the inserted compliant pins. In-Situ scanning electron microscopy was used to monitor the nucleation and growth of whiskers. In addition, a mechanical experiment and non-linear contact finite element analysis were used to estimate the magnitude of the stress in the HASL coating layer. It was found that the tin whisker formation with whisker size of more than 10 mum could occur on the surface of 60Sn-60Pb plating within less than 30 min at an ambient temperature under compressing stress conditions. The tin whisker initiation and growth were further studied at an elevated temperature of 70 degC to check if a higher temperature effects Tin whisker formation. It is believed that establishment of a quantitative relationship of whisker formation/growth under compressive stress and elevated temperature conditions could lead to better scientific methods for risk and reliability assessment and smooth transitions to lead-free assemblies.

Deformation and Fatigue Fracture of Pb-Free Solder Alloys Under Complicated Load Conditions

Fundamental study of deformation and fatigue fracture behavior of solder alloys under complex load conditions is a key to enabling implementation of sophisticated 3-D time-dependent nonlinear FEM stress and strain analyses and life assessment for electronic packages and assemblies. In this study, the rate-dependent deformation and fatigue fracture behavior of Sn3.8Ag0.7Cu Pb-free alloy and Sn-Pb eutectic alloy was investigated with thin-walled specimens using a bi-axial servo-controlled tension-torsion material testing system, with solder alloys subjected to a variety of complex load conditions: pure shearing at strain rates between 6.7×10−7 /sec to 1.3×10−1 /sec, creep at temperatures ranging from room temperature up to 125 °C, and cyclic loading with frequency of 0.001Hz to 3Hz. Bi-axial stress conditions were imposed for shearing tests to investigate effects of multi-axial stresses on deformation behavior. The effects of frequency and temperature on cyclic deformation and fatigue facture were investigated for lead-free Sn3.8Ag0.7Cu and Sn-Pb eutectic solder. Fractography of fatigue tested samples were also conducted to determine possible fatigue failure mechanisms.Copyright

Mechanical Properties of Lead Free Solder Alloy Measured by Nanoindentation

The elimination of lead from electronics due to its detrimental effects to environment is pushing component manufacturers to consider lead free solder alloy as an option. Application of lead free solder alloy requires a better understanding of its mechanical behavior at elevated temperatures and at small volume size. Such information is still lacking. In this study, nanoindentation technique was used to investigate the deformation behavior of the near eutectic ternary SAC387 (Sn 95.5 wt%, Ag 3.8 wt% and Cu 0.7 wt%) lead free solder alloy. The mechanical properties of each constituent phase in SAC387 were carefully measured and analyzed. Indentation tests were performed with different holding times to study the creep effects on the mechanical properties measured by nanoindentation. Microhardness tests were also carried out at temperatures above ambient temperature to investigate its influence on the mechanical properties.Copyright

Tin Whisker Formation Kinetics on Sn-Pb HASL Coating Layer Inside Plated Through Holes With Press-Fit Complaint Pins

Summary form only given. It has been long believed that residual stress is the root cause of tin whisker formation on pure tin-plated component leads. However, tin whiskering also happens to a lesser degree on other tin-based alloys under certain conditions. In this study, the tin whisker phenomena were reported on Sn-Pb HASL coating layer inside plated through holes with press-fit complaint pins. Scanning electron microscopy (SEM) was used to monitor the nucleation and growth of whiskers in-situ. In additions, a nonlinear contact finite element analysis was carried out to theoretically calculate the stress/strain distribution on HASL coating inside PTH, which was under compression due to the inserted compliant pins. Experimental and theoretical calculation results show that the whisker initiation and growth to 20 micrometers could be achieved with a matter of 30 minutes at ambient temperature under certain stress conditions. The tin whisker initiation and growth were further studies at higher temperature at about 80 deg. C and at cold around -30 deg. C. Nano-indentation is performed to measure hardness and elastic modulus of the Sn-Pb coating layer where whiskers initiate, then compared to location where there is no whiskers observed. It is believed that establishment of a quantitative relationship between stress level, temperature, and whisker formation/growth could lead to better scientific methods for risk and reliability assessment with tin whiskers to safeguard a smooth Pb-free transition

Indentation Induced Tin Whisker Formation on Tin Plated Component Leads

Tin whisker formation has been a serious concern for application of pure Tin as a Pb-free component lead finish. It has been long believed that residual stress is the root cause of whisker formation. A fundamental question is if stress produced by other than the plating processing and post-plating metallurgical reactions can induce whisker formation. In this study, micro indents were made on pure Tin plated component leads to induce stress for studying stress induced whisker formation. Nano-indentation was performed to measure hardness and elastic modulus of the Tin coating layer where whiskers initiate. Scanning electron microscopy (SEM) was used to study indentation deformation mechanisms and to monitor the nucleation and growth of whiskers in-situ. In additions, finite element analysis was carried out to theoretically calculate the stress/strain distribution around the indentations. Experimental and theoretical calculation results show that whiskers form at a certain stress level. This suggests that there might exit a critical stress threshold that governs the whisker formation. It is believed that establishment of a quantitative relationship between stress level and whisker formation/growth could lead to a breakthrough in risk and reliability assessment with pure Tin application in the electronic industry and in safeguard for smooth Pb-free transition.Copyright ?? 2005 by ASME

Analytical Expressions for Shear and Axial Joint Deformations in Area-Array Assemblies Due to Global CTE Mismatch

In a previous study the authors derived an analytical expression for calculating the maximum shear deformation, (Δu)max , in an area array of solder joints under global CTE mismatch loading. The result was expressed in the form (Δu)max = βsh (Δu)0 , where βsh fish is the “shear correction factor” and (Δu)0 is the commonly used and easily calculated estimate of shear deformation, which is based on the free thermal expansion of component and substrate. A key assumption in the previous model was that warping of the assembly was neglected. In the present work the companion problem of assembly warpage is treated, the results of which are analytical expressions for the maximum axial deformation in the array, (Δw)max . The present results are cast in the form of an “axial correction factor,” βax , to be applied to the same convenient reference deformation: (Δw)max = βax (Δu)0 . Exact solutions are presented for several cases of practical interest: (1) an assembly in which the component and substrate have the same plan dimensions, (2) a rigid-substrate assembly, and (3) a rigid-component assembly. In addition, approximate expressions are presented in simple analytical form for the case in which the array is relatively flexible in comparison with the component and substrate. When combined with the previous solution for array shear deformation, the present results may be viewed as furnishing the complete solution to the thermal deformation problem for area-array assemblies of the types considered. The analytical results clearly indicate the relationship between the correction factors and the physical parameters of the problem: (a) the dimensions and material properties (elastic and thermal) of the component and substrate; (b) the material properties of the interconnect material (effective Young’s modulus and Poisson’s ratio); (c) the array size and population; and (d) the geometric parameters of the individual joints. An interesting and potentially useful conclusion is that the sign of the maximum axial deformation (tension or compression) in the corner joints can be related to the sign of a very simple expression that depends on the assembly dimensions and the material properties of the component and substrate. This result could be used to design more reliable assemblies, as it enables one to minimize the amount of axial deformation in the corner joints or, for a given thermal loading condition, to create a desirable compression in the corner joints to inhibit shear-driven fatigue cracking. The solution is based on a theoretical model of two circular elastic disks connected by an elastic layer whose distributed axial and shear stiffnesses are related to the joint/array characteristics by means of the authors’ previously derived stiffnesses for a single joint.Copyright