Nader G. Dariavach

Microstructural analysis of lead-free solder alloys

Among the many issues related to the performance of lead-free solder alloys, the dependence of their mechanical properties on the microstructure and the stability of the microstructure stability are some of the most important issues. A comprehensive understanding of the process-microstructure-property relationships is essential. Toward that goal, a microtextural analysis is performed using orientation imaging microscopy (OIM) for alloy Sn-3.8Ag-0.7Cu (wt pct) processed at four different temperatures. Sn-3.8Ag-0.7Cu is one of the most promising lead-free solder alloys that has shown superior mechanical properties to other candidate lead-free solder alloys. However, a comprehensive understanding of their microstructure and the dependence of microstructure on processing conditions are still lacking. In the present work, a detailed microstructure characterization with respect to phase compositions, grain size and size distributions, texture, and orientation relationships between various phases are performed. The measured microstructural features are correlated with the soldering temperatures.

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 336 h. 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...

Deformation behavior of solder alloys under variable strain rate shearing & creep conditions

The rate-dependent deformation behavior of Sn3.8AgO.7Cu Pb-free alloy and Sn-Pb eutectic alloy under constant and variable shearing strain rates and creep conditions was studied systematically with thin-walled specimens using a biaxial servo-controlled tension-torsion material testing system. The shearing tests were conducted at strain rates between 10/sup -6//sec to 10/sup -1//sec. Variable strain rate tests were also conducted for sudden strain rates change and stress relaxation to study the post-yielding flow stress dependency on stress. Shearing creep tests were carried at room temperatures under a variety of stress level. It is believed that such a fundamental study of deformation behavior of solder alloys under complex stress conditions is important to understand effects of soldering processes and microstructures on solder interconnect reliability, and also to implement sophisticated 3D time-dependent nonlinear FEM analyses on electronic packages and assemblies.

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...

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

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