Lawrence Lehman

The microstructure of Sn in near-eutectic Sn–Ag–Cu alloy solder joints and its role in thermomechanical fatigue

During the solidification of solder joints composed of near-eutectic Sn-Ag-Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints

The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.

Effect of sample size on the solidification temperature and microstructure of SnAgCu near eutectic alloys

The degree of undercooling of Sn in near eutectic, SnAgCu solder balls upon cooling at a rate of 1 °C/s from the melt was examined and found to increase linearly with inverse nominal sample diameter (for balls of radius between 100 and 1000 μm). The mean undercooling for SnAgCu solder balls in a flip chip assembly was 62 °C. The microstructures of these different samples were examined by means of scanning electron microscopy. The Sn dendrite arm width was observed to monotonically increase with ball diameter, indicating a possible dependence of the mechanical response of such solder balls upon size.

Influence of Sn grain size and orientation on the thermomechanical response and reliability of Pb-free solder joints

The size and orientation of Sn grains in Pb-free, near eutectic SAC solder joints were examined. A clear dependence of the thermomechanical response of these solder joints on Sn grain orientation was observed. Solder balls with Sn grains of particular orientation (a-axis perpendicular to the substrate) were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn along the a-axis is half the value along the c-axis; joints observed to be damaged had maximum mismatch in the coefficient of thermal expansion between solder and substrate at the joint interface, as well as tensile stress modes during the hot part of the cycle

Microstructure and Damage Evolution in Sn-Ag-Cu Solder Joints

The implementation of no-Pb soldering is progressing rapidly, often without immediately obvious problems. However, our current understanding of pertinent materials issues is far from sufficient to prevent surprises, and potentially serious miscalculations. The fundamentally different nature of no-Pb from SnPb solder joints has serious consequences for the design and interpretation of accelerated tests. The present work addresses the behavior of ball grid array (BGA), wafer level chip scale package (WL-CSP) and flip chip assemblies with SnAgCu solder joints in long term thermal cycling. Components were assembled onto either Cu or Ni pads on high-Tg printed circuit boards and cycled between 0 o C and 100 o C. Samples were removed at various stages for cross sectioning and microstructural characterization at various stages. Careful examination of the formation of intermetallic compounds at the pad surfaces, the size and orientation of the Sn grains, the morphology and number of precipitates, and the growth of cracks, revealed the sensitivity of the damage evolution to the solder microstructure. The consequences of the observed pattern of damage evolution are discussed.

Grain Deformation and Strain in Board Level SnAgCu Solder Interconnects Under Deep Thermal Cycling

Digital image correlation and cross polarizer, optical microscopy were used to quantify the deformation behavior under deep thermal cycling of near eutectic SnAgCu (SAC) solder in board level interconnects. Maps with sub micron spatial resolution of the strain levels and von Mises strain were produced for selected cross sections. Large spatial variations in the thermo mechanical response of the solder joints were observed and were correlated with Sn grain boundaries or intermetallic precipitates. Such observations are consistent with the anisotropic nature of the mechanical properties of Sn, and the differences in the mechanical responses of Sn and the intermetallic precipitates in SAC solder. The demonstrated anisotropic thermomechanical response of many SAC solder joints sheds doubt on any model which considers these joints to be composed of isotropic material

Grain Formation and Intergrain Stresses in a Sn-Ag-Cu Solder Ball

The thermo-mechanical behavior of near eutectic lead-free SnAgCu (SAC) solder joints under Deep Thermal Cycling (DTC) and in-situ thermal loading was examined. Crossed polarizer, optical microscopy revealed that in ball grid array (BGA) solder joints, these Sn rich, Pb-free solders exhibit large grained Sn structures. After imaging, these SnAgCu solder joints were subjected to repeated thermal stresses under an inert atmosphere. Subsequent to this thermal loading, the samples were again examined with optical microscopy. Using both data sets, the intergrain strains and deformations were quantified by Digital image correlation, a full field optical measurement technique. The relations between the positions of grains as well as intermetallics compounds, their boundaries and Sn deformation fields were examined.© 2005 ASME

Failure analysis of contact probe pins for SnPb and Sn applications.

A study has been conducted to investigate the failure mechanism of pogo pin-type probe contacts. Probe pins are used for electrical test of microelectronic components in manufacturing. A false rejection of parts due to high probe contact resistance results in a penalty in cost and yield. The probe pin contact bears distinctive characteristics of failure compared to the conventional contact systems such as mechanical switches and interconnects. Moreover, the transition to Pb-free component leads demands understanding of different probe failure mechanisms between a SnPb and Sn surface. The objective of this study is to understand this unique failure mechanism and the effect of lead coating metals on probe pin life. This has not been clearly elucidated to date in spite of its significant impact on yield and cost of electronic package manufacturing. A simulated probe tester with 3-axes actuation capability was devised to mimic the actual test process. The force required to penetrate the surface oxide layer and develop electrical contact was measured. Contact resistance history revealed that probe pins mating to Sn surfaces failed earlier than pins used on SnPb surfaces. Through periodic inspection of probe pins using microprobe/EDS as a function of probe actuations, the general root cause of probe pin failure was found to be probe pin tip wear out associated with the Sn oxide growth on its surface. The matte Sn surface wears the probe pin more than SnPb due to the rough and abrasive nature of the matte Sn surface.

Examination of the utility of commercial-off-the-shelf memory devices as X-ray detectors

An examination was conducted of the use of standard memory devices as X-ray detectors. Commercial-off- the-shelf memory devices such as flash memory, UV-EPROM, DRAM, and non-volatile SRAM units were studied. The memory states of the devices were continuously monitored as a function of time and X-ray flux. It was found that in all configurations used, the devices were not practical X-Ray dosimeters; hard fails were nearly as prevalent as soft fails.

Failure Analysis of Contact Probe Pins for SnPb and Sn Applications

A failure mechanism of pogo-type probe pin is investigated. A probing tester with actuation capable in three-axes is used to simulate the actual inspection process experimentally. Force required to break in surface oxides and develop electrical contact is measured. Contact resistance history reveals that pins mating to Sn surfaces fail earlier than SnPb surfaces. Through periodic inspection of pin using microprobe/EDS as a function of probing count, the general root cause of pin failure is turned out to be pin tip wear out associated with Sn oxide growth on its surface. The cause of earlier failure of the pin probing matte Sn surface is identified as severe wear out by a rough and abrasive characteristic of matte Sn.Copyright