Rapid Whisker Growth Research Articles

Transmission electron microscopy study of the epitaxial association of hedenbergite whiskers with babingtonite

ABSTRACTOvergrowths of whiskers of hedenbergite (Ca(Fe2+,Mg)Si2O6) on the hydrous pyroxenoid babingtonite (Ca2Fe2+Fe3+[Si5O14(OH)]) have been observed at Arvigo in Switzerland and Kreimbach/Kaulbach in Germany, and we have studied them with transmission electron microscopy in order to understand their structural relationships and formation. The boundaries between babingtonite and hedenbergite are sharply defined, and the two minerals are in direct contact with no additional phases present. The relationships of babingtonite (Bab) and hedenbergite (Hd) were determined as Bab[100]//Hd[112] in the Arvigo specimen and Bab[$\bar 1$00]//Hd[1$\bar 1$2] in the Kreimbach/Kaulbach specimen. Diffraction derived from Bab(031) and Hd(02$\bar 1$) in the Arvigo samples and Bab(031) and Hd(021) in the Kreimbach/Kaulbach samples were observed in identical positions. The reciprocity between the babingtonite and hedenbergite structures is governed by the direction of the SiO4-tetrahedral chains, and the related configuration of octahedra. Thus, hedenbergite is apparently an epitaxial phase grown on a base of {010} plates of babingtonite. The defined orientation relationship is also consistent with that shown in topotaxial intergrowths of other clinopyroxenes and pyroxenoids. The topotaxial intergrowths may result from diffusion-controlled solid-state reactions, whereas rapid whisker growth is characteristic of supersaturated solutions or a vapour medium. The epitaxial growth of hedenbergite whiskers on babingtonite with an abrupt but coherent change of structure at the interface represents an ideal example where the similar chemical compositions of host and guest contribute strongly to the close structural relationship.

Electrochemical Migration and Rapid Whisker Growth of Zn and Bi Dopings in Sn-3.0Ag-0.5Cu Solder in 3wt.% NaCl Solution

Electrochemical migration (ECM) tests and rapid whisker growth on Sn-3.0Ag-0.5Cu solder candidates doped with Zn and Bi on Cu-plated FR-4 printed circuit board were conducted by applying constant voltage. The results showed that dendritic shape were different when the doped metals were different. When Zn was doped in SAC solder, dendrites looked like tree trunk, while that of Bi doping looked like rose, which was due to the different composition of SAC candidates. Comparing the length of dendrites at the same condition, it could be concluded that dendrite growth might be suppressed by Bi addition, which the contrary effect suitable to Zn addition. EDAX results showed that the main content on dendrites was Sn with or without Bi doping, while the main content was Zn, Sn, and Ag with Zn doping and Zn/Bi dopings. Whisker growth test verified that Sn accounted for a majority (larger than 95wt.%), no difference could be seen on whiskers of SAC solder candidates although the contents of SAC solder candidate were differently. The whisker growth rate were not different although the doped metals were different largely. Both dendrite growth and rapid whisker growth of SAC solder candidates doped with Zn and Bi were harmful to micro/nanoelectronic packaging attributing to bridge short circuit in PWBs industries.

Interfacial microstructure and bonding strength of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce– xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn–3Ag–0.5Cu–0.5Ce– xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce– xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn 3 intermetallics and Zn atoms, formed in the Sn–3Ag–0.5Cu–0.5Ce– xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu–Sn intermetallics growth in Sn–3Ag–0.5Cu–0.5Ce– xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn–Ag–Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.

The effect of 0.5 wt.% Ce additions on the electromigration of Sn9Zn BGA solder packages with Au/Ni(P)/Cu and Ag/Cu pads

It has previously been established that Sn-9Zn-0.5Ce alloy possesses mechanical properties superior to those of undoped Sn-9Zn alloy, and is free of the problem of rapid whisker growth. However, no detailed studies have been conducted on the electromigration behavior of Sn-9Zn-0.5Ce alloy. In this research, Sn-9Zn and Sn-9Zn-0.5Ce solder joints with Au/Ni(P)/Cu and Ag/Cu pads were stressed under a current density of 3.1 × 10 4 A/cm 2 at room temperature for various periods of time. Due to finer grain sizes, the electromigration effects were more severe in Sn-9Zn-0.5Ce solder joints than in Sn-9Zn solder joints when joint temperature was around 80 °C. In addition, both solder joints (Sn-9Zn and Sn-9Zn-0.5Ce) with Au/Ni(P)/Cu pads possess longer current-stressing lifetimes than those with Ag/Cu pads because Ni is more resistant than Cu to migration driven by electron flow.

Evaluations of Whisker Growth and Fatigue Reliability of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.05Ce Solder Ball Grid Array Packages

Sn-Ag-Cu solders doped with rare-earth elements were reported to exhibit beneficial mechanical properties; however, rapid whisker growth could be induced. In order to retain the advantageous effects of rare-earth doping without causing the formation of tin whiskers, a Sn-3Ag-0.5Cu-0.05Ce alloy was suggested. In the present study, metallographic observations indicate that whisker growth is indeed prevented in this alloy. However, tensile tests with the bulk materials show that the ultimate tensile strength and Young’s modulus of this Sn-3Ag-0.5Cu-0.05Ce alloy are only slightly higher than those of undoped Sn-3Ag-0.5Cu. Further reliability tests with actual ball grid array packages indicate that the Sn-3Ag-0.5Cu-0.05Ce solder joints have fatigue lives similar to those of undoped Sn-3Ag-0.5Cu specimens regardless of the various surface finishes, such as electroless nickel/immersion gold (ENIG), immersion tin (ImSn), and organic solderability preservatives (OSP). The results suggest that, although lowering the content of rare-earth elements in solders can inhibit the occurrence of rapid whisker growth, it cannot ensure the improvement of the tensile properties of bulk solders and the fatigue reliability of solder joints in actual packages.

Electromigration of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–0.2Zn solder joints with Au/Ni(P)/Cu and Ag/Cu pads

It has previously been established that adding 0.2 wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce alloy improves the mechanical properties and eliminates the problem of rapid whisker growth. However, no detailed studies have been conducted on electromigration behavior of Sn–3Ag–0.5Cu–0.5Ce–0.2Zn alloy. The electromigration damage in solder joints of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–0.2Zn with Ag/Cu pads and Au/Ni(P)/Cu pads was studied after current stressing at room temperature with an average current density of 3.1 × 10 4 A/cm 2. With additions of 0.5 wt.% Ce and 0.2 wt.% Zn, the electromigration processes of Sn–Ag–Cu solder joints were accelerated due to refinement of the solder matrix when joint temperature was around 80 °C. Since Ni is more resistant than Cu to diffusion driven by electron flow, solder joints of both alloys (Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–0.2Zn) with Au/Ni(P)/Cu pads possess longer current-stressing lifetimes than those with Ag/Cu pads.

Effect of adding Ge on rapid whisker growth of Sn–3Ag–0.5Cu–0.5Ce alloy

Although solders doped with rare earth elements have been reported to show many beneficial effects, tin whisker growth has been observed to grow at an extremely high rate on the surface of such novel solder alloys. This study shows that adding 0.5 wt.% Ge into a Sn–3Ag–0.5Cu–0.5Ce alloy effectively decreased whisker growth. This inhibition effect is attributed to alleviation of oxidation in the CeSn 3 intermetallic phase in this alloy as a result of Ge-alloying. Compressive stress in this case is insufficient to extrude tin atoms out of the solder to form whiskers.

Rapid whisker growth on the surface of Sn–3Ag–0.5Cu–1.0Ce solder joints

In spite of the many beneficial effects obtained from the addition of rare earth elements to solder alloys, rapid growth of tin-whiskers has been found in Sn–3Ag–0.5Cu–1.0Ce solder joints. The morphology of the whiskers changes from fiber-shaped to hillock-shaped when the storage temperature increases from 25 to 150 °C. The driving force for the whisker growth is the compressive stress resulting from the volume expansion of the oxidized CeSn3 phase, which is constrained by the surrounding solder matrix.

Whisker growth in A1 thin films

Whisker growth in aluminum thin films has been studied by transmission and scanning electron microscopy. The aluminum films were vacuum deposited on unheated substrates and were not annealed prior to whisker formation. The rapid whisker growth was promoted by covering the aluminum films with a layer of TiN. Growth rates of up to 500 Å/s have been observed. Whiskers nucleate at grain boundaries, single grains and hillocks. The whisker axis is parallel to <110>. The growth mechanisms, morphology and kinking of whiskers are described and discussed. Aluminum films which were annealed before TiN coverage did not show any whisker growth.

TRACER EXPERIMENTS ON THE GROWTH OF TIN WHISKERS

Tracer experiments on the growth of proper whiskers from evaporated tin layers indicate the existence of solid or nearly solid as well as hollow whiskers. The presence of oxygen during evaporation, resulting in a rapid whisker growth, inhibits large scale diffusion. This inhibition effect must be released, however, at small regions, corresponding to the roots of the whiskers.