Formation of Pure Tin Whiskers
In the different branches of industries specifically the involvement of the broad use materials of tin metal based along with its alloy accompanied by the copper metal also combination of lead. Basically, the solder materials that involve the combination of Sn-lead alloy usually act as surface finish of parts in the microelectronic manufacturing. This can be state that one of the application of the lead-Sn based alloys soldering process is attach the electronic parts on the printed circuit board (PCB) during the assembling of the electronic parts during the manufacturing process 13, 20-23.
With the Restriction of Certain Hazardous Substances (RoHS) by European Parliament and European Council towards the use of lead (Pb) in the electronic and electrical equipment during 2003 become the start point of use of lead-free in the production processing where the certain hazardous metal is excluded in the manufacturing such as hexavalent chromium, cadmium and mercury which are occupational hazard substances. This adaptation is practicable where the final finishing of the components and the solder materials must contain no trace of lead 24-26.
One of the disastrous phenomena that formed is the reliability issues which lead to the formation of voids 27-28, the formation of cracks in the thermal cycling 29,30-31, the whiskering mechanism 32-36, corrosion at elevated temperature and humidity conditions 37-40 which interconnected to each other. Formation of thin whiskers promote the short circuit system failure due to the formation of link made of single filament or cluster of the filament that form the connection formed in between the neighbor component of the electronic component parts. Thus, the mismatch of potential different value existed and the whiskers are conductive filaments. Besides that, the metal materials of low melting point as in zinc, indium, silver, gold, antimony, magnesium, bismuth, nickel, tin and cadmium may act as substrate for the spontaneous formation of whiskers which often lead to materials failure 41-45.
Pure tin whiskers formation is the formation of the abnormal growth where the extrusion of the single-crystalline grain structure grow out of the alignment of the grains arrangement structure can be manipulated by the diffusion of atoms at the grain boundaries and the grain size transformation. The morphology of crystal structure of whisker can be described into three types which are hillocks, nodules, columnar and filament which start with the formation of the hillock root then extrude out into nodule body and lengthen to the filament shape. The growth mechanism can detect by the lateral directions towards the axis of the whiskers formation.
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The structure of filament can be described as the single crystal needle-like with diameter of 1-5 µm with the length of tens to hundreds µm. Nodules are not straight, thick in diameter, bundled base, the shape and sizes vary greatly in size, diameter about 5-20 µm, length about 10-50 µm. Appear as base for filament type of whisker. Hillocks appear as in pyramidal structures with both lateral and vertical dimension in the range of a few µm. may act as the precursor for the whiskering. Columnar whisker is the common crystal on the matte with about 1 µm of the grain size which also known as thin whisker, the shorter and thicker than the filaments type where it diameter about 2-6 µm that depend on the size of the grain on the Sn layer and the length of the columnar whisker is about 0.2 mm.
The recrystallization temperature of tin is about 0.4 – 0.7 x Tm of 505 K and at the 0.58Tm, grain growth starts to evolve 17. The decreasing level of the grain boundary-free energy is for the movement of grain boundary occur when the atom moves across the interface in between the grains arrangement 17. The grain growth of the thin film consists of the additional orientation dependent on the surface energy which involves the movement and the growth against the alignment of the grain boundaries. The presence of the compressive stress as precursor induce the formation of the vertical and horizontal direction of shear stress which causes sliding of the grains boundaries 17,38.
The growth of the whiskers which out below the surface of the metal with the help of the compression stress that carries out the transportation of mass via atomic diffusion involves few stages. At stage one is the formation of the hillock which can be described as the pyramidal structure with both lateral and vertical dimension in the measurement of the µm. Hillock act as the precursor for the formation of other dimensions of whiskers. Characteristic of pure tin whiskers in the form straight, kinked, bent, forked, lumped, combine morphology, nodules, odd-shaped eruption. 17
Theory of the Whiskering Phenomenon.
There are few theories that has been practiced by others researcher such as the dislocation-based theory 3,4,19, recrystallization theory 1,5,13,46, compressive stress-based theory 6-7,18, catalyst-based theory 2, temperature and humidity 1, 13,19,46, corrosion 19,37. The whiskering process can be promoted by the formation of an abnormal grain growth on the grain layer surface. The whisker grows out of the surface of grain layer due to the migration of atoms by the formation of crystalline structure extruded out of the surface of metal in order to relieve stresses that formed within the atomic metal layers and in between the grains arrangement 17.
Dislocation-based theory involve the defect on the deposit orientation and grain structure localized. Where the localization of the grain structure may be modified by the secondary phase particles which lead to the reduction of the grain boundary diffusion rate and the blockage of the movement of the wall of grain boundary. This condition causes the increasing rate of the whiskers formation. The characteristic of the deposit such as the shape and size of the grain, the crystal orientation plane and the thickness of the deposit on the plating 17.
Besides that, the deposition of the fine grains size increases the rate of stress and susceptible value onto the formation of whiskers growth compare with the low susceptibility and stress value exhibit by deposition of the large size 55. This is why the formation of small grain sizes deposition is more preferable in order to reduce the stress level to avoid the sliding of the grain boundary occurs 47. At the condition of the high vacuum environment, the free surface of tin is full of voids and provide the pathways for the stress relieve for the elevated level of stress formed in between the atomic arrangement layers. This will contribute to the deformation of the whiskers. This is one of the factors for the certain localization of the whiskering formation 17,38.
The recrystallization theory involves the formation of the intermetallic compound such as Cu3Sn or Cu6Sn5.There is the increasing rate of the compressive stress gradient with the volumetric expansion within the deposition layers and the presence of the pyramidal growth of the intermetallic compound 36-37. This theory deduces that the oxidation of substrate deposit and the formation of intermetallic compounds will lead to the alteration of the new formation of the grain shape and alignment 19,46. However, there is decreases in the stress level yield around the altered grains formed compared to the vertical alignment of grain boundaries.
Thus, this can state that the formation of the residual stress is affected by the alteration of the grains boundaries in order to allow the formation of the intermetallic compound of the interaction in between the substrate deposit and the plating material 47-51. The reaction of the tin deposit solder with the copper plate forms the intermetallic compound of Cu6Sn5 at room temperature condition. The rate of the formation of the intermetallic compound will constant form as long the driving force existing. Formation of intermetallic compounds inside the tin at the film/substrate interface and with an alloying element at grain boundaries 54.
The catalyst-based theory deduces that the rate of whiskering can be manipulated by the types of the substrate which can be proven where the presence of titanium (Ti) control the growth of the whiskers on the milled Ti/Sn/C compound only while neither formation exists on the milled pure Sn and milled Sn/C compound 2.
Electrodeposition-based theory state that there are several factors that influenced the whiskering phenomenon such as the parameters of the plating as in the plating chemistry, rate of current density, temperature and movement of electrolytes around the substrate which affect both microstructural and chemical properties 28-29.
The orientation of the (211) and (321) Miller planes are more preferred compare to other planes, which is due to the corrosion propensity which depends on the crystallographic orientation of the substrate’s deposition. 1, 24. Among of all type of the depositions, the bright deposition substrate is one of the major factor that influence the formation of the whiskers 54. The whiskering mechanism can be explains with the presence of bright deposition on the plating material where the diffusion of the substrate into the atomic arrangement through the grain size of bright deposition 38,54.
This process causes the change in arrangement of the grains with the adjustment of the lattice spacing which also lead to the product of the intermetallic compounds formed 17. The change in the crystal lattice arrangement of the deposition starts to develop the extra stress which promote the formation of filaments extrude out of the surface of the deposit. The presence of the whiskers affects the value of the susceptibility of the deposition surface. This, can be conclude as the grains size of the atomic arrangement influence rate of stress level produced by some intrinsic stress originating from the deposition process and electro- migration movement 47,52.
Corrosion-based theory state that corrosion rate is one of the precursor for the formation of compressive stress gradient as in the presence of the flux residues and halides product which start the corrosion reaction on the surface of metal deposit. Basically, the presence of the intermediate elements that state above act as precursor for the whiskering mechanism.
Stress-based theory involve the formation of mechanical load as in the residual stresses, internal stress, compressive stress and chemical stress. The sources for development of stress gradient can be derive from the indentation mechanism as in the application of the microscale indentation which providing the continuous loading such as micro-indentation and Nano-indentation of ball indenter 18. Apart from the indentation, the plating stress development, changes in temperature, external application of mechanical load, mechanical scratching and corrosion mechanism also can act as precursor for the formation of pure tin whiskers 32,44-50.
The developed stress that form on the plating will lead to the change in alignment of the grains which lead to the formation of the nodules for the extrusion of the filament whiskers 38. The other factors are the formation of the residual stress formation due to the presence of organic additive as in the brightener in the bright tin deposit, the development of the substrate stress for the certain substrate materials as in steel, copper and brass with the application of conformation process as in annealing, etching and stamping 23.
In order to enhance the growth of the whiskers, the formation of oxide layer on the are required to effectively cover up all the voids source on the surface of the metal. Due to this requirement, only certain materials that can form the oxide layer would extrude out the whiskers as in aluminum and tin. Meanwhile, the whiskering phenomena are unusual to form the growth of the whisker on the surface of the metal with non-protective layer as in copper and iron 18.
The mechanism of the whiskering on the oxide layer can be describe as the influence of the thickness of the oxide layer affect the rate of the protrusion of the hillock out of the nodules. the thin oxide layer allows the faster rate of the whiskering process. As for the thick oxide layer, there is the need to form rupture at certain weak point on the surface of metal for the pathway of the growth of whiskers 19.
The formation of the oblique stress gradient in the tin matrix layer act as the precursor for the high distance of atomic diffusion of tin atoms that required for the formation of the whiskers. The localized and spontaneous whiskering formation at the room temperature condition can only be achieved with the presence of constant stress and the existing of the thin protective layer 17.
The formation of the intermetallic compound of Cu6Sn5 formation formed the biaxial compressive stress on the pure tin deposit extrude out the thin whiskers from the weak point of the oxide compound layer. At this, the center of stress relieve point for the high velocity rate of diffusion of atom is acting by the nodules. The size of volumetric expansion of the grain boundary influenced by the size of the differences in the coefficient of thermal expansion of each components of materials involved 29-31.
Temperature and Humidity Theory
Temperature and humidity-based theory state that the whiskering growth mechanism can be affected by manipulating the degree of the temperature as in the placing the deposit substrate soldering in the exposure of the high temperature and increasing rate of the humidity. Besides that, the formation of the compressive stress gradient for the extrusion of the whiskers can be create with the mismatch of the coefficient of thermal expansion (CTE) in between the substrate and plating materials 48, 53-56.
The other conditions that can be state are the factor of the thermomechanical effect 46, the formation of the oxide layer, elevated temperature aging process 19 and the formation of the tensile stress gradient through the mechanism of thermal cycling 37. increasing temperature will accelerate diffusion rate of atoms with tin plated surface finish. The elevated level of humidity circumstances promotes the several corrosion and oxidation process. The presence of the condensation process triggers the increasing level of humidity which increase the volume of the pure tin grain size and form the compressive stress on the surface finish 33-35.
Development of The Mechanism for The Formation of The Whiskers
The trend of the development of whiskering mechanism as shown, at year 2015, the method for the formation of the whisker is via the electroplating method as in electrodeposition on bright acid tin finishing, the application of the modified embedded atom method via molecular simulation, the microscale and nanoscale-indentation, addition of Ni on the tin plated surface finishes 1-37.At year 2016, the development of the whisker through the catalyst-based growth using the ball milling processing 2. At year 2017, vacuum deposited Sn layers on copper substrate 46, micro-electro-mechanical system (MEMs) testing device 19, 2-dimension analytical model for the study of the generation of applied stress and the whiskering mechanism 53.
The tin appears as shiny and silvery grey-white metal form that can be found in ores which commonly found as Sn oxide (SnO2) or cassiterite. Stannum (Sn) is the chemical name for tin which made up of tetragonal structure. The melting point and boiling point of the tin are 505.05 K (231.9 oC, 449.41998oF) and 2543.15 K (2270.0 oC, 4118.0 oF) respectively. Sn can coat the other metals as in providing the resistance against corrosion. Sn are ductile and soft providing the excellent conductivity that enhance the solderability of substrates. Unlike the lead, tin is non-carcinogenic and non-toxic material. tin alloys containing rare-earth elements should readily form an oxide layer that can give rise to the growth of tin whiskers 8.
The plating with steel sheets used to form cans for food containers. Besides that, tin is one of the materials that are used in the production of electronic devices. The tin also can undergo soldering process with the Indium to form solder and production of bronze with copper. The replacing of lead with the soldering of pure tin is due to the toxicity of the lead. Besides that, the higher melting temperature of the tin provide the maximum service temperature of the pure tin solders 1-17.
Table 2.1: Properties of Pure Tin 12
Physical state and appearance Solid
Molecular Weight 118.71 g/mole
Color Silver-white Grey
Boling Point 2507oC (4544.6 oF)
Melting Point 231.9oC (449.4oF)
Specific heat capacity 227 Jkg-1K-1
(Temperature (K), Pressure (Pa))
(1000 K,8.62×10-9 Pa)
(1400 K,0.207 Pa)
(1600 K,4.85 Pa)
(1800 K, 56.3 Pa)
Solubility Insoluble in cold water, hot water
Conditions of Instability Excess heat, incompatible materials
Incompatibility with various substances Reactive with oxidizing agents, acids, alkalis
Copper is the face centre cubic crystal structure that is yellowish red in physical appearance and become a bright metallic after being polished. The properties of copper are shown in Table 3.2 12.
Table 2.2: Properties of copper plate 12
Molecular Weight 63.546 g/mol
Melting Point 1356.6 K
Boiling Point 2840 K
Physical Appearance Malleable, ductile, reddish-brown
Specific heat capacity 376.812 Jkg-1K-1