The electronic component manufacturing, the use of lead (Pb) solder is the most common in as in the Pb-Sn intermetallic compound. However, there is the legal act upon the use of Pb in the soldering of the electronic component due to it hazardous toward the human 12. The development of the lead-free solder starts to revolve in all industries to promote the “green manufacturing where all part on the manufacturing industries start to use the lead-free material in replacing the use of lead in their productions.
Instead of only lead, there are also other materials that has been restricted to use as in cadmium, mercury and hexavalent chromium. This is to be said that the metal finish and solder materials of the component must be free of any trace of lead to produce the lead-free assembled parts of electronic component on the printed circuit boards. the spontaneous whiskers formation starts to increase in the industries along with increasing development in lead-free solders 14-16.
The spontaneous emerging of the thin filament out of the deposit plating can be in the form of single filament or a cluster of shape that lengthen to several 50 µm 12. In the other words, the formation of crystalline structure out of the surface of metal electrodeposition plating. There is certain type of materials that tend to extrude the whiskers such as indium, antimony, zinc and tin alloy, silver, gold, aluminum, magnesium, tungsten, bismuth and nickel. The formation of crystalline structure of whisker is in the form of nodules, hillock, filament, columnar, extrusion, hair-like shape protrusion of single filament, straight, kinked, bent, lumped and forked also with the combination of morphologies 17.
The whiskering phenomenon is one of the mechanism to relieve the stress by the deposit on the plating by the extrusion of electrically conductive thin filament. There are several problems arise such as the formation of bridge by the whiskers which form the interaction in between the component with different potentials value which then lead to short circuit. There are several theories that derive for the understanding of the whiskering phenomena such as dislocation-based theory 3-4, recrystallization theory 5, compressive stress-based theory 6-7, catalyst-based theory, temperature and humidity theory, corrosion-based theory and electrodeposition-based theory.
The formation of whiskers can be trigger by several conditions such as the application of mechanical stress, change in temperature, the formation of intermetallic compounds, corrosion rate, presence of catalyst, formation of substrate material stress, plating chemistry parameters, temperature cycle, different in coefficient of thermal expansion (CTE), the formation of oxide compound, expose to the increasing temperature, level of humidity and the orientation, shape and size of microstructure of grains. There are few ways to form the whisker such as the application of catalyst-based growth, electrodeposition 1, molecular simulation 13, indentation 18 and Micro-electro-mechanical system (MEMs).
The purpose of this research is to study about the originality of the spontaneous formation of the whiskers and how the stress and temperature effect on the whiskering process to solve the reliability problems that affect in most of industrial sectors. There are several researches that has been continued through the past years are commonly investigate on the mechanism of the whiskers, the precursor of whiskering and how the mitigation process of whiskering are successful. In this research, we are going to investigate on the effect of stress and temperature on the formation of pure tin whiskers. There are two phases that involve in this research which are the preparation of sample by the dip soldering method and the development of mechanical jig.
For the soldering of the deposit of pure tin on the copper plate, there are two types of processing method that applicable which are the casting processing and the dip soldering. The properties of the casting process are the production of the high dimension of tolerance of the final product with good finishing design. The casting process also an automated processing stages that provide faster production rate compare to dip soldering. The casting process usually involve the production of the complex and large size of parts. Due to the automated processing, the internal monitoring and process control are needed.
The level of stress exerted by the indentation of the steel ball is manipulated by the value of the load placed on the steel ball where the presence of the applied load produces the compressive stress that act as the precursor for the formation of pure tin whiskers which also trigger the growth of intermetallic compound. The reason for the use of the dip soldering for the sample preparation is due to the allocation of the precise view of IMC compound which one of the precursors of the pure tin whiskers growth.
The development of the 3-dimensional model of the mechanical jig model provide the source of the stress to be apply on the copper plate to trigger the formation of the pure tin whiskers. Among all the precursor for the whiskering phenomenon, stress is also one of them. The screw of the mechanical jig act as the controller for the applied stress by manipulating the level of the stress applied onto the copper plate. The accuracy of the mechanical jig model can be measure through the calculation of stress-strain value and the area of the whiskering formed.
In this research, the pure tin solder deposit is formed on the copper plate through the dip soldering process. This research is conducted to study the morphology of formation of the pure tin whiskers. The main objectives of this research are:
To design a mechanical jig for the for the applied stress controller to manipulate the level of stress applied on the copper plate holder.
To study the effect of stress on the formation of the pure tin whiskers.
To study the effect of temperature on the formation of pure tin whiskers.
The Scope of Study
In this study, the effect of the stress and temperature on the formation of the pure tin whiskers are studied. The sample preparation involves the application of dip soldering process to form the pure tin solder deposit. After that, the applied axial stress of the mechanical jig act on the pure tin solder deposit.
There are two source of driving force for the formation of the whisker which are the applied mechanical stress and thermal stress. Furthermore, the formation of the intermetallic compound is developed due to the presence of elevated stress and strain level which induced the dislocation of the grains across the grains boundaries. With the mechanism of the mechanical jig, the applied mechanical stress level can be manipulated along with the rate of formation of thin whiskers. Meanwhile, the strain level can be manipulated by the level of thermal stress via place the mechanical jig in the oven. After all the process is completed, the morphology and microstructure properties of pure tin whiskers are then characterized Scanning Electron Microscope and optical microscope.