http://www.am2rg.com/ijamm Aamir et al. IJAMM (2016), 01(1), 33-41
International Journal of Advanced Materials and Manufacturing
Relationships between Microstructure and Mechanical Properties in High Sn Content Pb-based and Pb-Free Solder Alloy after Thermal Aging M. Aamira,*, R. Muhammada, M. I. Hanifa, N. Ahmeda a
Department of Mechanical Engineering, CECOS University, Peshawar, Pakistan Corresponding Author (*M. Aamir): Department of Mechanical Engineering, CECOS University of I.T and Emerging Sciences, Peshawar, Pakistan Email:
[email protected]
Abstract: The microstructure and mechanical properties of solder joints in electronic components are of considerable importance when exposed to thermal aging. The current work is based on the impact of thermal aging on the microstructure examination and mechanical properties of high Tin (Sn) content Lead (Pb) based 96Sn-04Pb solder alloy and Pb-free 96.5Sn-3.0Ag-0.5Cu solder alloy. Both solder alloys are aged isothermally at 100◦C for 50 hours and their relationship between microstructure and mechanical properties are reported and compared. The microstructure investigation is carried out using Scanning Electron Microscopy (SEM) and chemical composition is investigated by Energy dispersive X-ray (EDX) technique. The average Intermetallic compounds (IMCs) particle size are measured using ImageJ and mechanical properties including Yield strength (YS) and Ultimate tensile strength (UTS) are investigated from stressstrain curves. The results show increase in average size of IMCs after thermal aging due to coarsening of the microstructure and decrease in the YS and UTS of both solder alloys. However, in comparison to 96Sn-04Pb, the mechanical properties of 96.5Sn-3.0Ag-0.5Cu have been investigated superior due to its refined microstructure. The ductility of 96.5Sn-3.0Ag-0.5Cu is also considered as more than 96Sn-04Pb. Keywords: 96Sn-04Pb; 96.5Sn-3.0Ag-0.5Cu; Intermetallic compound particles; Mechanical properties; 1. Introduction Solder joints are the most commonly used interconnection between chip and substrate with Printed circuit board PCB in board level packages. The solder joint is mainly consisting of three constituents; the solder, Copper (Cu) substrate and the intermetallic compounds (IMCs) formed between the solder alloy and the Cu substrate. All these constituents are strongly linked with the mechanical properties of the solder joint because mechanical properties altered when the solder joint is under service condition or isothermal aging process which is the key reliability issue [1, 2]. Therefore, it is significant to investigate the effect of thermal aging on the microstructure changes in
association with mechanical properties of the solder joint. The IMCs thickness is one of the key parameter for characterizing its microstructure and isothermal aging conditions has drawn a lot of attention [3]. A thin layer of IMCs is important for necessary metallurgical bonding but when these MCs become thick, it creates reliability issue due to their brittle nature and ability towards structural defects [4-6]. The thickness of these IMCs due the its growth is the result of prolong exposure to high thermal environment [7, 8]. Therefore, the effect of thermal aging can be used to simulate the changes of microstructure and properties of the soldered joint under working conditions, since at elevated temperature, the microstructure of the solder 33
Publishers: CECOS University of IT and Emerging Sciences, Peshawar, Pakistan
http://www.am2rg.com/ijamm joint changes significantly and thus affects the overall mechanical properties [9]. Recent reports have shown that eutectic 63Sn-37Pb solder has been used in electronic industry due its good solder ability, reliability, low cost, physical, mechanical and metallurgical properties [10]. But legislation to ban on lead (Pb) compels the electronic manufacturing industry to characterize different solder alloys in terms of their different properties and compatibility with components and PCB materials However, information about these Pb-free solders is limited due to their novelty [11]. Therefore, different solder alloys have been introduced, among all Sn-AgCu family is considered to be the most attractive
substitute for the conventional 63Sn-37Pb solder alloy [12]. 96.5Sn-3.0Ag-0.5Cu is considered to be the most significant members of the Sn-Ag-Cu family. However, investigation about their microstructure and mechanical properties for their reliability is very important [13]. The comparative properties of different elements in electronic materials are given in Table. 1 [14]. The current work is based to investigate the impact of thermal aging and relationship between microstructure and mechanical properties of high Sn content Pb-based and Pb-free solder alloys. The concentration of Pb in Pb-based solder alloy is reduced to 04% as compared to conventional 63Sn-37Pb to get the properties of Pb and make the environment as green as possible.
Table 1. Comparative properties of elements in electronic materials [14].
Properties
Pb
Sn
Ag
Cu
Melting Point, (°C)
327.5
232
962
1083
Electrical Resistivity,(µ Ω. cm) Thermal Conductivity, (W m-1. K-1)
20.8 33
13-20 63.2
1.55 419
1.7 385
Coefficient of thermal expansion, CTE (ppm K-1),~300K
29.1
15-30
19.6
16.4
Entropy of melting (J mol-1 K-1)
8.3
14
9.2
9.6
Elastic modulus (GPa)
10-40
22-69
44-120
67-192
Yield Strength (MPa)
