Lead-free Solders in Microelectronics
Introduction
Soldering is a well-known metallurgical joining method that uses a filler metal, the solder, with a melting point below 425°C [1]. In the immense electronic materials world, solder plays a crucial role in the assembly and interconnection of the silicon die (or chip). As a joining material, solder provides electrical, thermal and mechanical continuity in electronics assemblies. The performance and quality of the solder are crucial to the integrity of a solder joint, which in turn is vital to the overall functioning of the assembly. Solders are used in different levels of the electronic assembly sequence, as shown in Fig. 1. As a die bonding material, the solder provides the electrical and mechanical connection between the silicon die and the bonding pad. It also serves as a path for dissipation of the heat generated by the semiconductor. Bonding of the die to a substrate and its encapsulation is referred to as Level 1 packaging. While the predominant method of providing electrical connection to the silicon chip is through wire bonding, the use of solder bumps on the surface of the Si die, instead of wire bonding, has been gaining acceptance recently, due to the higher number of input/output terminals that can be attached to a given area. The flip chip configuration, a cross section of which is shown in Fig. 2, is such an approach. The Si die is turned ‘upside down’, hence flip chip, and mounted on an appropriate substrate.
The next level of assembly and interconnect, referred to frequently as Level 2 packaging, is where the component (encapsulated silicon die) is mounted on a printed wiring board (PWB). Solder is the primary means of interconnect in Level 2 packaging. Practically all microelectronic devices (also known as packages) are mounted on PWBs using solders. There are two primary means of attaching electronic components to PWBs — pin-through-hole (PTH) or surface mount technology (SMT), illustrated in Fig. 3, Fig. 4, respectively. Surface mounted electronic components can either be leaded, i.e. with leads, as shown in Fig. 4 or have solder balls that are called ball grid arrays (BGAs), as shown in Fig. 5. The locations where solders are used are also shown.
The assembly and soldering of Printed Circuit Board Assemblies (PCBA) can involve pure surface mount components or mixed technology assemblies where both SMT and PTH are used, either in single- or double-sided PCBA configuration. Soldering of surface mounted devices, commonly called reflow soldering is done by application of solder paste on one of the mating surfaces, usually on soldering pads located on the PCB, and heating the assembly to melt the paste, which upon solidification forms the joint. The solder paste is a mixture of solder powder, flux and other additives forming a thick cream. Additives are included in the paste to promote wetting (surfactants) and to control the property of the paste (tackiness, slump, viscosity, etc.). Reflow soldering is defined as ‘the joining of mating surfaces that have been tinned and/or have solder between them, placing them together, heating them until the solder fuses, and allowing them to cool in the joined position’ [2].
As shown in Fig. 6, PTH soldering is done by wave soldering, where the assembly is transported over a molten solder bath from which the solder rises and forms solder joints by capillary action. The properties of the solder that are important are invariably process dependent. The formulation and printability of the solder paste are critical parameters for reflow soldering, while viscosity and density of the molten solder significantly influences the performance of the wave soldering process.
Hitherto, solder usage had been restricted primarily to the board level assembly process, i.e. Level 2 packaging, with very little being used in Level 1 packaging. However, with the advent of area array packaging concepts (flip chip and ball grid arrays), usage of solders in Level 1 packaging is increasing sharply. In the ‘flip chip’ approach the silicon die has an array of solder bumps placed on it, usually by plating or vapor deposition.
For Si dies that have their terminals on the periphery, and are therefore wire bonded, the trend has been to attach the component to the board via the area array concept, by using solder balls, as shown in Fig. 5. The solder balls are attached to the substrate, and the entire assembly is processed through a reflow oven. The solder ball melts and forms a joint between the solder ball pad of the substrate and the ball itself.
In board level packaging the solder used is primarily 63Sn–37Pb, a eutectic composition, or 60Sn–40Pb, a near eutectic composition. With a melting eutectic temperature of 183°C, the Sn–Pb binary system allows soldering conditions that are compatible with most substrate materials and devices. As one of the primary components of eutectic solders, Pb provides many technical advantages, which includes the following to Sn–Pb solders:
- 1.
Pb reduces the surface tension of pure tin, which is 550 mN/m at 232°C, and the lower surface tension of 63Sn–37Pb solder (470 mN/m at 280°C) facilitates wetting [3].
- 2.
As an impurity in tin, even at levels as low as 0.1 wt.%, Pb prevents the transformation of white or beta (β) tin to gray or alpha (α) tin upon cooling past 13°C. The transformation, if it occurs, results in a 26% increase in volume and causes loss of structural integrity to the tin [4].
- 3.
Pb serves as a solvent metal, enabling the other joint constituents such as Sn and Cu to form intermetallic bonds rapidly by diffusing in the liquid state.
These factors, combined with Pb being readily available and a low cost metal, make it an ideal alloying element with tin. The board level soldering system that is mainly based on eutectic and near eutectic Sn–Pb solders has been well developed and refined with many years of experience. A relatively well-established knowledge base about the physical metallurgy, mechanical properties, flux chemistries, manufacturing processes and reliability of eutectic Sn–Pb solders exists. Board level assembly and soldering equipment are almost exclusively engineered with Sn–Pb solder in mind. A good understanding of the behavior of Sn–Pb solders has enabled current board level technology to assemble and create small geometry solder joints, approaching 75 μm in size, in high volume, and at competitive cost. Nevertheless, there are legal, environmental and technological factors that are pressing for alternative soldering materials and processing approaches. These factors include:
- 1.
Legislation that tax, restrict or eliminate the use of Pb due to environmental and toxicological concerns.
- 2.
The continued trend towards packaging and interconnect miniaturization in SMT that is stretching the physical capability of Sn–Pb solder to provide sound and reliable solder joints. The natural radius of curvature of molten solder, R, as determined by surface tension, (R=(γ/ρg)1/2=2.2 mm) [5] is already larger than the sizes of the solder joints of SMT devices with less than 0.5 mm pitch. This means that forcing the solder to form joints with a smaller radius of curvature can result in runaway of solder from desired locations due to high internal liquid pressure.
- 3.
The need for better paste printing capability that is required for fine pitch SMT. The minimum distance between adjacent soldering pads for optimum paste application is dependent on the edge definition of the print itself. This depends on the granular nature of the paste, which in turn is dependent on the natural radius of curvature.
- 4.
The need for cascade soldering of complex assemblies that require different types of solders with different melting temperatures.
There are several Pb-free solders, such as Sn–Au, Sn–In, Sn–Ag, Sn–Bi, that have been in use in the electronics industry for special applications. Major solder paste vendors have research programs targeted at developing new alloys that can be ‘drop in’ replacements for eutectic Sn–Pb. Many of these new alloys are ternaries and quaternaries, rather than binaries, and information regarding the characteristics of these alloys is frequently proprietary; there is relatively little information available in the open literature.
In this paper a review of the existing literature on the metallurgy of Pb-free solders for microelectronics applications is presented. To this end, the physical, electrical, chemical and mechanical properties, including the physical metallurgy, of alternative solders are discussed. The corrosion behavior of Pb-free solders as applicable to microelectronics is also described. First, a brief description of the legislative and regulatory actions advocated to restrict the use of Pb is provided. However, prior to that, in order to avoid confusion, a brief description of the two drastically different usages of the word ‘lead’ is provided.
Due to possible confusion arising from the use of the word ‘lead’, which could have two different meanings, the chemical symbol Pb is used when the metallic element lead is being referred to. When fully spelt out as ‘lead’, it stands out for ‘a self-supporting path which connects the electrical component to the outside world’, as shown in Fig. 4.
Section snippets
Adverse health effects of lead
Lead and Pb compounds have been cited by the Environmental Protection Agency (EPA) as one of the top 17 chemicals posing the greatest threat to human life and the environment [6]. When Pb accumulates in the body over time, it can have adverse health effects. Lead binds strongly to proteins in the body and inhibits normal processing and functions of the human body. Nervous and reproductive system disorders, delays in neurological and physical development, cognitive and behavioral changes,
Performance characteristics of solders
There are strict performance requirements for solder alloys used in microelectronics. In general, the solder alloy must meet the expected levels of electrical and mechanical performance, and must also have the desired melting temperature. It must adequately wet common PCB lands, form inspectable solder joints, allow high volume soldering and rework of defective joints, provide reliable solder joints under service conditions and must not significantly increase assembly cost.
When trying to
Solder alloy costs
The microelectronics industry is extremely cost conscious. The history of the industry has been to continuously produce higher performance at lower costs. Since cost of the product is the resultant of the cumulative cost of the components, the cost of Pb-free solder alloys can impact the cost of the finished product. Cost competitiveness in the electronics industry is maintained by reducing the cost of individual components to a minimum, in order to maximize the overall cost reduction.
The unit
Pb-free solder alloy compositions
A relatively large number of Pb-free solder alloys have thus far been proposed, and are summarized in Table 5, with their elemental compositions. The solder alloys are binary, ternary and some are even quaternary alloys. A total of 69 alloys were identified from the literature. It can be noticed that a very large number of these solder alloys are based on Sn being the primary or major constituent. The two other elements that are major constituents are In and Bi. Other alloying elements are Zn,
Melting/liquidus temperature
From a manufacturing perspective the melting temperature, i.e. the liquidus temperature, is perhaps the first and most important factor. The eutectic temperature of Sn–Pb is 183°C, and most of the assembly equipment in use today is designed to operate using 183°C as a base reference. While some variation in the baseline temperature, for example 50°C, can be accommodated by the equipment currently in place, if the melting point of the replacement Pb-free solder is significantly higher, then new
Wetting characteristics
To form a proper metallurgical bond between two metals, wetting must take place. This means that a specific interaction must take place between the liquid solder and the solid surface of the parts to be soldered. The ability of the molten solder to flow or spread during the soldering process is of prime importance for the formation of a proper metallic bond. The term ‘wetting’ is often used when discussing soldering processes. The phenomenon of spreading is also frequently referred to as
Solder–substrate interactions
During soldering, molten metal (solder) comes into contact and reacts with the Cu pads on the substrate or PCB. The nature of this interaction is dependent on the composition of the solder, and how each of the constituents interacts with the Cu pad. Whether or not the Cu pad is coated, which it frequently can be, also affects the nature of the interaction with the constituents of the solder. Of fundamental significance here is the long-term reliability of the solder joint, which is directly
Mechanical properties
When an electronic device is in operation, the solder connections are subjected to mechanical stresses and strains. The primary cause of these stresses and strains arise from the fact that the electronic component and the board have different coefficients of thermal expansion. An example of how these stresses are generated, in the case of flip chip packages, between the silicon die and the substrate, is shown in Fig. 9. If room temperature represents the unstrained conditions, then as the
Coefficient of thermal expansion
A typical microelectronics assembly is made up of a large variety of materials, most notably, metals, polymers, polymer based composites and sometimes ceramics. During its service life the device goes through heat cycles because every time the device is powered, heat is generated due to IR heating. Localized temperatures on the Si die itself can be as high as 300°C. If all of the components within the device have identical coefficients of thermal expansion (CTE), and heat transfer is
Electrical properties
When the microelectronics device is functioning, the solder also serves as an electrical interconnect, i.e. all the electrical currents going into and out of the silicon device must pass through the solder connection.
In order to function adequately as an electrical interconnect, the electrical resistivity is the property of interest. The electrical conductivity of the solders chosen must be sufficiently high to permit current flow for the geometry chosen, without IR heating. While this is very
Chemical properties
Three major chemical properties can affect the usage and long-term reliability of solders. They are: (a) solubility of Cu in the solder, (b) resistance to corrosion, and (c) oxidation behavior.
Toxicity
The extent to which Pb-free solders themselves are ‘environmentally friendly’ is also relevant. There are two basic requirements here: toxicity and recyclability. While a solder might be Pb-free, if it contains other toxic metallic elements such as Cd, then the whole motivation behind the drive for Pb-free solder development will be violated. Of particular importance here are the acceptable concentrations in ground water, and potable water, and the leachability of the solder alloys. The alloys
Other manufacturing issues
In the previous sections, the properties of an array of Pb-free solders have been presented. In cases where experimental data were not available, estimates were made based on fundamental considerations. The availability of engineering data, although a prime consideration, is still only one of the factors that are important for the deployment of Pb-free solders. The engineering data are crucial for the design engineer, to enable the design of interconnects, and to estimate its lifetime. The
Discussion
The information on Pb-free solders that are available thus far falls primarily in the category of basic properties. Two further items that are necessary before concrete steps towards across-the-board adoption of Pb-free solders are still missing, namely (a) data on actual performance and reliability testing, and (b) a systematic materials selection procedure that is based on decision theory. After surveying several customers (users of solders), Witt found that the most critical of the desired
Conclusions
Pending environmental legislation worldwide has provided an impetus towards the development of Pb-free solders. A relatively large number, approximately 70, of Pb-free solders have been proposed so far, by a combination of researchers and manufacturers. While a few of the alloys are patented, most are not. The majority of the alloys are based on Sn, In, and Bi being the primary component, with Sn being by far the most dominant. Materials selection studies thus far, albeit on a limited scale,
Acknowledgements
The authors wish to acknowledge the assistance of the following individuals: Mr. Charles Barnhardt, of SCI Systems Inc., Daniel Pfahl who began construction of the database, Libin Chang and Jayamalar Vijayen who assisted in preparation of this manuscript, and Nicholas Dugbartey who provided some of the graphics.
References (99)
Interdiffusion and reaction in bimetallic Cu–Sn thin films
Mat. Chem. and Physics
(1996)- H.H. Manko, Solder and Soldering, 2nd Edition, McGraw-Hill, New York,...
- ASM International, Electronic Materials Handbook, Vol. 1, Packaging, Materials Park, OH, 1989, p....
- T.P. Vianco, Development of alternatives to lead-bearing solders, in: Proceedings of the Technical Program on Surface...
- R.E. Reed-Hill, Physical Metallurgy Principles, PWS Publishing Company, Massachusetts, 1994, pp....
- K.J.R. Wassink, M.M.F. Verguld, Manufacturing Techniques for Surface Mounted Assemblies, Electrochemical Publications...
- et al.
In search of new lead-free electronic solders
J. Electron. Mater.
(1994) - E.R. Monsalve, Lead ingestion hazard in hand soldering environments, in: Proceedings of the 8th Annual Soldering...
- D. Napp, Lead-free interconnect materials for the electronics industry, in: Proceedings of the 27th International SAMPE...
- et al.
An assessment of the use of lead in electronics assembly
Circuit World
(1993)