Intermetallic compounds consist of a homogenous phase of two or more materials that form prior to soldering, after soldering, and during service. There are some key facts about intermetallic formations that should be illustrated in order to prevent reductions in solderability, reliability, and yield. Intermetallics are necessary, but can result in embrittled joints and unsolderable components or circuit boards. The EMPF has participated in studies of intermetallic formations and their effect on solder joint fidelity. This article outlines the basic science of intermetallic formation and includes guidelines for improving solder joints through intermetallic management.

Dissolution
Dissolution is necessary for forming the initial intermetallic compounds found in solder joints. Dissolution is the chemical change that takes place as solid materials melt into liquid materials. Dissolution of metal substrates and their protective metal coatings occurs during soldering operations including wave soldering, reflow, rework, and repair.

Precious metal surface finishes (i.e. silver, gold, and palladium) and base substrates (copper and nickel) are involved in the dissolution process. The rate of dissolution is dependant upon the composition of the base metal and solder, cleanliness, and solder velocity. The dissolution rate also varies exponentially with temperature. The most common solder-substrate intermetallic compound is tin-copper (Sn-Cu). This compound forms due to tin from the solder and copper from the substrate during soldering operations. While the solder is molten, tin and copper both dissolve and become one homogeneous Cu6Sn5 compound. On average, the typical tin-lead soldering process will produce approximately 1-2 µm of Sn-Cu intermetallic compound. The amount of intermetallic formed will vary with the amount of time the solder joint is held above liquidus temperature. Therefore, tight process controls are necessary when soldering tin-based solders to copper leads or pads. The compound is both mechanically and electrically stable, and provides the necessary mechanical, thermal, and electrical connection between the substrate, lead and the solder.

The dissolution rate of nickel into tin is a lot slower than that of copper. This results in a nickel-tin (Ni3Sn4) intermetallic that is only 0.25-0.75µm thin after a normal soldering process. The Ni-Sn intermetallic compound tends to be more brittle than Sn-Cu compounds. Nevertheless, Ni is often used as a diffusion barrier because of its slow dissolution rate.

Alloying effects that result from molten solder-substrate dissolution cause embrittlement of some solder joints. Particularly sensitive to leaching and the resulting alloy are solder joints with smaller cross-sectional area and rigid substrates. Gold (Au) atoms fall out of solution and form a gold-tin intermetallic when the gold concentrations in tin-lead solders are in excess of 3-4 wt%. AuSn4 is a brittle intermetallic that will form in the bulk solder and at the interface reducing the strength of the joint. Figure 3-1 depicts an X-ray spectroscopy image of a tin-silver solder joint microstructure with severe gold embrittlement. The alloying that occurs not only reduces the overall compliance of the joint, but can also create stress concentration areas where solder joint failure can occur.

Small additions of other alloying elements can counteract the leaching effect. Silver is commonly added to solder applications that utilize silver or gold plated wires. Utilization of solder containing 1-3 wt% silver can significantly reduce the dissolution rate in tin-lead alloys . The use of silver-palladium (Ag-Pd) and platinum-gold (Pt-Au) surface metallizations significantly decreases the dissolution of silver and gold metallizations. Silver and cadmium (Cd) are also used to reduce the dissolution rate of copper into solder. Ag-Sn-Cd intermetallics are formed at the joint/pad or joint/lead interface that acts as a diffusion barrier to the copper.

Intermetallic Growth
The solid state growth of intermetallic compounds is diffusion controlled. The growth rate is linear with the square root of time at a given temperature and exponential with temperature. Excessive intermetallic growth provides sites for crack initiation and propagation. During aging, Cu6Sn5 intermetallics that are formed immediately after reflow develop Cu3Sn layers that grow adjacent to copper pads or leads. These intermetallic formations are brittle and should be avoided. Figure 3-2 shows copper-tin intermetallic growth in a high-temperature environment; specifically, interfacial intermetallics between solder and copper substrate immediately after reflow (left), and after aging at 170ºC for 21 days (right).

In addition to the embrittling effect of interfacial intermetallic growth, Kirkendall porosity often leads to reduced mechanical strength of the solder joint at the interface. Kirkendall porosity occurs when the diffusing rates of two or more elements are not the same. When one element diffuses faster than the others, vacancies are formed in the material with the higher diffusion rate. The vacancies accumulate to form a line of voids that severely diminishes mechanical stability.

The common failure mechanism for boards that have been thermally cycled or thermally aged is fracturing of the solder in the Pb-rich areas where Kirkendall voiding has occurred. This should be considered when the assembly or system is stored or used in high-reliability, high-temperature, or uncontrolled environments.

The reduced solderability of component leads caused by intermetallic layers is of major concern for those that use pre-tinned components or substrates. Intermetallic growth occurs as plated components or substrates are stored above ambient temperature for extended periods of time. As the intermetallic layer grows and consumes the fusible layer, it may penetrate the surface. The exposed surface intermetallic quickly oxidizes and provides a surface that does not wet easily. The time needed for the intermetallic to overcome the surface finish is dependent on the thickness of the coating and the storage atmosphere. Storing components in an inert atmosphere and specifying adequate coating thickness are the most common methods for preventing poor solderability caused by intermetallic growth. Diffusion barriers, such as Ni plating beneath the surface finish, are also used because of the slower growth rate of the intermetallic formed.

Intermetallic growth has proven to be a barrier for some rework applications. At the time of rework, intermetallic layers from previous soldering operations have already formed. Upon the removal of solder, intermetallic areas may be exposed causing a severe reduction in solderability. Consequently, the typical response to non-wetting or de-wetting pads has been to increase temperatures, pressure on the pad, and holding times. This results in the degradation of board materials and reduces reliability.

Lead-free Solders and Intermetallics
With the implementation of lead-free solders and surface finishes there has been an increase in concern about intermetallic formation and solder joint contamination. Fortunately, many of the intermetallic systems found in lead-free solders and surface finishes are similar to that observed in leaded systems. There are, however, some slight differences.

Where tin-lead alloys exhibit distinguished tin-rich and lead-rich grains, the majority of tin-based lead-free alloys exhibit intermetallic structures within the tin matrix. These intermetallic structures are composed of a ratio of tin and some other elemental constituent of the alloy (i.e. Ag3Sn). Because of the relatively low fraction (3-5 wt%) of alloying elements, these intermetallic structures comprise a small portion of the area within the solder joint. The morphology varies, exhibiting a round, lathlike, blocky, or needle-like structure. Studies have shown, however, that intermetallic structures at the interface, such as tin-copper, grow slower in some tin-based lead-free solders than with their leaded counterpart. It is believed that lead plays a part in enhancing intermetallic growth when subjected to thermal exposure.

In conclusion, reductions in solderability, reliability, and yield, caused by intermetallic formation and growth, can be prevented by following three generic guidelines:

1) In situations where the solder joints are exposed to high
temperature service conditions, diffusion barriers will slow the growth of intermetallics and increase the reliability of the joint.

2) To prevent solder joint enbrittlement, careful control over the surface finish is required when soldering to gold and silver.

3) When using tin-based surface finishes such as plated tin or
solder coatings, the storage environment should be controlled to curb intermetallic growth that can affect solderability.