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SMT Perspectives and Prospects: Can Microstructure Indicate a Good Solder Joint? Part II
October 24, 2012 |Estimated reading time: 4 minutes
Editor's Note: This column originally appeared in the September 2012 issue of SMT Magazine.Last month’s column ended with: “Additionally, the microstructure of the solder joint is affected by the process used.” This month I’d like to address the impact of that process.
With all other conditions being equal--the same solder alloy, the same substrate surface finishes, and the same components and PCB--the microstructure of solder joints can indeed vary with the process parameters. For a given system, the parameters affecting the formation of microstructure during the solder joint-making process include both heating and cooling conditions. Let’s look at the heating stage and the cooling stage separately.
Microstructure Versus Solder Joint Making Heating Parameters
For the purpose of this discussion, we separate the PCB assemblies into two groups--one group of assemblies that are prone to the formation of intermetallic compounds (IMCs) at the interface and another group that are relatively sluggish in the formation of IMCs under commonly used soldering conditions. For a given solder alloy, the two groups are demarcated by the material elements of the surface finish on the PCB and components. Immersion Sn, immersion Ag, OSP, and HASL belong to the former group, and Au/Ni and Au/Pd/Ni fall in the latter group, regardless of the architecture and type of component or the structure of the PCB in an assembly. The nature of the substrate and its metallurgical affinity to solder composition can affect the development of the microstructure of solder joint.
In the heating stage of a process, key parameters are the peak temperature and the time above liquidus. A higher peak temperature and/or a prolonged heating above liquidus temperature will produce excessive intermetallic compounds at the interface and in the interior of a solder joint for those material systems that are prone to metallurgical reactions. The excessiveness can be considered in three structural areas: The interface, the bulk of the solder joint, and the surface of the solder joint. The conditions that promote the formation of excessive IMCs increase the IMC thickness at the interface. When the peak temperature is high enough, and when the in-liquid state is prolonged, IMCs will grow and migrate toward the interior of solder joint. As an illustration, in the case of Sn-based solder on OSP, the CuxSny compounds (commonly Cu6Sn5, Cu3Sn) formed at the interface may migrate into the interior of solder joint, resulting in additional CuxSny phases in the microstructure.
In extreme cases, IMCs may emerge on the free surface of the solder, causing the change in solder joint appearance. This change in appearance is the direct reflection of change in microstructure. All three mechanisms and phenomena are expected to adversely affect the solder joint, either aesthetically or mechanically. The mechanical properties in relation with microstructure will be discussed in a future column.
Microstructure Versus Solder Joint Making Cooling Parameters As to the cooling effect, it is understood that the faster the cooling rate is applied, the finer the microstructure will result. For a SnPb eutectic alloy, a slow cooling rate renders the microstructure to approach equilibrium conditions. The microstructure of eutectic composition normally consists of the characteristic lamellar colonies as exhibited in 63Sn37Pb. As the cooling rate increases, the degree of degeneration of lamellar colony structure increases and colonies eventually disappear. For lead-free, such as SnAgCu, a faster cooling rate also results in finer Sn grains.
Although it is generally accepted that a faster cooling rate creates finer grain (phase) structure in bulk solders, this general rule is often complicated by the interfacial boundary and metallurgical reaction at the interface of solder joints. Figure 1 is an SEM micrograph of a 63Sn37Pb solder joint, comprising the light Pb-rich phase and the dark Sn-rich phases, and exhibits the compositional gradient from the interface toward the interior bulk of the solder joint. Figure 2 shows an SAC305 solder joint consisting of the Sn grains and the CuxSny and SnxAgy IMC precipitates.Figure 1: An SEM micrograph of a 63Sn37Pb solder joint.
Figure 2: An SAC305 solder joint consisting of the Sn grains and the CuxSny and SnxAgy IMC precipitates.
A Picture is Worth a Thousand Words
A microstructure obtained in the form of a high-quality SEM or metallographic micrograph fits well the adage: “A picture is worth a thousand words.” It provides sights and insights into the state of solder joint integrity and anticipated behavior Reference: 1. Dr. Jennie S. Hwang, “Modern Solder Technology for Competitive Electronics Manufacturing,” Chapter 6, McGraw-Hill, ISBN-0-07-031749-6.
Dr. Hwang, a pioneer and longstanding contributor to SMT manufacturing since its inception as well as to the lead-free development, has helped improve production yield and solved challenging reliability issues. Among her many awards and honors, she has been inducted into the WIT International Hall of Fame, elected to the National Academy of Engineering and named an R&D Stars to Watch. Having held senior executive positions with Lockheed Martin Corporation, Sherwin Williams Co., SCM Corporation and IEM Corporation, she is currently CEO of H-Technologies Group providing business, technology and manufacturing solutions. She is a member of the U.S. Commerce Department’s Export Council, and serves on the board of Fortune 500 NYSE companies and civic and university boards. She is the author of 300+ publications and several textbooks and an international speaker and author on trade, business, education and social issues. Contact her at (216) 577-3284; e-mail JennieHwang@aol.com.