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SMT Perspectives and Prospects: Can Microstructure Indicate a Good Solder Joint? Part I

Editor's Note: This column originally appeared in the August 2012 issue of SMT Magazine.Microstructure Versus Dimensional Scale

On the subject of microstructure, let’s first look at what constitutes a microstructure in terms of the dimensions of the object to be examined and its detectability in relation to the analytical instruments.

When dealing with physical objects in the linear dimension larger than 102 microns, we work on structure engineering. If we desire to view the object in the scale of 10-10 to 102 microns, we study material science and nuclear physics. This includes nanotechnology, which is scoped as the study of manipulating matter on an atomic and molecular scale, and generally deals with developing materials, devices, or other structures with at least one dimension sized from 1 to 100 nanometers, which covers the quantum mechanical effects.  As the scale shrinks further, to less than 10-10 microns, the object to humans becomes intangible, immeasurable, and fussy; we then extend it to perhaps philosophy. Under this premise, the microstructure essentially falls in the range of 10-2 to 102 microns. The understanding of the solder joint within this range of dimensions is generally considered adequate in relating material properties to end-use applications, unless a deep dive into the science is the intended purpose.

Microstructure Examination

Solders are normally a polycrystalline that consists of an aggregate of many small crystals or grains. Most solder compositions contain multiple metallurgical phases in various sizes that are physically distinct and formed and distributed according to given thermodynamic and kinetic conditions. The finer structure, with features smaller than grains and phases, is referred to as submicrostructure, which exhibits atomic arrangement within grains and is associated with dimensional scale of angstroms. Coarser than microstructure is macrostructure which is discernible to human eyes without the aid of instrumentation. Under macro-examination, an instrument (microscope) up to 50X would be adequate; micro-examination requires higher than 100X, often 500X to 2,000X. Solder joint microstructure can be examined optically by means of an optical microscope or electronically by using a scanning electron microscope (SEM). By either technique, prior to examination, the samples need to be properly polished and etched in an established etching solution that is capable of revealing relevant metallurgical phases. When using optical or electronic microscope, the respective contrast of phases is opposite to each other.

For example, the well-understood 63Sn37Pb composition is typically composed of lead-rich phase and tin-rich phase in solid state below its eutectic temperature (183°C) in the scales of microns. Under SEM examination, the lead-rich phase is shown as a lighter phase and tin-rich phase as a darker phase; under an optical microscope, the reverse is true. The microstructure of lead-free solders, however, varies with the specific elemental compositions, which will be subsequently discussed in this series of columns.

To assess solder joint, an appropriate, strategically selected cross-section is important to the accuracy and meaning of the examination. In some cases, multiple cross-sections are required to reveal the state of solder joint.

Solder Material Versus Solder Joint

Before we discuss microstructure, another important note is the differentiation between solder material and the solder joint. The microstructure of solder materials and solder joints is each determined by its elemental makeup, yet the added effects in the solder joint are the natural and elemental content of substrates from which the solder joint interfaces are formed. It would not be surprising to see the microstructure of 63Sn37Pb solder joint interfacing with Ni-substrate differ from that with Cu-substrate. For example, a solder joint made with a PCB having Au/Pd/Ni surface finish may deviate from the microstructure or submicrostructure of a PCB that made with an OSP surface finish.

From an application and reliability perspective, one has to consider the microstructure of solder joints comprising the bulk solder, the interface with the top substrate (usually the component side) and the bottom substrate (usually PCB or hybrid circuits).

Additionally, the microstructure of the solder joint is affected by the process used.  Dr. Hwang will present two lectures on “Array Package Interconnection: Forward/Backward Compatibility and Reliability” and “Preventing Production Defects and Failures” at SMTA International Conference on October 15, 2012, in Orlando, Florida.

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.