February, 2020
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tech.com
Nihon Superior SN100CV Provides Reliability in Simplicity
By Keith Sweatman, Senior Technical Advisor, Nihon Superior Co., Ltd. I
n the search for lead-free solder alloys that can deliver high reliability for electronics that oper- ate in harsh environments, there has been a
trend to formulate complexity. The distinguishing feature of the new Nihon Superior alloy, SN100CV, is that it can deliver cost-effective relia- bility, without the microstructural instability of more complex formulations.
Failure Mechanism in Solder Joints The driver of the failure mechanism in lead-
free solder joints subjected to cyclic strain has been identified as damage accumulation, as the tin crys- tals that form the matrix of the joint respond to the forces that result from differences in the CTW of the various materials that make up the assembly. That damage takes the form of
pileups of dislocations on the slip planes of the tin crystal that, in a phenomenon known as “work hard- ening,” make it more difficult for fur- ther deformation to occur. The distortion of the crystal
structure associated with these pile- ups means that the area in which these dislocation pile ups occur is in a high energy state. Thermo - dynamics drives the diffusion of atoms to lower the energy state in the process, known as recovery. This recovery process results in the growth of new grains smaller than those in the as-soldered microstruc- ture.
Subsequent strain in these
recrystallized areas tends to be accommodated by grain boundary sliding and grain rotation that results in separation at boundaries between the grains, which forms the cracks that eventually lead to an increase in the electrical resistance of the joint and mechanical failure. Given this failure, the emphasis in the development of solder alloys for harsh environments has been on increasing the resistance of the alloy to deformation.
Strengthening Solder The two mechanisms available
for increasing the resistance of the tin matrix of lead-free solders to deformation are particle and solid solution strengthening. Fine particles dispersed through
the microstructure act as obstacles to dislocation. In solid solution strength- ening the obstacle to dislocation is the distortion in the crystal lattice of the tin matrix, caused by solute atoms that are larger or smaller than the tin atom that they replace. A trend in lead-free solder for-
mulation has been to add as much as possible of other elements, such as silver, copper and nickel, forming strengthening particles, or elements such as bismuth and antimony that have some solubility in the tin. At higher concentrations they can pre- cipitate from the tin to form particles and provide both strengthening mechanisms. While in the as-soldered condi-
tion solders with all of these addi- tions have formidable resistance to deformation, there are two disadvan- tages from which these alloys can suffer — lack of compliance and microstructural instability. An electronic assembly is made
up of a disparate collection of materi- als with a wide range of CTEs. A con-
Additional solder alloys: SAC305 / SAC0307 / Sn63 / 99C / NT100GeX
www.nathantrotter.com
Compete at a higher level with NT100Ge. Available in solder bar and wire, NT100Ge has an identical chemical composition and is a seamless drop-in replacement for the well-known SN100C® solder. Produced with the highest quality raw materials and delivering the industry’s best price-performance ratio, NT100Ge lowers cost of ownership, raises competitiveness and produces high-integrity interconnects.
Figure 1: (left) SN100CV, (right) SN100C+8Bi. Reflowed microstructure, peak temperature 460.4°F (238°C) (Photo courtesy of Sergey Belyakov, Imperial College London).
sequence is that in the thermal cycles to which the assembly is exposed as a result of on/off cycles or
changes in ambient conditions components that do not have flexible leads that can accommodate such strain can be subjected to forces sufficient to cause mechanical failure. In such circumstances, compli- ance, the ability to accommodate strain at stresses below those that could cause mechanical failure is a useful property in a solder.
Microstructural Instability in Solder Joints Because solder freezes relatively quickly, the
microstructure of the soldered joint is usually far from thermodynamic equilibrium. The system that constitutes a solder joint includes the substrates,
Continued on next page
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NT100Ge
Solder Smarter A seamless replacement for SN100C®
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