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Reliability of Sn/Cu/Ni Solder Joints
Reliability of Sn/Cu/Ni Solder Joints
Paper covers the electronics industry search for a single lead-free alloy that can be used throughout the manufacturing line.
Analysis Lab

Authored By:
Brian Roggeman
Universal Instruments Corporation
Binghamton, NY, USA

Ursula Marquez de Tino and Denis Barbini
Vitronics Soltec
Binghamton, NY, USA
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Transcript
The electronics manufacturing industry has continued its search for a single lead-free alloy that can be used throughout the manufacturing line. The ability to employ one alloy would reduce complexity and cost while increasing yield.

The tin/copper/nickel alloy has been widely used in wave soldering applications for many circuit board types. This alloy is characterized by lower rates of reaction with base materials; the lack of precious metals makes this alloy less expensive; and, the cosmetics of the final solder joint is similar to that of tin lead.

However, this alloy's focus and integration into reflow soldering applications has been hampered by its higher melting point. Before widespread use of the alloy is adopted for surface mount applications, extensive qualification is required.

In this study, the reliability is further quantified through both thermal cycling and further mechanical test.
Summary
The electronics manufacturing industry has continued its search for a single alloy that can be utilized throughout the manufacturing line. To date, there is minimum success in the implementation of an alloy across the manufacturing process as compared to the tin lead based processes. The ability to employ one alloy will allow manufacturers to reduce complexity and cost while increasing yield.

The Sn/Cu/Ni alloy (SCN) has been widely used in wave soldering applications due to its applicability in achieving acceptable soldering results for many printed circuit board types. In addition, the SCN alloy is characterized by lower rates of reaction with base materials such as copper and iron [1]; the lack of precious metals makes the SCN alloy less expensive; and, the cosmetics of the final solder joint is similar to that of tin lead. However, the SCN alloy's focus and integration into reflow soldering applications has been hampered by its higher melting point temperature of 227°C. At temperatures required for complete and homogeneous mixing of the paste deposit with the component lead/bump, there are concerns such as possible damage of heat-sensitive components and joint reliability.

Before widespread use of the alloy is adopted for surface mount applications, extensive qualification is required [2]. Through a previous study [3], it was shown that typical SAC assembly profiles achieved satisfactory SCN solder joints, and the vibration reliability results indicated similar performance of SCN solder joints to SAC305. In this study, the reliability is further quantified through both thermal cycling and further mechanical test. Drop testing was chosen as the mechanical test and a comparison in the solder's performance is made to SAC305 and SAC105.

The results show that, while SCN better matches SAC105 in solder ball strength testing, the performance in board level drop is very similar to SAC305. Thermal cycling results also appear very similar to SAC305.
Conclusions
As a continuation to a previous study, the reliability of tin-copper-nickel solder alloy in surface-mount assemblies was examined. The results were compared to typical SAC based assemblies.

Both studies showed that SCN solder paste can be reflowed using SAC profiles. Peak temperatures of 2380C and TAL (227°C) of 30 seconds are sufficient to obtain good solder joint formation.

The appearance of SCN solder joints is similar to SAC joints.

Ball shear testing showed that SCN solder joints are generally weaker than SAC305 and SAC105 joints, but at the highest test speeds the differences are negligible.

Drop testing results showed that SCN assemblies were very similar to SAC305, however reliability of each was less than SAC105.

Thermal cycling showed that SCN assemblies were very similar to SAC305.

The mechanical testing indicates that solder strength alone cannot indicate the reliability in a repeated high strain rate shock environment. Because the SCN alloy has a markedly different Sn-grain structure, the actual mechanical behavior and stress/strain relationships are also likely to be different from SAC based systems.

SCN alloy appears to be well suited to use in surface mount applications with minimal adjustments being necessary to the assembly process. Reliability can be expected to be similar to SAC305,
Initially Published in the SMTA Proceedings
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