SMTA International Conference Proceedings


Authors: Jorge A. Manriquez, Ph.D. et al.
Company: Tecnologico de Monterrey
Date Published: 9/25/2005   Conference: SMTA International

Abstract: This paper reports the results of an experimental study designed to evaluate the effects on solder joint reliability using lead-free solders and tin-finished components on boards with lead-base metallizations. Variables in the study were solder chemistry (Sn63Pb37, Sn95.5Ag3.8Cu0.7, Sn96.3Ag2.5Cu0.7Sb0.5) and thickness of nickel barrier (50 µin, 30 µin) below the tin finish of 0603 BaTiO3 ceramic capacitors. Test vehicles were HASL-Sn90Pb10 finished.

SMT boards were assembled with printing parameters and reflow profiles optimized for each condition, and fillet formation inspected according to ANSI/J-STD-001B. The first portion of the study is an isothermal test at 125 °C and 25V DC (MIL-STD-202G) to evaluate formations of Cu-Sn and Ni-Sn intermetallic compounds (IMC) at solder-metal interfaces. Twenty-four boards with 20 capacitors each were evaluated. IMC thickness measurements were taken from samples after 0, 65, 130 and 264 hours, using a SEM/EDX unit. Capacitance measurements were taken at the start and the end of the isothermal aging test.

The second portion of the study used a thermal cycling stress condition (IPC-SM-785) using a +25/+100/+25°C cycle with 15 minutes dwell time at each temperature extreme and a 15 °C/minute heating/cooling step. Forty-two boards with 20 capacitors each were evaluated according to failing criteria by loss of electrical continuity and by maximum variation of capacitance. It was found that growth of Cu3Sn and Ni3Sn IMCs along the solder-metal interfaces was very similar for all type of solder chemistries. All tested conditions produced assemblies that complied with expected reliability.

Two capacitors presented a higher than 25% reduction in capacitance before 1000 hours, both from the antimony containing lead-free solder joints when capacitors had a 30 µin Ni barrier. F-test showed that capacitance variance increased with cycling for samples with the 30 µin nickel barrier. The paper discusses results and proposes possible relationships between joint-level reliability and studied variables.

Key words: reliability, lead-free, solder joints, SAC.

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