|Eutectic SnPb (Tin-Lead) solder has been used in electronics since early days. This universal
alloy combination was used in all soldering applications because it had good mechanical
and electrical properties and hence had good reliability. But, by the end of 20th century, harmful
effects of lead were identified. Since then the industry has been pushing toward lead free
electronics. In 2,000, the European Union put forward two directives towards lead free electronics.
The Waste of Electrical and Electronic Equipment (WEEE) directive stipulated that
lead should be removed from all electrical and electronic components at the end of life. The
Restriction of Hazardous Substances (RoHS) directive prohibits the use of lead in electrical
and electronic components manufactured after July 1, 2006. As a result of these and similar
directives, the industry went through an intense search for replacements. Research led to a
series of near eutectic alloys based on tin (Sn)-silver (Ag)-copper (Cu), commonly known as
SAC solder alloy.
With the advent of new materials, the reliability of solder joints became a major concern,
especially in harsh environment. In electronics, the reliability is typically limited by the fatigue
failure of a single solder joint. Aging makes the situation worse by altering the mechanical
and physical properties of the solder material. The precipitate coarsening and the growth of the
brittle intermetallic compound layer over time weakens the solder joint and hence deteriorates
the reliability. Several elements such as bismuth, nickel, antimony, cobalt and indium have
been microalloyed with the SAC based solder alloy to mitigate the adverse effects of aging.
In this study, thermal cycling reliability of aged SAC based solder alloys is examined.
Twelve solder pastes from the leading manufacturers with three solder spheres, namely, SAC105,
SAC305 and match (where solder paste is same as solder sphere) and three surface finishes
(ENIG, ImAg and OSP) are examined. For certain solder pastes, matching spheres could not be
used due to their unavailability. The test vehicle consists of three 15mmx15mmCABGA208s,
three 6mmx6mm CABGA36s, three 5mmx5mm MLFQFNs and a bank of six 2512SM resistors
connected in series. A printed circuit board was made of four layers of FR-4 glass epoxy
substrate with non solder mask defined pads. Several boards were aged at 125 C for a period of
twelve months starting immediately after the assembly. The aged boards were subjected to thermal
cycling in a temperature range -40 C to +125 C with 15 minutes dwell at +125 C and 10
minutes dwell at -40 C. The profile had a ramp time of 50 minutes, which corresponds to a rate
of about 3.3 C per minute. The components were continuously monitored throughout the test.
The failure data collected was used for statistical analysis using Weibull and DOE-ANOVA
methods. After the test, the failed components were cross-sectioned and analyzed using optical
and scanning electron microscopes to have an understanding about different failure modes.
From the study, it could be concluded that SAC based alloys with elements such as Bi,
Sb, and Ni had better fatigue resistance and therefore better reliability than SAC305 alloy.
ENIG finish with its Ni layer performed better than the other surface finishes. It could also
be concluded that the reliability depends on the combination of different factors considered in
the study (solder paste, sphere and finish). For example, just because of the fact that ENIG
performs better than ImAg and OSP, it does not guarantee that ENIG has better reliability in all
combinations of solder paste/sphere/finish as the material properties of the whole solder joint
vary with the combination (interaction effect of factors).