|With the ban on lead (Pb) by the European Union (EU), the electronics industry has sought alternatives to replace the long used Sn-Pb solder. Although Sn-Ag-Cu (SAC) alloys with Sn3Ag0.5Cu (SAC 305) have been considered as a replacement option, a clear understanding of the mechanical and reliability aspects of these alloys is very important. With the increasing use of mobile electronics, the reliability of SAC 305 alloys is under question due
to its poor drop/shock performance. Even though suggestions to use less silver content to increase the alloy performance are shown in the literature, they suffer from increased failure rates when exposed to higher operational temperatures and thermal cycles.
A search for an optimized Sn-Ag-Cu (SAC) alloy with good drop performance (SAC 105) and a better thermal cycling reliability (SAC 305) is ongoing to increase reliability. Many lead-free solder pastes with additives such as Mn, Cu, Ni etc are being tested by numerous researchers to determine their drop/shock and thermal cycle reliability.
In this work Sn/0.7Cu/0.05Ni/Ge paste, manufactured by Nihon, was used to assembly various SMT packages and its assembly characteristics were studied. On a 60 mil thick board, land grid arrays (LGA), quad flatpack no lead (QFN) and resistors were assembled using SAC 305, SAC 105 and Nihon paste. Boards were also built using Sn-Pb paste to serve as a reference. Assembly characteristics of all the pastes were studied, identifying ideal
parameters for successful surface mount technology (SMT) implementation.
The assembled boards where subjected to accelerated thermal cycle testing from -55C to 125C, with 10 minutes dwells at high temperature, 5 minutes dwells at low temperature and 30 minutes ramps. The total cycle time was 90 minutes. The thermal cycle results as a function of both alloy and the package type were studied.