Complexity of Problem

Solder joints of electronic assemblies are complex elements that cannot be studied using the traditional techniques of structural analysis or fatigue of engineering metals. The acknowledged complexity of the mechanics of solder joints arises from the following:

• The problem is three-dimensional (3D) with solder joints subjected to a system of distributed, multiaxial forces and moments exerted by the interconnected parts. Even when taking advantage of symmetries, all joints are not equal because of varying distances to the neutral axis of an assembly and variability in joint geometry and metallurgy.
• A solder joint is a multi-layered, non-homogeneous structure. Reflowed solder is sandwiched between thin layers of intermetallic compounds. In the case of SnPb, solder itself is made up of lead- and tin-rich phases with variations in composition near the intermetallic layers, e.g., tin-depleted regions on the board side due to the formation of Cu-Sn intermetallic compounds during reflow. Moreover, the SnPb microstructure evolves in service. The microstructure coarsens due to thermally activated grain growth, a phenomenon that takes place under stress or at constant temperature.
• The mechanical behavior of solder is highly non-linear and temperature dependent. Solder creeps readily at ambient temperature (and below) and creep rates increase with temperature.
• Failure of SnPb solder joints is a complex sequence of events involving microstructural coarsening, matrix creep, grain boundary sliding, micro-void formation and linking, crack initiation and crack growth. In the case of SAC and SnAg solder joints, the damage accumulation process leads to much less coarsening of the microstructure, if any.
• Most often, electrical opens resulting from solder joint failures are intermittent and may be difficult to detect accurately. Electrical continuity may still be maintained when a solder joint is fully cracked because of contacts between asperities on the opposite surfaces of the crack. This may result in hard to detect failures and “No Trouble Found” (NTF) diagnosis during troubleshooting.

In spite of all this, significant progress has been made in the understanding of SnPb solder joint mechanics, fatigue and failure. Although the study of SnPb assembly reliability is a semi-empirical science, a vast body of knowledge, simplified engineering models, test data and experimental findings has accumulated that provides useful insight into the mechanical behavior of solder joints of real assemblies.