Meaning
Reduction in the mechanical load bearing capacity of a solder joint under a shear force over time due to thermal aging, environmental stress, or mechanical fatigue is a primary reliability concern in electronic assemblies. This material degradation, known as shear strength degradation, occurs as the microstructure of the solder joint changes during operation, with processes like grain coarsening, intermetallic compound growth, and microvoid formation weakening the metallurgical bond. It is measured by performing shear tests on solder joints at periodic intervals during thermal cycling or isothermal aging tests, comparing the measured force required to fracture the joint against the initial, unaged strength.
The analysis of this degradation is limited to the mechanical performance of the solder joint under shear loading, and it does not directly address other failure modes like tension or bending failures.
Fracture Mechanism
Mechanisms that drive this loss of mechanical strength are closely linked to the microstructural evolution of the solder joint under thermal and mechanical stresses. During the life of the electronic device, the solder joint is exposed to elevated temperatures that promote the diffusion of atoms, leading to the growth of the intermetallic compound layer at the solder-to-pad interface. While some intermetallic growth is necessary to form a bond, an excessively thick layer can act as a stress concentrator and a source of brittle fracture.
At the same time, the grain structure of the bulk solder coarsens, reducing its resistance to plastic deformation and creep, and allowing microvoids to coalesce into microcracks that propagate under shear loads, eventually leading to joint failure.
Thermal Aging
Aging at elevated temperatures is a major environmental factor that accelerates the degradation of the joint’s shear strength. In many operating environments, electronic assemblies are subjected to continuous high temperatures or cyclic temperature changes that drive the diffusion of metals and the development of internal thermal expansion stresses. These stresses are particularly severe in lead-free solder joints like SAC305 due to the high rigidity of the alloy and the mismatch in the coefficients of thermal expansion between the silicon components and the fiberglass circuit board.
This thermal exposure causes the shear strength of the joint to decay over time, with the rate of decay being a function of the temperature and the duration of the exposure, making thermal design crucial for long-term reliability.
Performance Test
Testing of the joint’s shear strength is performed using specialized testing machines that apply a controlled, lateral force to the component until the joint fractures. The maximum force recorded before fracture is the shear strength of the joint, which is compared against the requirements of industry standards to evaluate the reliability of the connection. By performing these tests on assemblies that have undergone different levels of thermal aging or thermal cycling, reliability engineers can plot the rate of strength loss and estimate the remaining useful life of the solder joints in the field.
This quantitative data is used to validate material selections, refine reflow profiles, and improve the design of future electronic products.
