Measuring Intermetallic Phase Growth in Lead Free Solder Joints
Accurate intermetallic phase growth measurement requires bevel-free metallographic polishing, etching chemical selection, and kinetic modeling to set supplier liability limits.
Shear Force Degradation
Meaning
Gradual reduction in the maximum load a solder joint can withstand before failure indicates the cumulative effect of thermal stress and microstructural changes over an operational period. This shear force degradation is a primary indicator of the aging process in electronic interconnects, where the initial strength of the bond is lost due to the growth of brittle intermetallic layers and the coarsening of the solder grains. As the device is used, the repeated heating and cooling cycles cause the solder to undergo plastic deformation, leading to the formation of micro-cracks and the accumulation of fatigue damage.
Measuring the shear force at different intervals during reliability testing allows engineers to plot a decay curve and predict when the joint will become too weak to survive a mechanical impact. This parameter is particularly important for large components like ball grid arrays, where the peripheral joints are subjected to the highest strain due to the mismatch in thermal expansion between the package and the board.
Strength Reduction
Physical changes in the solder and the interface are the underlying causes of the loss of load-bearing capacity. The shear force degradation is often driven by the thickening of the intermetallic compound layer, which is more brittle than the bulk solder. As this layer grows, it consumes the soft, ductile tin and creates a more rigid interface that is prone to cracking.
Additionally, the grains of the tin-silver-copper alloy tend to grow larger over time, a process known as coarsening, which reduces the yield strength of the material. In some cases, the migration of alloying elements or the formation of Kirkendall voids can further weaken the bond. These microstructural changes occur faster at higher operating temperatures, making the thermal management of the device a critical factor in maintaining joint integrity.
Quality teams use high precision shear testers to quantify this reduction in strength after accelerated aging tests at elevated temperatures.
Fatigue Stress
Repetitive mechanical loading from thermal expansion and contraction is the most common source of damage that leads to lower shear resistance. This fatigue-driven shear force degradation is a major concern for electronics used in harsh environments, such as under-the-hood automotive applications or outdoor telecommunications equipment. Each thermal cycle adds a small amount of damage to the joint, which gradually coalesces into larger cracks.
These cracks usually start at the corners of the joint where the stress is highest and propagate along the intermetallic interface or through the bulk solder. As the crack grows, the remaining intact area of the joint decreases, leading to a direct drop in the measured shear force. Testing protocols often include hundreds or thousands of thermal cycles followed by a shear test to see how much of the original strength remains.
This data is used to validate that the design will meet the expected service life of the product without catastrophic failure.
Operational Limit
Establishing a minimum acceptable strength for the interconnect is necessary to ensure that the product can survive the shocks of transportation and daily handling. Once the shear force degradation reaches a certain level, the joint becomes a reliability risk, even if it still provides an electrical connection. Engineers define a failure threshold based on the mass of the component and the expected acceleration levels the device will face.
If the shear strength drops below this point, the risk of a field return increases significantly. Manufacturers use the results of these tests to compare different solder alloys and surface finishes to find the combination that offers the best resistance to aging. For high reliability products, a safety margin is added to the minimum strength to account for the variability in the manufacturing process.
Regular monitoring of the shear force in production samples is a common practice to ensure that the process remains in control and that no unexpected degradation is occurring. The final analysis of the shear data provides a clear picture of the long term mechanical health of the electronic assembly.