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.
Micro-Drop Shear Testing
Meaning
Mechanical testing procedures that apply a high speed lateral force to individual solder spheres evaluate the resistance of the joint to sudden impact and brittle fracture. This micro-drop shear testing is specifically designed to simulate the stresses that an electronic device experiences when it is dropped onto a hard surface. Unlike traditional low speed shear tests, which measure the static strength of the bond, this method uses a high strain rate to identify weaknesses at the intermetallic interface.
The test is performed using a specialized high speed bond tester that can drive a shear tool at velocities up to several meters per second. The force required to break the joint is recorded, and the resulting fracture surface is examined to determine if the failure was ductile, brittle, or a combination of both. This technique is a critical part of the qualification process for new solder alloys and surface finishes, particularly for mobile electronics where drop resistance is a primary requirement.
Mechanical Assessment
Execution of the test requires precise alignment of the shear tool with the solder ball to ensure that the force is applied consistently across all samples. The micro-drop shear testing equipment must be calibrated to handle the high speeds and rapid data acquisition needed to capture the peak force during the impact. A small tool, often tipped with tungsten carbide, is positioned just above the surface of the printed circuit board and then driven into the side of the solder ball.
The height of the tool above the board, known as the shear height, is a critical variable that must be kept constant to ensure repeatable results. If the tool is too high, it may cause the ball to deform excessively rather than shearing at the interface. If it is too low, it may strike the board itself and damage the equipment.
Engineers use this test to compare the performance of different manufacturing batches and to verify that the assembly process is within specification.
Force Measurement
Data collected during the impact provides a detailed profile of how the solder joint absorbs and resists energy. The micro-drop shear testing software calculates the peak force, the energy to failure, and the displacement at the point of fracture. These values are used to assess the overall toughness of the interconnect and its ability to withstand the shocks of daily use.
A high peak force combined with a brittle fracture mode often indicates that the intermetallic layer is too thick or has a poor crystal structure. Conversely, a lower peak force with a ductile failure mode suggests that the bulk solder is absorbing most of the energy, which is generally preferred for drop resistance. Manufacturers often set a minimum threshold for the shear force based on the size of the solder ball and the intended application of the product.
This quantitative data allows for a scientific comparison of different material combinations and process parameters.
Evaluation Criteria
Interpretation of the results goes beyond the simple force numbers and involves a careful inspection of the fracture surface. After the micro-drop shear testing is complete, the samples are examined under a microscope to categorize the failure mode. A “Type 1” failure occurs within the bulk solder and is considered a sign of a strong interface.
A “Type 4” failure, on the other hand, occurs entirely at the intermetallic interface and is a clear indicator of a brittle bond. Most industry standards, such as those from the IPC or JEDEC, provide guidelines for what constitutes an acceptable failure mode for different classes of electronics. By tracking the percentage of brittle failures across a large sample set, quality teams can identify trends and implement corrective actions before the product reaches the customer.
This rigorous approach to mechanical testing is what allows modern electronics to survive the rigors of a mobile lifestyle. The final report from these tests is a key document in the approval of any new product design or manufacturing site.