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.
Microsection Polishing Tilt Error
Meaning
Geometric distortion occurring when the viewing plane of a cross section is not perfectly perpendicular to the interface leads to inaccurate measurements of the intermetallic layer thickness. This microsection polishing tilt error is a common analytical artifact that can result in the overestimation of the size of critical features in a solder joint. When a sample is ground and polished at an angle, the projected image of the intermetallic layer appears wider than its true physical dimension.
This is similar to how a shadow becomes longer as the light source moves closer to the horizon. In the high precision world of electronic manufacturing, even a few degrees of tilt can cause a significant error in the reported data. For intermetallic layers that are only a few micrometers thick, a small tilt can lead a technician to believe that a process is out of specification when it is actually perfectly fine.
This makes the control of the polishing angle a primary concern for any laboratory performing quality audits or failure analysis.
Geometric Artifact
Mathematical relationship between the true thickness and the measured thickness is governed by the cosine of the tilt angle. If the microsection polishing tilt error is present, the observed width is equal to the actual thickness divided by the cosine of the angle of inclination. For example, a ten degree tilt results in a measurement that is approximately one point five percent larger than the real value.
While this may seem small, a twenty degree tilt increases the error to over six percent, which can be the difference between a passing and a failing grade for a critical component. The error is always in the direction of overestimation, meaning that a tilted sample will never appear thinner than it actually is. This bias can lead to unnecessary changes in the manufacturing process or the rejection of high quality parts.
Ensuring that the sample is mounted perfectly flat is the only way to eliminate this geometric distortion and provide reliable data for engineering decisions.
Measurement Accuracy
Verification of the polishing angle is a standard practice in advanced failure analysis labs to ensure the integrity of the results. Technicians can check for microsection polishing tilt error by looking at other features on the sample, such as the shape of the plated through-holes or the symmetry of the solder ball. If the circles appear as ovals, it is a clear sign that the viewing plane is not perpendicular to the features.
Some labs use specialized mounting fixtures that lock the sample in a fixed orientation during the grinding process. Others utilize digital image analysis software that can detect and correct for small amounts of tilt by comparing the known dimensions of certain features with their observed dimensions. High resolution optical systems and laser alignment tools are also used to verify that the sample surface is flat and level before any measurements are taken.
These steps are essential for maintaining the high level of precision required for the development of next generation electronic devices.
Correction Protocol
Procedures for addressing suspected errors involve either re-polishing the sample or applying a mathematical correction based on the observed distortion. If the microsection polishing tilt error is large, the best course of action is to re-mount the sample and start the grinding process again to ensure a true cross section. If the tilt is small and the sample cannot be replaced, engineers may use a correction factor based on the aspect ratio of a known circular feature, such as a copper via.
This requires a high degree of confidence in the original dimensions of the reference feature. Documentation of the polishing process, including the tools used and the alignment checks performed, is a key part of the quality record for any failure analysis report. By being aware of this potential artifact, laboratories can provide more accurate and defensible data to their clients.
This rigor is especially important when investigating the root cause of large scale field failures or when qualifying a new supplier in a competitive market. The final measurement of the intermetallic layer is only as good as the preparation of the sample surface.