Post Holiday Wave Solder Joint Reliability Evaluation Methods
Post-holiday wave solder defects stem from pot contamination, substrate moisture, and profile drift; lab microsectioning and strict lot debits prevent field failures.
Metallographic Microsectioning
Meaning
Destructive analysis involves the preparation of a cross sectional sample from a printed circuit board to examine the internal structure of through holes, traces and solder joints. This technique of metallographic microsectioning allows engineers to measure the thickness of copper plating, the quality of the intermetallic layer and the presence of internal defects like cracks or voids. The process requires cutting a small piece from the board, mounting it in a plastic resin and then grinding and polishing the surface to a mirror finish.
Once prepared, the sample is viewed under a high power microscope to verify that the board meets the technical specifications and reliability standards. It is an essential tool for failure analysis and for the initial qualification of a new manufacturing process or supplier.
Preparation Method
Creating a useful sample requires a series of precise steps to ensure that the internal features of the board are not distorted or damaged. First, the area of interest is cut using a diamond saw, and then it is placed in a mold filled with a liquid epoxy or acrylic resin that hardens over time. This resin provides the necessary support for the thin copper layers during the subsequent grinding and polishing stages.
The technician uses increasingly fine grades of abrasive paper followed by diamond paste to remove all scratches from the surface. In many Chinese high tech factories, automated polishing machines are used to ensure the consistency and speed of the sample preparation. The final step often involves a chemical etch that highlights the grain structure of the metals and makes the different layers easier to distinguish under the microscope.
Analytical Value
Measurement of the plating thickness in the barrel of a through hole is one of the most common uses for this technique. The ipc standards specify minimum thicknesses for the copper wall to ensure that the board can handle the electrical current and the thermal stress of soldering. Metallographic microsectioning also reveals the presence of “knee thinning” where the copper is thinner at the entrance of the hole, a common site for failure.
Engineers also look for “inner layer separation” where the copper trace pulls away from the plating in the hole, which is a major reliability risk. The quality of the intermetallic bond between the solder and the board is also evaluated to ensure that the soldering process was performed at the correct temperature and for the right duration.
Industrial Application
Regulatory bodies and major brands in China use microsectioning as a primary method for auditing the quality of printed circuit board fabricators. The Ministry of Industry and Information Technology requires that manufacturers of high reliability electronics maintain the equipment and expertise to perform this analysis on site. For a foreign party, reviewing the microsection reports from a supplier is a necessary part of the due diligence process when starting a new production run.
These reports provide a permanent record of the internal quality of the boards that cannot be seen with the naked eye or through automated optical inspection. While the test is destructive and consumes a board, the information it provides is essential for preventing large scale field failures. Accurate cross sectional analysis is the most reliable way to verify the internal integrity of a complex electronic assembly.