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.
Pinholes
Meaning
Small apertures appear in the surface of a solder joint or in the copper plating of a printed circuit board, indicating the escape of gas during the manufacturing process. These pinholes are a smaller version of blowholes and are often the result of the same outgassing mechanism where moisture or volatile chemicals are released from the substrate. In a solder joint, a pinhole is a tiny opening that goes through to the base metal or into a larger internal void.
In the context of board fabrication, they can also refer to tiny gaps in the copper plating that leave the underlying laminate exposed. These defects are a quality concern because they can provide a path for moisture and contaminants to enter the board, leading to corrosion or electrical failure.
Detection Method
Visual inspection under magnification is the primary way to identify these tiny defects on the surface of a finished assembly. Automated optical inspection systems are programmed to look for the characteristic dark spots that indicate a hole in the reflective surface of the solder. For pinholes in the copper plating, more sophisticated methods like backlight testing are used during the board manufacturing stage.
In this test, a light is placed behind a sample of the board, and any points where light shines through the plated hole indicate a defect. Metallographic microsectioning is also used to determine the depth and origin of the hole by looking at a cross section of the affected area. This analysis helps to distinguish between a harmless surface irregularity and a significant structural defect.
Manufacturing Cause
Moisture absorption is the most frequent cause of gas release during the soldering phase, especially in high humidity environments. If the printed circuit board has not been stored in a dry environment or baked before assembly, the trapped water will vaporize and push through the liquid solder. Another cause can be the presence of organic contaminants or residues from the plating chemicals that were not properly rinsed away by the board fabricator.
In some cases, the pinholes are caused by the flux itself if it is not given enough time to evaporate during the preheat stage. Chinese factories often implement strict environmental controls and moisture barrier packaging to reduce the occurrence of these issues. Foreign manufacturers must ensure their local partners have these controls in place to avoid receiving batches with latent reliability problems.
Reliability Impact
While a single small pinhole may not cause an immediate failure, it is often a sign of a larger internal problem like a void or poor wetting. For high reliability applications, such as automotive or industrial control systems, these defects are treated as a sign of an unstable process and may be grounds for rejection. The presence of these openings allows corrosive elements from the atmosphere to reach the copper traces, which can lead to the long term degradation of the circuit.
Standards like ipc-a-610 provide specific limits on the size and number of allowable pinholes depending on the class of the product. Manufacturers must balance the need for high throughput with the need for the slow, controlled heating that prevents gas from being trapped. Regular audits of the soldering and plating processes are necessary to keep the rate of these defects at an acceptable level.