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.
Outgassing Blowholes
Meaning
Gaseous emissions create voids or craters in a solder joint when trapped moisture or volatiles in the printed circuit board escape during the heat of the soldering process. These outgassing blowholes typically appear in through hole joints where the gas pushes through the molten solder, leaving a hole that remains after the metal solidifies. The source of the gas is often moisture absorbed by the epoxy resin of the board or chemicals trapped in the plating of the holes.
If the gas cannot escape through the top of the hole, it creates a bubble that weakens the mechanical and electrical connection. Preventing these defects requires careful control of the board manufacturing process and thorough preheating of the assemblies before they reach the solder wave.
Formation Mechanism
Moisture trapped inside the laminate material expands rapidly when it hits the high temperatures of the molten solder, which can exceed two hundred and sixty degrees Celsius. This expansion creates internal pressure that forces the gas through any small path of least resistance, such as a thin or porous area in the copper plating of a through hole. When the gas enters the liquid solder, it forms a bubble that may burst at the surface, creating a visible crater.
In some cases, the gas remains trapped inside the joint as a void, which can only be seen using x-ray inspection. The severity of the outgassing blowholes is often related to the quality of the drilling and plating process at the board fabricator. If the holes are not drilled cleanly or if the plating is too thin, it is much easier for the gas to escape from the laminate into the solder.
Process Control
Manufacturers in the Chinese electronics sector use pre-soldering baking cycles to drive out moisture and reduce the risk of outgassing. The duration and temperature of the bake must be sufficient to dehydrate the boards without damaging the surface finish or making the laminate brittle. During the soldering process itself, the preheat stage must be long enough to allow any remaining volatiles to escape before the board contacts the solder wave.
Monitoring the quality of the incoming boards from the fabricator is also a necessary step in the control process. Companies often use microsectioning to check for thin plating or cracks in the barrel of the hole that could allow gas to enter the joint. Foreign companies sourcing boards from local vendors often include specific requirements for plating thickness and laminate quality in their procurement contracts to mitigate this issue.
Quality Consequence
Large blowholes can significantly reduce the cross sectional area of the solder joint, leading to a weak mechanical bond that is prone to failure under vibration. These defects are also a major concern for electrical reliability because they can lead to intermittent connections or localized heating. While some small pinholes are allowed under the ipc-a-610 standard for class one products, they are strictly limited for class three high reliability electronics.
Reworking these joints is difficult and time consuming, as the source of the gas must be completely eliminated to prevent the hole from forming again. Identifying the root cause of the outgassing, whether it is moisture in the board or an issue with the plating, is necessary for an effective solution. Constant attention to the moisture content of the materials is the best defense against the formation of these voids.