Quantitative Depth Profiling Procedures for Multi-Layer Circuit Board Surface Finishes
Quantitative depth profiling via AES or XPS accurately measures sub-nanometer interfacial oxidation and phosphorus enrichment to prevent joint failures.
Pad De Wetting
Surface Defect
Withdrawal of molten solder from a metal surface pad during reflow results in irregular, droplets of solder and exposed pad areas on a circuit board. This soldering defect, known as pad de wetting, occurs when the solder initially wets the surface but then recedes due to contamination or a lack of solderability. The resulting finish displays thick, rounded solder mounds separated by a thin, non-solderable surface film.
Soldering Impact
Loss of a continuous metallic bond reduces the electrical and mechanical strength of the component attachment. When pad de wetting occurs, it leaves the copper-tin intermetallic layer exposed to the atmosphere, which can lead to rapid oxidation and weak joint failures. This defect is a common cause of intermittent circuit connections and high-resistance joints in surface mount assemblies.
Quality Verification
Inspection of the finished solder joint using optical or automated inspection systems is used to detect the exposed pad area. If pad de wetting is identified during quality checks, the assembly is rejected to prevent field failures from occurring. Remedying this issue requires evaluating the cleanliness of the solder paste and checking the storage conditions of the board.
This testing protocol helps engineers identify whether the root cause lies in contaminated raw materials or in process variations during the reflow stage.