Nonwetting is a condition whereby a surface has contacted molten solder but has had part or none of the solder adhere to it. Nonwetting is recognized by the fact that the base metal is visible. It is usually due to the presence of contamination on the surface to be soldered.
Process and design-related causes:
• Solder paste flux not aggressive enough for level of oxidation present on part or PCB
• PCB pad contaminated
• Component lead contaminated
• Excessive moisture absorbed by paste
• Paste exposed beyond worklife
• Ambient humidity and temperature beyond paste work envelope
• A palladium lead finish which requires higher reflow (liquidus) temperature
• Peak reflow (liquidus) temperature not being attained
• Malfunctioning oven which is impeding attenuation of proper temperatures
• Flux activity too weak for the level of oxidation present on the PCB and/or components
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