SMT assembly humidity: which band suits paste, boards and components
6 November 2026Application note6 min read
Too dry and static builds on feeders and boards; too damp and solder paste absorbs water, components pop in reflow and leakage current climbs. The working band is narrower than most halls assume.
An electronics assembly hall sits between two failures that pull in opposite directions. Dry air raises static voltage on belts, feeders and bare boards; damp air lets solder paste take up water and lets moisture-sensitive packages absorb it before reflow.
Most specifications settle around 40–55% RH for the line itself, with storage held at the lower end or in a dedicated dry area. The exact limits depend on the paste chemistry, the component moisture sensitivity level and the flooring.
Why halls drift out of band
The load changes with the season and the shift. Door openings, vehicle traffic and the air-handling balance move more moisture than the process does, and a hall that holds its band in winter can sit well above it in the humid season.
This is why the plant is sized on the outdoor design condition rather than on a comfortable average, and why the sensor position matters as much as the machine.
Storage is a separate problem
Component reels and bare boards are usually protected by packaging rather than by the room. Where the room itself has to protect them, the specification moves from a comfort band to a dry-room figure, and the equipment moves from a condensing unit to a desiccant rotor platform.
Treating storage and assembly as one zone is a common error: the storage area is small, needs a lower dew point and is far cheaper to dry on its own.
Measuring what you control
A single wall-mounted sensor reports the air next to the wall, not the air at the print head. Logging at two or three positions across the line shows whether the band is held where it matters, and gives the enquiry a real starting point.