Standard HVAC systems are designed to heat and cool air. They are not optimized to manage humidity with the precision or responsiveness required by modern surface mount technology (SMT) processes. In many cases, they create humidity fluctuations as a byproduct of temperature control.

For example, a facility may maintain a steady 72°F across shifts. But when outdoor humidity changes or internal heat loads shift from one production area to another, relative humidity inside the room begins to drift. If these changes go unnoticed or uncorrected, they introduce risk.

Even small RH swings can influence:

Solder paste consistency




Flux activation timing




Adhesive behavior during component placement




Electrostatic discharge (ESD)




Conformal coating quality and curing

Each of these factors can affect first-pass yield or long-term reliability, even if the temperature reading on the thermostat appears stable.

Seasonal Shifts and Daily Variability

During colder months, ambient humidity drops. HVAC systems running in heating mode dry the air further, often pushing RH levels below 30 percent. This increases the likelihood of static buildup, especially in high-friction zones like feeder banks or conveyor surfaces.

In summer, high outdoor humidity pushes indoor RH upward. Without proper dehumidification, paste viscosity can become inconsistent and coatings may not cure properly.

Even within a single shift, humidity can vary. A line that runs cleanly in the morning may experience issues by afternoon as heat from machines and foot traffic alters the microclimate.

Real Costs of Inconsistency

What are the consequences of “almost controlled” environments?

Increased rework rates: Defects such as cold solder joints or bridging may result from subtle changes in paste behavior or board conditions




Material spoilage: Shelf life for solder paste, adhesives, and moisture-sensitive components depends on proper humidity storage




Shift-to-shift variation: RH fluctuations can cause different results between operators using the same setup




Hidden ESD events: Dry air is a major contributor to electrostatic buildup even in well-grounded facilities

These issues are often misdiagnosed. Engineers may adjust stencil designs or change reflow settings without realizing the root cause lies in unstable humidity.

Creating a Truly Controlled Environment

Solving these problems does not require a facility-wide overhaul. It starts with recognizing that temperature control alone is not enough.

A reliable electronics manufacturing environment should include:

Real-time RH monitoring




Defined humidity targets based on process sensitivity




Zoning in critical areas like reflow ovens, inspection stations, or PCB storage




Year-round control that responds to seasonal swings and operational changes

Technologies like Dry Fog humidification help create stable conditions without wetting surfaces or overshooting targets. When paired with responsive controls and localized adjustments, they support greater consistency and reduce variation across all stages of production.

Final Thought

In high-precision manufacturing, the air itself becomes part of the process. While HVAC systems offer a good starting point, they often fall short when it comes to tight humidity control.

By taking a more deliberate approach to environmental stability, manufacturers can reduce process variation, improve quality outcomes, and eliminate costly rework. “Almost controlled” may look good on paper, but in practice, it can cost more than you think.

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