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SMT Temperature and Humidity Control

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One of the most important ways to prevent PCB defects is controlling factory environmental conditions. If the manufacturing floor’s humidity and temperature levels are not properly controlled, very expensive components—and possibly entire assemblies—could be damaged, resulting in quality issues and unnecessary costs.

A manufacturing floor’s environmental conditions can be affected by the factory’s geographical location, or even the type of equipment being used to manufacture the boards. Yet even manufacturers in the most temperate regions of the world have to observe, and control, the conditions on their floor—namely: temperature and relative humidity.

Relative Humidity

Humidity is measured by a room’s “relative humidity” (Rh), which is the ratio of partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. Simply put, Rh is an analysis of the amount of water vapor in the air.   

High Humidity

High levels of humidity in a manufacturing environment can cause a number of serious problems:

  • Slumping: solder paste accepts too much water and causes bridging during reflow.
  • Solderballing (or “popcorning”): too much water absorption in solder paste and causes poor coalescence.
  • Out-gassing: too much water creeps under surface mounts, especially BGAs, and causes pressure build up. In some cases, covers can be blown off.

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Low Humidity

Flux evaporates too quickly, which causes solderpaste to dry out. This in turn creates poor release from stencils and insufficient solder joint defects.

High Temperature

With high temperature comes decreased solder paste viscosity. This can cause a number of problems: mainly, paste smearing and slumping – which, furthermore, can cause bridging and solderballing defects such as voiding. High temperatures might also result in extra oxidation of the solder, which can compromise solderability.

Low Temperature

If temperature is too low, solder paste viscosity might increase. This can result in poor printing behavior, such as release and rolling, as well as print voiding, in which the paste is too solid to print correctly.  

Acceptable Range & Conditions

Expert opinions vary on Rh and temperature ranges. Some recommend a wider range (35-65%, 40-70%, 20-50%), while others say anything above or below 60% Rh could result in the aforementioned defects, as well as lifecycle issues. However, Rh range is really a matter of experience and preference—what works best for your products.

The same goes for temperature, although expert opinions are less varied. The general consensus is that solder paste performs best in 68 – 78 degrees F, a normal human comfort zone. However, it should be noted different solder pastes act differently in different conditions. It’s always good to allow some flexibility depending on the product.

Monitor and Control

Some geographical locations, such as very humid or very dry regions, may require a higher level of environmental control. But no matter where a factory is located, certain climate control methods remain the same.

Humidity sensors: Not only is it important to invest in a quality Rh sensor, it’s crucial that sensors are placed properly to ensure accuracy. Otherwise, unnoticed fluctuations in humidity and temperature could turn into big, expensive problems. It’s also important to inspect the sensors regularly. In high humidity regions, especially, Rh sensors tend to malfunction.

Air conditioning/heating unit: Invest in good air conditioning and heating. This is much of the battle. If you’re able to effectively control the temperature, then defects caused by temperature should be an afterthought. Also important is a dehumidifier, especially in high humidity regions.

Nitrogen in ovens: Too much humidity tends to cause unneeded oxidization in the solder paste. Introducing nitrogen tends to quell that oxidization.

Moisture Sensitive Components

Moisture sensitive components are another important consideration. In high humidity environments, moisture sensitive components should spend the least amount of time possible outside of their packaging, depending on their sensitivity levels. But if proper humidity is maintained, this should be a concern.