What is the Best Humidity Level for PCB Assembly
From soldering precision to preventing corrosion, one often overlooked yet crucial element in electronics manufacturing is humidity control. Too much moisture in an assembly environment can lead to issues like corrosion, while too little can increase the risk of ESD. It’s up to manufacturers to control humidity with certain ranges, the values for which are specified in various industry standards.
If you’re auditing a manufacturer, or you are planning a captive assembly operation, what humidity level should your facility have? The guidelines below outline the main standards specifying humidity requirements and some reasons to ensure humidity is kept within appropriate limits.
Industry Standards on Humidity During Assembly
The main standard that specifies requirements for preventing humidity from damaging PCBs is the IPC-1602 standard. This standard encompasses various aspects, such as preventing contamination from moisture, preventing physical damage, and ensuring solderability. The requirements in the standard include:
- Production environment: This includes considerations related to temperature, humidity, and other environmental factors that can impact the quality of printed boards.
- Handling and transport: Proper handling techniques and transport methods are specified as best practices that should be adopted to ensure the safe movement of printed boards.
- Baking profiles for moisture removal: The standard establishes guidelines for baking profiles that are effective in removing moisture from PCBs that have been exposed to excess humidity or were handled improperly.
For humidity testing, there is the J-STD-075 standard. This standard provides test methods that help determine the maximum thermal process sensitivities of electronic components. This is relevant when components are subjected to wave and reflow soldering, including lead-free processing.
Based on these standards, some best practices have been developed relating to humidity in PCB assembly environments.
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Moisture sensitivity level |
Component MSL classifications will impact the appropriate humidity in the production environment. Maintaining a consistent environment between 40% and 60% humidity is considered good practice. |
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Low humidity |
If humidity is too low, production may be shut down. An appropriate lower limit on relative humidity is 30%. |
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Handling of paste and fluxes |
Solder paste and fluxes might have separate specifications based on manufacturer recommendations. Typical humidity requirements are 40%-50% at 65-75 °F. |
How Humidity Affects Electronics Assemblies
When humidity strays too far from ideal levels, you may experience multiple problems at different points in the assembly process.
Electrostatic Discharge
One of the primary concerns associated with humidity in electronics manufacturing is ESD. When ESD occurs, the discharge current and electric field causing the discharge both depend on the humidity level in air. A low humidity environment carries greater risk of ESD. To help prevent ESD during handling of assembled PCBs and components, it is recommended to maintain humidity above 30%.
Desoldering During Rework
High or low humidity can create difficulties in soldering during rework. For example, when humidity is too high, excess moisture will infiltrate solder paste and lead to incomplete soldering with leftover paste under typical reflow profiles. Ideally, the area where solder paste is being applied should be maintained between 40% to 50% and at temperatures ranging from 65 to 75 °F. These values are typical in temperate climates, but tropical or arid climates will deviate significantly from these levels.
Condensation
Condensation by itself is not a problem when no moisture sensitive components are present. But if condensation remains on the board when it is powered on, electrochemical reactions could begin that lead to corrosion. This would appear on surface pads that are visible to the environment, and especially between closely-spaced pads where there could be voltage difference when the device is powered on.
Moisture-Sensitive Devices
Some components have a moisture sensitivity level (MSL) rating, as was mentioned above. MSL values are defined based on allowable exposure time to a humid environment, and not necessarily by the specific humidity level. For moisture-sensitive components, the typical acceptable relative humidity level in temperate climates is 40% to 60% without requiring a reduction in allowed exposure times.
Storage
Even if the design passes through assembly perfectly and there are not problems from excess moisture, the assembled PCBs must be packed and stored correctly to prevent infiltration of excess humidity. Once properly packed with desiccants and stored in a dry location, the assembled PCBs should have long enough shelf life before entering another process or being shipped to a customer.
Whenever you are working with moisture sensitive designs and you need to call out specific handling practices for your assembly, make sure you create your design data package using the best set of PCB design features in OrCAD from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.
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