What is MSL? A Guide to Moisture-Proof Storage in PCB SMT Workshops

In the SMT (Surface Mount Technology) production process, moisture sensitivity issues with PCBs and components directly affect soldering yield and product reliability. The Moisture Sensitivity Level (MSL) is the core indicator for defining protection standards. Coupled with standardized workshop storage conditions, it can effectively prevent production failures caused by moisture absorption. Why are PCBs afraid of moisture? What is the MSL rating?

PCB substrates (such as FR-4) easily absorb moisture from the air. During the high temperatures of SMT reflow soldering (>220°C), internal moisture rapidly vaporizes and expands, which may lead to board delamination or micro-cracks in solder pads (known as the "popcorn" effect), resulting in electrical failure. The industry uses the Moisture Sensitivity Level (MSL) standard to quantify this risk, divided into levels 1–6. The higher the number, the more sensitive the component, and the shorter the allowable workshop exposure time:

• MSL Level 3: Must be soldered within 168 hours (7 days) after opening.

• MSL Level 6: Must be soldered within 24 hours and often requires baking for moisture removal before use.

Storage and management specifications in SMT workshops are based on MSL requirements. Modern SMT workshops must establish a strict moisture-sensitive material control system:

1. Incoming Storage: Clearly label materials with their MSL level and store them separately. Standard materials are stored in a controlled environment (typically temperature <30°C, humidity <60% RH), while high-level materials (e.g., MSL 5 and above) must be stored in ultra-low-humidity nitrogen cabinets (humidity <10% RH).

2. Packaging and Identification: Use moisture-proof vacuum-sealed bags with built-in humidity indicator cards and detailed labels (including MSL level, workshop lifespan, and baking conditions). When opening, check the humidity card and record the opening time.

3. Lifespan Tracking: Once materials are opened, their "workshop lifespan" countdown begins. A tracking system must strictly monitor and ensure soldering is completed within the specified time. Materials exceeding the time limit must undergo baking for moisture removal (e.g., 125°C for 8–48 hours) and be confirmed as qualified before reuse.

4. Environmental Monitoring: Continuously monitor and record the temperature and humidity of the SMT workshop to ensure a stable and controlled production environment and prevent accidental moisture absorption by materials.

A precise understanding of PCB Moisture Sensitivity Levels (MSL) and strict management of SMT workshop storage conditions form an invisible yet critical "process defense line" in modern electronics manufacturing. It is not only a direct requirement for controlling the production process and reducing scrap costs but also a core engineering capability for fundamentally preventing early product failures and ensuring end-product reliability. From design and selection to production execution, attention to moisture sensitivity at every stage reflects the manufacturing industry's deep commitment to quality.

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Source: PCB Circuit Board Information Sharing

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