While there is no direct link between protection from ESD and protection from moisture, many devices and components require protection from both. Statshield Moisture Barrier Bags (MBB) are designed to protect the contents from both moisture and ESD events. MBBs protect ESD susceptible devices by forming a faraday cage around the contents and they protect the contents from moisture with specialized layers of film that control the Moisture Vapor Transfer Rate (MVTR). In addition to the bags, Desiccant Packs and Humidity Indicator Cards must be used for proper moisture protection. The bag must be heat sealed with a vacuum-sealer to eliminate the amount of “moisture laden air” within the package.
Most manufacturers of the Moisture Sensitive Devices (MSD) will dictate how their product should be stored, shipped, etc. However, the IPC/JEDEC J-STD-033B standard describes the standardized levels of floor life exposure for moisture/reflow-sensitive SMD packages along with the handling, packing and shipping requirements necessary to avoid moisture/reflow-related failures. The ESD Handbook ESD TR20.20 mentions the importance of MBB in section 5.4.3.2.2 Temperature.
“While only specialized materials and structures can control the interior temperature of a package, it is important to take possible temperature exposure into account when shipping electronic parts. It is particularly important to consider what happens to the interior of a package if the environment has high humidity. If the temperature varies across the dew point of the established interior environment of the package, condensation may occur. The interior of a package should either contain desiccant or the air should be evacuated from the package during the sealing process. The package itself should have a low WVTR.”
Further information on using MBBs:
Surface Mounted Devices
Surface Mounted Devices (SMDs) absorb moisture and then during solder re-flow operations, the rapid rise in temperature causes the moisture to expand and the delaminating of internal package interfaces, also known as “pop corning.” The result is either a circuit board assembly that will fail testing or can prematurely fail in the field. For more information on SMDs and moisture sensitivity, see the following articles:
- Guidelines for Handling and Processing Moisture Sensitive Surface Mount Devices
- (SMDs) – Handling of Highly-Moisture Sensitive Components – An Analysis of Low-Humidity Containment and Baking Schedules
Lead-Free Solder
The introduction of lead-free soldering has increased the problem because of the increased reflow temperatures required. A recent article from www.circuitnet.com called “Procedure for handling moisture sensitive PCBs” details the new risks.
PCB Laminate
The moisture problem is no longer only isolated to the SMD, but is also a problem for the PCB laminate itself. Moisture in the PCB laminate can result in delaminating problems. For further information about moisture concerns in PCB laminates see Ray Prasad’s post on SMTonline.com – Moisture Sensitivity Concerns in PCBs for Lead-free Assemblies.