Moisture-Sensitive Devices in PCB Assembly

Moisture-Sensitive devices or MSDs are electronic components that are encapsulated with plastic compounds and other organic materials. These components are particularly sensitive towards atmospheric moisture as humidity can penetrate permeable packaging materials by diffusion and collect at material interfaces. If not handled properly during the solder reflow attachment process to PCBs, the rapid heating of absorbed moisture can lead to serious issues like internal package cracking and delamination between internal package interfaces.

Sensitivity Classification

Sensitivity in electronic components can be governed by various factors like internal and external dimensions of the package, type of passivation, and relative humidity. It’s important to reach common ground among manufacturers regarding the classification of moisture sensitive components as it helps to issue suitable guidelines enhancing the life of finished electronics products. Few widely accepted specifications that provide important information regarding classification, storage and handling for such components are JEDEC STD22B, IPC/JEDEC standard J-STD-033A, Test Method A112-A, and IPC-SM-786A.

The classification procedure typically exposes the component for a specified soaking duration at the stated floor life conditions, following which it is subjected to three reflow cycles that involve vapor phase or IR reflow. The component is then visually inspected, put through an electrical test along, and finally analyzed with acoustical microscopy to determine its moisture sensitivity.

Lead-Free Challenge

With several manufacturers shifting towards lead-free components, moisture control in PCB assembly becomes more critical than before. The lead-free stance of government and regulatory organizations is influenced by rising environmental concerns and the advantages of lead-free components. The lead-free components require higher energy and greater reflow temperature that makes PCB assembly more prone to moisture issues.

Dry Solutions

As PCB assembly defects continue to rise significantly, manufacturers and suppliers are investing actively to safeguard the assembly lines. While there are various ways to tackle this, Dry solutions involve ensuring moisture sensitive components are rarely exposed. Dry packing or Moisture sensitive Level (MSL) packing is a particular packaging style that uses tough moisture resistant bags for shipping such components. Upon arrival to ensure there is no moisture diffusion, the components are baked for 24 hours at 125 Degree Celsius. Following the bake cycle, the components are again packed in dry packs along with a predetermined amount of desiccant and humidity indicator (Hic) card. The bag is sealed and affixed with a moisture sensitivity label that should indicate to everyone that the dry pack should be handled appropriately. The component might be needed to be rebaked if the sensitivity card indicates more than 20 percent relative humidity.

Hic card, desiccant and vacuum sealing,

When receiving an MSL package you must always look for 3 things. First is the HIC card, it will indicate if there has been any exposure to humidity. Second is desiccant which is used to keep the contents of the package dry. Lastly, you should look for the vacuum seal. If the package is properly vacuumed sealed, it will have a clean nice heat seal plus it will keep its vacuum seal appearance.

When buying an MSL part you must be careful. If the parts are not properly packaged it will cause the part to popcorn off the circuit board when it goes to reflow. This could cause $ 1000’s of rework. When buying MSL parts think of Summon electronics. We have a team that handles MSL parts daily. We can safeguard that nothing happens when sourcing sensitive parts.