What is Moisture Sensitivity?

Moisture Sensitivity is the sensitivity of electronic components to absorb the moisture or wetness from the atmosphere.

What Electronic Components are Sensitive to Moisture?

Moisture sensitivity does not depend on type of electronic component but depends on type of package. Mostly, semiconductor surface mount devices are prone to be moisture sensitive. In general, surface-mount devices (SMD’s) are more prone to popcorn cracking because:

  • They are thinner and therefore has lower fracture strength
  • They absorb and retain moisture more easily
  • SMD board mounting also subjects the molding compound to the high temperature experienced by the leads.

How Moisture Sensitivity Affects the Component Failure?

Improper storage, handling, or packaging of plastic encapsulated semiconductor devices can allow the introduction of moisture. Moisture trapped inside plastic encapsulated packages can damage them during soldering, as the moisture vaporizes and tries to expand. The expansion of trapped moisture can result in internal separation (delamination) of the plastic from the die or lead-frame, wire bond damage, die damage, and internal cracks. Most of this damage is not visible on the component surface. In extreme cases, cracks will extend to the component surface. In the most severe cases, the component will bulge and pop. This is known as the “popcorn” effect.

A plastic package contains moisture, can also result in external steam jets from the package, which may displace other nearby components on the circuit board during the solder process.

What are Moisture Sensitivity Levels (MSL)?

Moisture Sensitivity Level relates to the packaging and handling precautions for semiconductors.

The MSL is an electronic standard for the time period in which a moisture sensitive device can be exposed to ambient room conditions (approximately 30°C/60%RH).

Different package types exhibit different sensitivity levels to moisture ingress and its effects. The older, through-hole, bulkier packages absorb moisture per volume at a slower rate than the thinner, surface-mount packages of recent times.

In recognition of the varying degrees of popcorn cracking tendency of various package types, IPC/JEDEC defined a standard classification of moisture sensitivity levels (MSL’s). The MSL’s are expressed in numbers, with the MSL number increasing with the vulnerability of the package to popcorn cracking. Thus, MSL1 correspond to packages that are immune to popcorn cracking regardless of exposure to moisture, while MSL5 and MSL6 devices are most prone to moisture-induced fracture.

IPC/JEDEC MSL Classifications

From the above table, the floor life is the time allowed for a component to be unpacked before assembling on board. For example, MSL 3 components are allowed to keep unpacked upto 168 hours (with the condition of < or = 30C/60% RH) before it is soldered on board.

If a device is classified as MSL 1, it is not moisture sensitive and the device does not have to be dry packed again after the pack has been opened.

If a device is classified at a higher numerical level, it is moisture sensitive and must be dry packed in accordance with J-STD-033.

If a device is classified as Level 6, it is extremely moisture sensitive and the dry pack will not provide adequate protection.

How to Handle the Moisture Sensitive Components?

It is important to handle dry pack bags very carefully to prevent plastic packages from absorbing moisture.

The following guidelines are helpful in avoiding the component failure due to moisture sensitivity.

  1. Moisture barrier bags are sealed at manufacturer during component packing. These bags must be handled with care to avoid puncture of tearing of the bag’s material.
  2. Upon receipt, moisture barrier bags should be inspected for punctures or holes of any kind. If openings in the bag are found and the maximum humidity indicator has been exceeded, the parts shall be baked according to the conditions specified by manufacturer.
  3. Bags should remain sealed until parts are ready to be used.
  4. Inspect the humidity indicator card (HIC) immediately after opening the package and evaluate the colors of the dots. The HIC card color should be blue (dry).
  5. Proper handling of storage, board mounting assembly and rework is critical to avoid over-exposure of the package to moisture.

IPC created and released IPC-M-109, Moisture-sensitive Component Standards and Guideline Manual.