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Moisture absorption in connectors

The key parameter in blistering of connector housings is the control of moisture absorption in the molded parts. Blistering occurs upon super-saturation of the housing resin with water vapor.  Much of the SMT assembly for PCBs is done in the Pacific Rim region of the world.  The high temperature and humidity levels to which connectors can be exposed can result in saturation of these connectors with water.

The blister onset temperature is dependent upon a number of factors that include wall thickness, percentage of moisture in the part, the speed of connector heating, and the peak temperature of the reflow process.  The higher the peak temperature, the lower the allowable moisture content before blister occurs.  Lead-free solders require higher peak temperatures and the incidence of blistering will be greater.  Current connector handling procedures will not be sufficient to prevent blistering as the trend to lead-free soldering progresses.
Moisture absorption rate

Moisture absorption for a plastic part is determined by:
•   Part thickness
   -   Thin parts reach moisture equilibrium faster than thicker parts
•   Relative humidity
   - The higher the relative humidity the faster the moisture absorption
•   Temperature
   - The higher the temperature the faster the moisture absorption at a given RH
•   Time at temperature and relative humidity

Protecting parts from high temperatures and high relative humidities will extend their shelf life and minimize blistering.  Reducing the exposure time to such environments will lower the occurrence of blistering.
Part thickness

Part thickness affects the moisture absorption time and level as well as the rate of desorption and part heating.
•   Thicker parts absorb moisture at a slower rate than thin parts
•   Thicker parts do not heat up to blister onset temperatures as quickly as thin parts
•   Depending on the resin, some connectors that are sufficiently thin may not exhibit blistering as they give up moisture very quickly, reaching threshold levels below those where blister onset occurs.

Stanyl molded parts with a maximum wall-thickness of ≤ 0.5 mm have a very high moisture desorption rate and do not exhibit blistering, even at high lead-free soldering PCB temperature profiles.

The moisture sensitivity levels identified for a component and the corresponding standard (IPC/JEDEC J-STD-033A) defines part handling procedures (packing, shipping, and storage plan).  

The majority of non-LCP connectors fall into the IPC/JEDEC J-STD-020B MSL levels of 2 – 5a and require the following packaging to conform to the J-STD-033A standard:
•   Moisture barrier bags
•   Desiccant
•   MSID (Moisture-sensitive identification label)
•   Caution label

This industry standard provides direction for the connector manufacturer and board assembler, and minimizes assembly and productivity issues due to connector resin blistering during the lead-free SMT process.

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