Moisture barrier bags for connector packaging

Moisture barrier gags are defined in the IPC/JEDEC J-STD-033A standard as,
“A bag designed to restrict the transmission of water vapor and used to pack moisture sensitive devices.”
Water vapor transmission rate (WVTR) is the measurement of the effectiveness of these bags in the restriction of moisture permeability.

High-density polyethylene (HDPE) bags are better for preventing moisture ingress than low-density polyethylene (LDPE) bags and the thicker the bag, the lower the WVTR.

10 kgs of SCSI connectors in a 30% glass-reinforced, flame-retarded Stanyl resin were packaged in the above suggested plastic bags with a surface area of 1 meter2 and subjected to 2 different storage conditions:
1. 26°C @ 65% RH
2. 35°C @ 90% RH

Previous tests have shown that, at a 260°C peak temperature  lead-free solder condition, the blister onset moisture level for this Stanyl resin is » 1.5 wt%.  The results below show that, at the storage conditions of an air-conditioned warehouse in Southeast Asia (Condition 1), even the thinnest tested LDPE bag would provide enough moisture prevention to keep the connectors below the blister threshold level for 90 days.  At the higher thickness LDPE or at either of the 2 HDPE thicknesses, the storage time would be substantially longer.

If the packaged connectors were left exposed to the natural environment (Condition 2), only the HDPE bags would provide any real safety factor and then only the 100 µm thickness would allow for 90 day storage without blistering.

These results show that:
•   Any packaging is better than no packaging
•   The thicker the moisture barrier bag the better for retarding moisture absorption
•   HDPE moisture barrier bags retarding moisture absorption better than LDPE bags

According to the IPC/JEDEC J-STD-033A standard, parts having MSLs higher than 1 require desiccants be used as an integral part of the packaging.

Silica gel desiccant, added to moisture barrier bags, is very effective at retarding moisture uptake in the connectors.

In the test results shown below, the effectiveness of desiccant with moisture-sensitive connectors (MSLs > 1) was measured in moisture barrier bags measuring 21.5 cm X 21.5 cm in 2 different thicknesses (0.06mm & 0.2mm) were used with 11 grams (»1/2 unit ) of silica gel.  The environment to which the packaged connectors were exposed was 35° @ 90% RH.  

Effectiveness of desiccant on moisture absorption of packaged connectors

According to the IPC/JEDEC J-STD-033A standard, desiccants must meet the MIL-D-3464, TYPE II standard. There are a number of approved options including clay and silica gel.  Various websites can be found that give information on the amount of desiccant required to meet the spec based upon the package size.

For standard moisture barrier bag sizes used for shipping connectors (loose or on reels) the amount of desiccant required is less than 2 Units.

A desiccant Unit is determined by the following equation found in IPC/JEDEC J-STD-033A .
U = (0.304 * M * WVTR * A)/D
where:
U = Amount of desiccant in UNITS
M = Shelf life desired in months
WVTR = Water vapor transmission rate in grams/100 in2 in 24 hrs
A = Total surface area of the MBB in square inches
D = The amount of water in grams, that a UNIT of desiccant will absorb at 10% RH and 25°C.

Depending on the kind of desiccant used, the cost/package can be < $0.15.

If moisture absorption precautions are taken and connectors still exhibit blistering, the amount of moisture in the parts should be measured to determine what steps are required to solder without blistering.