Selecting a Moisture Barrier Bag

The need to keep surface-mount devices (SMDs) dry between the time of manufacture and the point of reflow soldering has driven the development of moisture barrier bags. Also known as vapor barrier bags, these bags are made from multiple layers of plastic and aluminum that control moisture vapor leakage.

Barrier bags are not moisture vapor-proof, nor do they remove moisture. Over time, moisture vapor will leak into the bag. Desiccant is put into the bag to reduce humidity and scavenge moisture that penetrates the bag.

A humidity indicator card (HIC) also may be put into the bag. HICs indicate the relative humidity with moisture-sensitive, color-changing chemical spots. HICs provide assurance the devices are dry when received.

As a final moisture impediment, vacuum removes air-containing moisture before the bag is heat-sealed.

Tests and Test Methods

Several tests are important to describe the performance of a moisture barrier bag. Resistance to moisture penetration for a barrier material must be defined. Also, the bag must be free of pinholes and voids in the side seams. It must pass a bag integrity test which includes submersion and puncture-resistance tests.

MVTR

The moisture vapor transmission rate (MVTR) is the rate that water vapor passes through a specific area of barrier material. As the MVTR reduces, dry storage time increases, and desiccant loading decreases. MVTR is measured in grams of water vapor per 100 square inches of barrier per 24 hours (g/100 in.²/24 h).

There are two primary test methods for MVTR:

ASTM F1249

In this test, a sample of barrier material is placed between wet and dry compartments. Infrared light is used to detect water vapor leaking through the barrier material.

Complete barrier bags also can be tested. The sealed bag is placed in a large container. Probes inside the container and bag are used to measure the MVTR.

Federal Test Method Standard 101 Method 3030

Using FTMS 101 MTH 3030, a sealed barrier bag with a desiccant pouch inside is weighed and placed in a chamber at 100°F and 90% relative humidity for 64 hours. Weight gain of the bag indicates moisture gain. The MTVR can be calculated using weight, bag area, and time.

Which Method?

Advocates of MTH 3030 claim that ASTM F1249 cannot measure MVTR low enough for foil barrier bags. Champions of ASTM F1249 disagree, adding that MTH 3030 allows too much variation in the procedure and is too technique-sensitive to produce results that can be compared from lab to lab. While a definitive study has not been made, virtually every moisture barrier bag supplier reports data from ASTM F1249 testing.

Samples

Whether a flat sample or bag is tested, the material should be from a factory- made barrier bag. The process of bag making can degrade barrier properties. Some bag users require barrier bags to be mechanically flexed prior to testing. This procedure is described in FTMS 101 MTH 2017.

Bag Integrity Tests

MIL-P-117 and -116 describe general procedures for making bags. In MIL-P-116, techniques for leak detection are specified.

Submersion

FTMS 101 MTH 5009 is a test method for finding leaks in bags. A bag is inflated and sealed. The test operator submerges the bag in water and applies pressure to the bag. Air bubbles coming from the material or seals indicate the location of any leaks. MIL-P-116 does not allow any leaks.

Hanging Weight

In FTMS 101 MTH 2024, a 1-in. segment of the bag’s side seam is stressed with a 3.5-lb hanging weight. MIL-B-81705 refers to this test method and does not allow any separation of the sealed material.

Puncture Resistance

This test determines a material’s resistance to puncture with a steel probe. MIL-B-81705 requires a minimum of 10-lb resistance. In FTMS 101 MTH 2065, a specimen of bag material is placed in a flat cage with a hole through the center. A 5-in. long rod with a 1/8-in. radius is pushed through the bag material. An electronic load cell measures the force (in pounds) required to puncture the material.

Barrier Technologies

Two primary moisture barrier technologies are used for bags. Barriers of aluminum foil and aluminized polyester are used where low MVTR is required. Most SMDs are packaged in metal barrier bags. Thick layers of plastic also can provide limited barrier for very short-term applications.

Foil/Polymer

This is the oldest and most effective barrier technology. A thin sheet of aluminum foil, usually about 0.00035-in. thick, is laminated to nylon or Tyvek™ for support and protection.

Aluminized Polymer

This newer technology reduces material cost. Aluminum is vapor-deposited onto polyester. The metal is so thin that multiple layers of aluminized polyester are laminated together. Voids in one layer are covered by another.

Engineered Polymer

Clear plastics that do not use metallic layers provide limited moisture barrier. They primarily are used for food packaging. Applications in electronics tend to be for very short-term dry-storage and clean-room situations. Clear barrier bags do not meet the electrical or MVTR requirements of MIL-B-81705C.

Moisture Barrier Bag Structures

Nylon/Foil/Poly

Typically, this structure consists of 60-gauge nylon laminated to 0.00035 aluminum foil laminated to heat-sealable polyethylene (Figure 1). This is the most common foil/polymer laminate. The MVTR for this structure, when properly converted into bags, is about 0.0005 g/100 in.²/24 h.¹

Tyvek/Foil/Poly

This structure consists of Tyvek laminated to 0.00035 aluminum foil which is laminated to heat-sealable polyethylene. Tyvek is a white, textured sheet made by spinning hot plastic onto a moving belt.

Tyvek foil is the oldest barrier structure and, with few exceptions, has been replaced by nylon/foil or metallized polyester structures. When properly converted into bags, the MVTR for this structure is about 0.0005 g/100 in.²/24 h.¹

Aluminized Polyester/Poly

Typically, this structure consists of two layers of 48-gauge aluminized polyester laminated to sealable polyethylene (Figure 2). This is the newest technology for barrier materials. These low-cost bags are suitable for the medium to short term dry-packaging users. For 3.6-mil materials, the MVTR is about 0.02 g/100 in.²/24 h¹. Structures that are 7.0 mils thick can achieve 0.005 g/100 in.²/24 h.¹

Other Performance Considerations

ESD Properties

Moisture barrier bags should provide dissipation, antistatic properties, static shielding, and some measure of EMI/RFI attenuation. The generic specifications in Table 1 should be met.

Standards

MIL-B-81705 Type I Barrier Materials, Flexible, Electrostatic Protective, Heat Sealable provides test methods and limits for MVTR, mechanical, and electrostatic properties for barrier materials. While the standard is comprehensive, it requires special military printing and lab qualification that add to material cost and do not contribute to material performance.

A material that meets the requirements of MIL-B-81705, as opposed to a material on the Qualified Products List (QPL), should suffice for all applications except military or military contractor. Both foil and aluminized polyester structures are listed on the QPL.

EIA 583 Packaging Material Standards for Moisture-Sensitive Items defines a Class 1 barrier as having an MVTR of <0.02 g/100 in.²/24 h. A Class 2 barrier is set at <0.08 g/100 in.²/24 h. EIA 583 also defines a puncture limit at 10 lb and provides desiccant loading calculations.

EIA/JEP 124 Guidelines for the Packaging, Handling, and Repacking of Moisture-Sensitive Components is little more than “EIA 583 Lite.” It provides some general suggestions regarding vacuum sealing, receiving, and repacking barrier bags.

Thickness

Barrier bags are available in thicknesses of 3.2, 3.6, 4.0, 5.0, 5.5, 6.0, 6.1, 7.0, and 10 mils. In general terms, only large thickness differences change bag performance and cost.

Three gauges are somewhat standard: 3.6, 6.0, and 10 mils. If we compare different gauges of the same structure, thicker materials usually provide greater puncture resistance.

Let’s use an aluminized polyester structure as an example. A 3.6-mil material will have a puncture resistance of about 20 lb/in. A 7.0-mil version of the same structure may exceed 32 lb/in.

MVTR

Industry standards require an MVTR of <0.02 g/100 in.²/24 h. A lower MVTR will provide low interior humidity for a longer period.

For example, a 16″ × 18″ barrier bag with an MVTR of <0.02 g/100 in.²/24 h and a maximum interior humidity of 20%, sealed for 12 months, requires 6.6 units of desiccant per EIA 583. A bag of the same size and conditions with an MVTR of <0.0003 g/100 in.²/24 h needs only 0.01 unit of desiccant.

This illustrates the difference in MVTR values. It also shows that desiccant costs can be reduced by using a bag with lower MVTR. Of course, bags with lower MVTR are more costly.

Bag Supplier

It may seem a bit odd to discuss suppliers here; however, the best barrier material can be rendered useless by leaking side seals or holes in the bag. Remember that barrier materials require special handling to avoid pinholes and proper bag-converting equipment to heat-seal thick barrier materials. Your supplier should perform some type of bag integrity testing on an ongoing basis.

Selecting a Moisture Barrier Bag

The list of items to consider when selecting a barrier bag is substantial: MVTR, puncture, term of usage, cost, ESD properties, and supplier reliability. Table 2 will help you compare the choices.

References

1. Tested per ASTM F1249. Flat specimens cut from machine-made bags.

2. Clear barrier bags do not meet the electrical or MVTR requirements of MIL-B-81705C.

About the Author

Brent Beamer is the technical director for the ESD Products Division at Static Control Components. He has authored articles, papers, and tutorials about ESD for industry publications, the British Electrostatic Control Association, and the EOS/ESD Symposium. His activities in the ESD Association include chairing ESD standards development Work Groups for packaging and wrist straps. Mr. Beamer also is a former board member of the ESD Association. Static Control Components, 3010 Lee Ave., P.O. Box 152, Sanford, NC 27331, (919) 774-3808, e-mail: [email protected].

Table 1.

Surface Resistance (interior/exterior): ANSI-EOS/ESD S11.11

<1.0 × 10¹¹ W

Static Shielding: EIA 541 Appendix E

or EOS/ESD S11.31

Tribocharging Film: EOS/ESD ADV11.21

Lower than virgin poly

EMI/RFI Attenuation: MIL-B-81705 Type 1

>25 dB

Table 2.

Copyright 1998 Nelson Publishing Inc.

October 1998