Fresh PCB Concepts: Part 5—How to Handle Possible Moisture During Shipping, Handling, and Storage


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David_Duross_NCAB.jpgThis is the fifth part in a series titled “What Damage Does the Assembly Process do to a PCB?” In part four of this series, I discussed the effect moisture has on the printed circuit board at soldering temperatures. I explained the material properties of FR-4 laminate and how they are hygroscopic. We also covered an acceptable practice known as dry baking used to force moisture from the product just prior to being exposed to soldering temperatures. I thought it appropriate expand further on part four in this column.

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Here, I will discuss further sources of moisture that may come about from shipping, handling, and storage. We need to be aware of these sources not only once product ships from your supplier, but also when it’s placed into your inventory waiting to assemble. The important thing to add here is that common packaging materials used by the PCB industry do not act as a 100% vapor-proof barrier. The shrink wrap film used only slows down the process of moisture absorption by the product. Moisture in the environment outside the shrink-wrap material shall migrate through the protective film but at a much slower rate.

Considerations

What types of materials are used for shrink-wrapping film and how much moisture resistance shall they provide?

Different manufacturers use different materials that offer various levels of protection. The material datasheet for these materials shall list a water vapor transmission rate (WVTR). The WVTR is a measure of the passage of water vapor through a substance. The thicker the material the greater the resistance. For example, a 10-mil thick clear shrink-wrap material I have used in the past for PCB packaging has a WVTR of < 0.1 g/24 hr M2 @ 23°C 50% RH. If your shrink-wrapped bundle of boards is sitting on the shelf in your warehouse for six months or more, moisture is still migrating through that 10-mil thick layer.

How well was the product shrink wrapped and sealed?

Some boards have edges routed or scored to a sharp angle or point at the corner of the board or the array. The sharp corner may puncture the film thus compromising the vacuum seal. Care also needs to be taken when handling the shrink-wrapped bundle to ensure you don’t damage and rip the shrink-wrap and break the vacuum seal.

How well are the shrink-wrapped bundles boxed up?

In the past some of you may have received board shipments from other suppliers where the box had burst open at the corners. Maybe your supplier used a single walled cardboard box without strapping reinforcement applied to the outside of the box. A box like that can get wet and weaken while in transit. Maybe the shrink-wrapped bundles were not tightly packed and were loose within the box. Weight placed on top of your delivery crushed the box and split it open. If the shrink-wrapped bundles were loose, they could move around potentially causing the vacuum seal burst open.

Does the product need to pass through customs?

Product from sources located in another country may be randomly selected for inspection by a customs official. Boxes may be opened and the containing environment compromised as a result.

Is desiccant provided within the shrink-wrapped bundles?

Desiccant absorbs moisture from the air it comes into contact. If the shrink wrap is compromised the desiccant may reduce the amount of moisture being absorbed into the PCB.

The following are some recommendations that may be specified as part of a shipping specification:

  • Shrink wrapped bundles should be packaged in a way that prevents the bundles from shifting around within the shipping box. Either rigid foam, extra bubble wrap or preferably, pieces of corrugated cardboard should be placed around the shrink-wrapped bundles for support. Movement within the box should be prevented. For example, NCAB Group has this as part of our internal specification for packing at all of our main factories.
  • Double walled shipping boxes should be used. We recommend a double wall of corrugated cardboard with a laminated covering on the outside of the box. The double walled corrugated cardboard adds a level of durability required for global shipping. The laminated covering is one more layer against the elements. Boxes should have all open seams closed and covered by shipping tape. A close and tightly sealed box adds much more material between the printed circuit boards and available moisture.
  • Strapping reinforcement should be added to the outside of the shipping box.
  • A weight limit should be specified on the boxes. Something to consider for weight limit determination is what are your people comfortable lifting. Also, the more weight allowed in a single box the more mass it has. When a box is tossed, an overly heavy box will have a lot of momentum. A heavy stack of boards not tightly packed in the box wants to keep going after the box stops. For example, we at NCAB Group specify a maximum weight limit of 25 Kg at all of our main factories.
  • There should be a stack height limit on the parts being shrink-wrapped. For example, we specify a stack height of 10 or 25 pieces.
  • You should have an option is to request that shrink-wrapped bundles be provided with a desiccant pack appropriate for the solderable surface finish. In fact NCAB’s specifications require we ship product from our main factories with desiccant.

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Are there shipping products available that offer a 100% vapor barrier?

No. However, there are packaging materials available that offer a near 100% vapor barrier. These packing materials are a hybrid combination of polymers and a metallic layer. The common term for these packaging materials is moisture barrier bag (MBB). A common MBB consisting of nylon/foil/polyethylene shall have a typical WVTR of ≤0.0310 g/m² (≤ 0.0002 g/in2) / 24 hrs. There are other types of MBBs available that offer similar or slightly less levels of protection.

If moisture barrier bags are so much better, why are they not used every time?

MBBs are very expensive and require special equipment to partially remove air from the bag and thermally seal the bag closed. Once the bag is opened you cannot reseal the bag with shipping tape. The shipping tape does not offer the same WTVR level of protection of the properly sealed MBB. You have the expensive MBB without the benefit of the MBB. You need to thermally reseal the MBB with the appropriate equipment to gain the WTVR of the MBB. Most companies do not have the equipment to do this.

To further explore and understand best shipping, handling, and storage practices we recommend consulting with your PCB supplier.

David Duross is an engineering director for NCAB Group USA.

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