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

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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2022

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

07-21-2022

This 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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In order to meet the increasing demand for smaller, more powerful, and complex electronics, PCB designers are under pressure to create boards that are not only reliable but also sustainable. This can be a challenge, as many of the design choices that maximize performance can be less environmentally friendly. However, with a little extra effort, it is possible to create circuits that are both high-performing and sustainable. In this column, I will explore some of the key factors to consider when designing sustainable PCBs.

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Fresh PCB Concepts: The Right Board for the Flex Job

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Fresh PCB Concepts: What Damage Does the Assembly Process Have on a PCB? Part 4

02-08-2022

In Part 3 of this series, I discussed how phenolic cured laminates can be mechanically weaker than their dicey cured laminate counterparts. I pointed out some of the material properties listed on material data sheets that explain and support this point. Whereas the phenolic systems are better at thermal management, the dicey systems are better under mechanical stress. There is no right or wrong here. The systems just perform differently under different circumstances. Understanding the differences and how they relate to the applied assembly process are important to ensure success. For this post I would like to discuss the effect moisture has on the printed circuit board.

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2021

Fresh PCB Concepts: Does the Assembly Process Damage a PCB? (Part 3—Phenolic Epoxy Systems)

12-16-2021

In part 2 of this series, I explained how the T260 and T288 material datasheet values could be used as an indicator of how durable a laminate system (FR-4) shall be when exposed to heat. The higher the temperature applied, the less time it takes to delaminate the FR-4. Traditional dicey cured epoxy systems do not stand up to lead-free assembly temperatures as well as one would think. The phenolic cured epoxy systems are much better suited and able to withstand the higher temperatures applied with lead-free assembly temperatures for longer periods of time.

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09-30-2021

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Fresh PCB Concepts: HDI Microvia Features in Illustrations

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Fresh PCB Concepts: Does the Assembly Process Damage a PCB? (Part 1—Soldering)

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Fresh PCB Concepts: Mitigating the Increasing Prices of PCBs

03-18-2021

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Fresh PCB Concepts: UL Certification for the Safety of PCBs

02-18-2021

Security is essential in the electronics industry. It is vital that users can rely on the finished products when considering factors such as fire and electrical safety, which means that both the PCB and the materials they contain must measure up to the highest standards. To ensure that the boards do conform, it has become common practice to UL certify the constituent materials or the PCB itself. In this column I am going to discuss UL certification, what’s involved and why you need it.

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2020

Fresh PCB Concepts: How 5G is Influencing PCB Technology Trends

12-17-2020

We have all heard about the Internet of Things (IoT) and artificial intelligence (AI). In combination with the increased data transfer rates available through 5G, they can open up a whole new level of connectivity and communication between devices and things.

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11-05-2020

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Fresh PCB Concepts: 4 Characteristics to Consider When Selecting PCB Base Materials

10-08-2020

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Fresh PCB Concepts: Advantages of Application-Engineered PCBs

09-17-2020

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Fresh PCB Concepts: How Do You Calculate Finished Copper?

08-13-2020

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Fresh PCB Concepts: 7 Options for Via Treatment

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Fresh PCB Concepts: What You Should Know About Your Board’s Solder Mask

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Fresh PCB Concepts: The Benefits of Being a PCB Producer Without Owning Any Factories

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With NCAB's infrastructure and factory management team based in China, many customers ask why they don’t own any factories. Jeffrey Beauchamp explains how it's part of the company's long-term strategy and an advantage to our customers that they don’t own any factories. Instead, they “own” what is most important: the relationship with the factory.

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2019

Fresh PCB Concepts: Designing a PCB for Telecom Applications

12-12-2019

Jeff Beauchamp and Harry Kennedy discuss PCBs for telecommunication applications, including key factors to consider, such as design and material considerations. They also recommend involving your PCB supplier at the time of design to help ensure manufacturability at the lowest possible cost.

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Fresh PCB Concepts: The Current Material Situation

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We have all heard about the component crisis in the circuit board industry, and maybe you heard about the CCL shortage, but how many are aware of the bare board material shortage? Ruben Contreras explains the current material situation and tips to address this issue.

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When faced with critical time-to-market situations, it is all too easy to say, “It doesn’t matter because this is just the prototype; we can fix this later.” However, if the design is perfected from the beginning, cost savings can be applied, and manufacturability can be ensured. Perhaps most importantly, the design can be adapted with reliability in mind, leaving a seamless transition from prototype to production. How do we get it right from the start?

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Fresh PCB Concepts: Why Material Selection Matters

10-02-2019

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