Introduction
Moisture sensitivity is a major reliability concern for many electronic components and devices. Even small amounts of moisture can lead to corrosion, short circuits, and device failures. This is especially problematic for moisture sensitive components that are hygroscopic, meaning they absorb moisture from the surrounding environment. Common moisture sensitive devices include integrated circuits, sensors, printed circuit boards, LEDs, and more.
Managing moisture exposure is critical throughout the entire supply chain, from manufacturing to shipping, storage, and end use. An effective strategy for preventing moisture issues requires tight control of materials, processes, storage and handling. A bill of materials (BOM) details all the parts and components that make up a product. When designed properly, the BOM can be a powerful tool to mitigate moisture risks. This article will examine how bill of material management enables proactive control of moisture sensitive materials.
What Makes a Component Moisture Sensitive?
Moisture sensitivity arises from the materials and construction of electronic components and devices. Key factors that contribute to moisture absorption and related failures include:
Hygroscopic Materials
Many materials used in electronics naturally absorb water from the air. Examples include organic materials like epoxy resins and polymers, as well as some ceramics and metal oxides. The rate of moisture absorption depends on material composition, porosity, and surface area.
Permeable Housings
Plastic encapsulants and housings can allow passage of water vapor and gases. Moisture permeation increases with higher temperatures.
Delamination and Cracking
Poor adhesion or cracks in chip coatings, wire bonds, and package seals provide pathways for moisture ingress. Thermal cycling and mechanical stresses can cause delamination over time.
Thin Form Factors
As components get thinner, moisture can diffuse more quickly through the material. This makes ultrathin ICs, flexible circuits, and chips-on-film especially vulnerable.
Interfaces and Contamination
Any interface between materials, even glue or epoxy, can provide a route for moisture diffusion. Ionic contaminants left from manufacturing can attract and absorb moisture through capillary action.
High Surface Area
Miniaturized components with more complex geometries have greater surface area compared to their volume. This amplifies the effects of moisture absorption.
Corrosion-Prone Metals
Many electronic devices contain copper traces, iron-nickel leadframes, aluminum bondwires, and other metals prone to galvanic corrosion when exposed to moisture.
Moisture Sensitivity Levels
To help device manufacturers assess the risks of moisture damage, the electronics industry has established moisture sensitivity levels (MSLs) for certain common component packages. The MSL scale ranges from 1 (unlimited shelf life in ambient humidity) up to 6 (highly moisture sensitive). Here are the classifications:
- MSL 1: No special handling required
- MSL 2: 1 year shelf life in humidity up to 60% RH
- MSL 2a: 4 weeks shelf life in humidity up to 60% RH
- MSL 3: 168 hours shelf life in humidity up to 60% RH
- MSL 4: 72 hours shelf life in humidity up to 60% RH
- MSL 5: 48 hours shelf life in humidity up to 60% RH
- MSL 5a: 24 hours shelf life in humidity up to 60% RH
- MSL 6: Time on label only, up to 6 hours exposure
Parts are tested to determine their MSL rating. However, MSL classifications have limitations:
- They only cover packaged ICs and discrete semiconductors.
- Testing uses standardized conditions that may not reflect actual usage.
- MSLs indicate sensitivity, but don't guarantee a shelf life.
- Absorbed moisture and corrosion may still occur before the time limit.
- MSL expiry dates can be invalidated by handling and storage errors.
So while MSLs provide a general guideline, they cannot replace proper moisture control practices.
How Moisture Causes Electronics Failures
To prevent moisture issues, it helps to understand the failure mechanisms:
Galvanic Corrosion
When surface moisture forms electrolytic films, it allows current to flow between adjacent metal conductors and traces. This galvanic corrosion erodes metals and increases contact resistance. Corroded traces and vias can cause open circuits.
Electrochemical Migration
Ionic contaminants dissolved in moisture enhance electrochemical transport between conductors. Metal ions migrate and grow into dendrites that short circuit traces. This occurs more readily at higher temperatures and voltages.
Swelling/Delamination
Moisture absorption causes hygroscopic materials like epoxy resins to swell in volume. The swelling strains interfaces between materials, leading to delamination and cracking. These create pathways for more moisture ingress.
Popcorning
As moisture vaporizes during soldering or other heating, it expands rapidly. The resulting pressure can cause package cracks, wire bond lift-off, and interfacial separation between chip and substrate. This damage is known as "popcorning."
Electrolytic Growth
When a bias voltage is present, moisture enables electrolytic growths of conductive metal filaments. These form inside insulating materials, eventually bridging conductors and short-circuiting components.
Oxidation and Hydrolysis
Moisture reacts with many materials, altering their electrical, chemical, and mechanical properties. For example, moisture oxidizes metals, degrades adhesives, and hydrolyzes silicon oxides.
Dendrite Formation
Certain materials dissolve and form dendritic growths when exposed to moisture. Silver migration is a common cause of short circuits and corrosion on printed circuit boards and component leads.
These moisture-induced degradation processes can be difficult to detect. They may only cause intermittent faults or latent damage. But over time, the effects accumulate and ultimately lead to circuit and device failures.
How Bill of Materials Ties into Moisture Control
An electronics bill of materials contains a complete parts list and product structure for an assembly or system. A well-constructed BOM is vital for managing moisture control. Here's why:
Links Materials to Design
The BOM connects specific materials and components to their placement in the product architecture. This enables assessing the risk based on which parts are moisture sensitive and their location.
Facilitates Process Controls
With the BOM linking parts to process steps, manufacturers can select appropriate drying, baking, storage, and handling for each material during production.
Allows Part Substitution
When high-risk parts are identified by the BOM, alternate components with lower moisture sensitivity can be substituted to reduce the likelihood of failures.
Provides Traceability
Item-level serialization and revision data in the BOM supports traceability and tracking of moisture exposure. This contained visibility aids failure investigation and root cause analysis.
Assists Quality Control
Digital BOM formats allow automated checks of moisture indicators like MSLs during component purchase and kitting. Manufacturing execution systems can then enforce process controls per the BOM specifications.
Supports Documentation
BOM data aids generating process instructions, training materials, and control documents. It also provides evidence of moisture management procedures for customer requirements and industry standards compliance.
For these reasons, an accurate, well-structured BOM is a prerequisite for proactively avoiding moisture issues. Let's examine BOM best practices to enable robust moisture control.
Best Practices for Managing Moisture Sensitive Items in BOMs
Assign Unique Item Numbers
Every component, subassembly, and material requires a distinct item number in the BOM. Unique IDs prevent confusion between similar parts and provide tracking throughout the product lifecycle.
Include Revision Levels
BOMs must show revision levels for all items to allow comparing moisture sensitivity among different versions of parts and assemblies.
List Manufacturing Data
Relevant manufacturing details in the BOM improve moisture analysis, such as date/lot codes, serial numbers, and fabrication processes.
Specify Package Types
The component package type indicates configurations more susceptible to moisture, e.g. QFN versus DIP ICs. Call out package information in the BOM when available.
Indicate Encapsulation
Note which parts are conformal coated, potted, or otherwise encapsulated to exclude moisture. Encapsulants help determine the actual moisture risk.
Add Moisture Content
Listing moisture content specifications for hygroscopic materials identifies parts needing precautions. For example, nylon typically absorbs 2-4% water by weight.
Show MSL Ratings
Including known MSL ratings in the BOM provides a quick gauge of item sensitivity, with lower ratings signifying higher risk. However, MSLs are not available for all component types.
Highlight Critical Items
Use visual indicators like color coding in the BOM to distinguish particularly moisture-sensitive parts requiring utmost care in processing and handling.
Include Handling Requirements
Specify any moisture precautions needed in fabrication, storage, or packaging, such as dry box storage, vacuum sealing, or desiccant bags.
List Approved Substitutes
Identify alternative parts with equivalent form, fit, and function that have lower moisture absorption. This facilitates part replacement when availability issues arise.
Detail Applied Coatings
Note use of conformal coatings, sealants, potting compounds, or other moisture barriers added during production. Include type of coating and location on assembly.
Provide Design Guidelines
Add general design guidelines for mitigating moisture in the BOM appendix, covering topics like hermetic sealing, avoiding buried vias, and limiting electrolytic contamination.
Following these guidelines optimizes the BOM information to support robust moisture control planning, reduce risks in production, and enable traceability.
Manufacturing Process Controls Guided by BOM
With a well-constructed BOM, manufacturers can implement process controls to avoid moisture pitfalls. Common methods at each stage include:
Materials Management
- Follow MSL expiry dates for moisture sensitive components
- Store parts in dry boxes or cabinets when not in use
- Reduce handling to minimize exposure; use tweezers or gloves
- Keep workspaces and equipment surfaces clean to avoid contamination
###PCB Fabrication
- Apply soldermasks, conformal coatings, and sealants to PCBs after etching
- Use finishing processes like Immersion Silver plating to avoid oxidation
- Control storage humidity levels for finished boards before population
###SMT Assembly
- Set up separate feeders for moisture sensitive components
- Perform selective baking/drying of parts right before use per BOM
- Use no-clean fluxes to avoid trapping moisture in residue
- Reflow profile and cooling rate tuned to minimize popcorning
###Post Solder Cleaning
- Selectively clean just high-risk fluxed areas to lower moisture entrapment
- Optical inspection of solder joints and underfill fillets to check for cracks/voids
- Localized application of sealants and encapsulants based on BOM design
###Final Testing
- Perform electrical tests at elevated temperature/humidity to screen for latent moisture damage
- X-ray inspection of BGA/CSPs to identify potential delamination or cracks under ICs
- Hermetically seal finished assemblies as needed per BOM requirements
###Shipping/Storage
- Vacuum seal parts and assemblies shown in BOM to need extra protection
- Desiccants or humidity indicator cards aid monitoring moisture levels
- Controlled warehouse storage conditions (climate, ESD, etc) based on BOM
Executing test and inspection points during manufacturing guided by the BOM details enables catching potential moisture issues early. Tying the right precautions to specific items in the BOM limits moisture exposure.
Applying BOM Analysis to Assess and Mitigate Product Risk
In addition to manufacturing process controls, BOM analysis helps assess design vulnerability to moisture and plan appropriate risk mitigation actions:
Identify Highly Absorbent Materials
Scrutinize BOM for parts made of hygroscopic materials like nylon, cellulose, polyurethane, PVC, etc. Assess potential for swelling-related damage.
Recognize High Moisture Diffusivity Parts
Thin components and those with higher surface area to volume ratios can quickly absorb ambient humidity. Target these items from the BOM for mitigation.
Evaluate Interface Integrity
Interfaces between disparate materials like chip coatings and leadframe adhesives are prone to delamination as moisture penetrates.
Review Exposed Surfaces
Parts and traces on the exterior of assemblies have greater exposure to humidity. The product architecture in the BOM facilitates identifying these.
Check for Buried Vias
PTH vias enclosed within PCBs can trap process chemicals and moisture, inducing corrosion. BOM indicates layer usage.
Assess Thermal Properties
Coefficient of thermal expansion mismatches highlighted in the BOM reveal locations prone to fatigue and fracturing as assemblies heat and cool.
Estimate Failure Consequences
BOM structure maps critical functions to components. This helps gauge the severity if certain parts fail from moisture issues.
Select Alternative Components
Substitute plastic parts prone to moisture absorption with lower-risk materials like ceramics or glass. Reduce exposed trace lengths.
Apply Protective Coatings
Conformal coatings, potting compounds, and sealants prevent moisture contact with susceptible areas based on BOM layout.
Improve Encapsulation
For high-risk semiconductors, choose moisture-resistant packages like LGA rather than exposed QFN. Enhance sealing and adhesion.
Add Ventilation Paths
Provide vent holes in housings indicated in BOM to allow moisture release during heating, reducing internal vapor pressure.
Introduce Redundancy
Add redundant contacts on at-risk interfaces prone to delamination. Diverse power and data paths prevent single-point failures.
Taking a systematic approach to moisture analysis based on bill of materials details helps gauge risk, design robust products, and implement targeted mitigations. Ongoing BOM maintenance and revision then sustains control over the product lifecycle.
Conclusion
Even trace amounts of moisture can lead to catastrophic failures of sensitive components and devices. Tight control of moisture exposure throughout manufacturing and service life is essential to product reliability. An intelligent bill of materials creates the foundation for robust moisture control programs. Linking materials properties and design data to specific items provides targeted visibility for process controls and risk mitigation. Developing moisture avoidance practices guided by the BOM will significantly improve manufacturing yields, product quality, and customer satisfaction.
Frequently Asked Questions
How does temperature affect moisture absorption in components?
Higher temperatures accelerate moisture uptake in hygroscopic materials. Elevated temperatures provide more thermal energy to overcome moisture diffusion barriers. However, absorption eventually saturates as materials become fully saturated.
How long can moisture sensitive devices be exposed to ambient air?
Maximum exposure duration depends on the item's moisture sensitivity level. MSL 1 components have no limits, while MSL 3-6 parts range from 72 hours down to just 6 hours. Parts within a sealed moisture barrier bag and desiccant can safely exceed usual time limits.
How is moisture removed from sensitive components?
Baking and dry box storage removes moisture from components. Baking applies heat to drive off absorbed water, while dry boxes maintain very low humidity levels. However, baking can only be done a limited number of times for plastic parts before damaging the material.
Can coatings completely prevent moisture damage?
While coatings help reduce moisture penetration, they cannot fully eliminate it under prolonged exposure. Moisture diffusion still gradually occurs over time, and imperfections in coatings provide pathways for moisture ingress. Proper handling and storage along with coatings provides the best protection.
How small of an opening is needed for moisture to damage an electronics assembly?
Even microscopic cracks or pores with widths measured in microns permit enough moisture ingress over months or years to cause corrosion. This makes imperfect seals and encapsulants particularly concerning. Reliable long term protection requires hermetic sealing.
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