Outgassing on a Printed Circuit Board: A Comprehensive Analysis

 

Introduction to PCB Outgassing

Outgassing is a critical phenomenon in printed circuit board (PCB) manufacturing and performance where materials release trapped gases or volatile compounds under specific conditions. This process can significantly impact the reliability, functionality, and longevity of electronic devices, making it a crucial consideration in PCB design and manufacturing.

Understanding the Fundamentals

Definition and Basic Concepts

Outgassing, also known as offgassing, occurs when materials release previously trapped, absorbed, or adsorbed gases or volatile compounds. In PCB context, this typically happens under:

• Elevated temperatures
• Reduced pressures
• Environmental stress
• Chemical reactions

Common Sources of Outgassing

Material Category
Common Sources
Typical Volatile Compounds
Base Materials
FR-4, ceramics
Water vapor, organic solvents
Adhesives
Die attach, bonding
Cure byproducts, solvents
Solder Masks
Coating materials
VOCs, cure residues
Components
Plastic packages
Plasticizers, moisture
Conformal Coatings
Protective layers
Solvents, cure products

Mechanisms of Outgassing

Physical Processes

Temperature-Dependent Release

Temperature Range (°C)
Primary Mechanisms
Common Compounds Released
25-100
Moisture desorption
Water vapor
100-200
Solvent evaporation
Organic solvents
200-300
Material decomposition
Complex organics
>300
Thermal degradation
Breakdown products

Chemical Processes

Common Chemical Reactions

Reaction Type
Mechanism
Products
Hydrolysis
Water reaction with materials
Alcohols, acids
Oxidation
Oxygen interaction
Oxides, CO2
Decomposition
Thermal breakdown
Various gases
Polymerization
Cure reactions
Reaction byproducts

Impact on PCB Performance

Immediate Effects

Effect
Impact Level
Consequences
Delamination
High
Structural failure
Void Formation
Medium
Signal integrity issues
Surface Contamination
Medium
Connection problems
Component Displacement
High
Assembly failures

Long-term Consequences

Reliability Issues

Time Frame
Issue Type
Potential Impact
Short-term (0-6 months)
Assembly defects
Immediate failures
Medium-term (6-24 months)
Performance degradation
Gradual deterioration
Long-term (>24 months)
Material breakdown
System failure

Prevention and Control Measures

Design Considerations

Material Selection Guidelines

Material Type
Recommended Properties
Verification Method
Base Material
Low moisture absorption
TGA analysis
Solder Mask
Low VOC content
Outgas testing
Adhesives
High temperature stability
DSC analysis
Components
Moisture sensitivity level
MSL testing

Manufacturing Controls

Process Parameters

Process Step
Control Parameter
Acceptable Range
Baking
Temperature
125°C ± 5°C
Baking
Duration
4-24 hours
Storage
Humidity
<30% RH
Storage
Temperature
20-25°C

Testing and Measurement

Standard Test Methods

Common Testing Procedures

Test Method
Standard
Application
TGA
ASTM E1131
Weight loss analysis
FTIR
ASTM E168
Chemical composition
GC-MS
ASTM D7599
Volatile compounds
RGA
ASTM E595
Condensable materials

Acceptance Criteria

Industry Standards

Industry
TML Limit
CVCM Limit
WVR Limit
Space
<1.0%
<0.1%
<0.5%
Medical
<0.5%
<0.05%
<0.3%
Automotive
<1.5%
<0.15%
<0.7%
Consumer
<2.0%
<0.2%
<1.0%

Industry-Specific Requirements

High-Reliability Applications

Space and Aviation

Requirement
Specification
Test Method
TML
<1.0%
ASTM E595
CVCM
<0.1%
ASTM E595
Vacuum Stability
24-hour test
Custom
Thermal Cycling
-55°C to +125°C
MIL-STD-883

Medical Devices

Parameter
Requirement
Standard
Biocompatibility
ISO 10993
Multiple tests
Sterilization
EtO compatible
ISO 11135
Outgassing Limits
Application specific
Custom

Mitigation Strategies

Process Optimization

Pre-Assembly Treatment

Treatment
Parameters
Benefits
Baking
125°C, 4-24h
Moisture removal
Plasma cleaning
Low pressure, RF
Surface activation
Vacuum treatment
<1 torr, 2-4h
Volatile removal

Material Modifications

Modification
Approach
Impact
Surface treatment
Plasma/Corona
Improved adhesion
Barrier coating
Metallic/Ceramic
Reduced outgassing
Material substitution
Low-outgas alternatives
Prevention

Future Trends and Developments

Emerging Technologies

Technology
Application
Benefit
Smart materials
Self-monitoring
Early detection
Nano-coatings
Barrier enhancement
Better protection
Green materials
Environmental
Reduced VOCs

Frequently Asked Questions

Q1: What are the primary causes of outgassing in PCBs?

The main causes include trapped moisture, volatile organic compounds (VOCs) in materials, chemical reactions during curing processes, and thermal decomposition of materials at elevated temperatures. Environmental conditions like temperature, pressure, and humidity can accelerate outgassing.

Q2: How can I detect outgassing problems in my PCB assembly?

Common detection methods include:

• Visual inspection for delamination or blistering
• Weight loss measurements
• Thermal analysis (TGA)
• Gas chromatography-mass spectrometry (GC-MS)
• Residual gas analysis (RGA)

Q3: What are the best practices for preventing outgassing issues?

Key prevention strategies include:

• Proper material selection and qualification
• Implementation of moisture management procedures
• Controlled storage conditions
• Appropriate baking procedures before assembly
• Process optimization during manufacturing

Q4: How does outgassing affect PCB reliability in space applications?

In space applications, outgassing can be particularly problematic due to the vacuum environment. Released volatiles can condense on sensitive surfaces, causing:

• Optical surface contamination
• Electrical connection failures
• Thermal control degradation
• Component malfunction

Q5: What are the industry standards for outgassing testing?

The most widely recognized standards include:

• ASTM E595 for TML and CVCM testing
• NASA SP-R-0022A for space applications
• IPC-4101 for base materials
• MIL-STD-883 for electronic components

Conclusion

Understanding and controlling outgassing in PCBs is crucial for ensuring reliable electronic performance across various applications. Through proper material selection, process control, and testing procedures, manufacturers can minimize outgassing-related issues and produce high-quality PCBs that meet increasingly stringent industry requirements. As technology advances, new materials and methods continue to emerge, offering improved solutions for outgassing challenges.