Humidity and Electronics: Corrosion Reliability Issues and Preventive Measures

Front Cover
Humidity and Electronics
Copyright Page
Contents
Preface
1 Humidity and electronics: corrosion perspectives
1.1 Scope of this book
1.2 History of electronics
1.2.1 Early electronics
1.2.2 Modern electronics
1.2.3 Miniaturized electronic devices
1.3 Use of materials in modern electronics
1.4 Electronics and corrosion reliability today
References
2 Basic theory of corrosion and important failure mechanisms connected to corrosion in electronics
2.1 Definition of corrosion
2.2 Corrosion in electronics versus conventional corrosion scenario
2.3 Electrochemical corrosion of metals under wet conditions
2.3.1 Thermodynamics of corrosion
2.3.1.1 Electrode potential
2.3.1.2 Nernst equation
2.3.1.3 Nonequilibrium conditions and corrosion reactions
2.3.1.4 Galvanic series
2.3.2 Practical application of electrochemical and galvanic series
2.3.3 Galvanic and electrolytic cells, and importance in relation to corrosion in electronics
2.3.4 Importance of potential bias and current
2.3.5 Translating electrochemical theory to events on PCBA under humid conditions
2.3.6 Visualizing electrical equivalence of water film as an electrical circuit superimposing on electrical aspects of PCBA...
2.3.7 Importance of solution conductivity
2.3.8 Importance of solution pH, stability of dissolved metal ions, and solubility product
2.4 Corrosion failure modes for electronics and related mechanisms
2.4.1 Reduction in surface insulation resistance and leakage current
2.4.2 Electrochemical migration
2.4.2.1 Under DC conditions
2.4.2.2 Models describing ECM failures
2.4.2.3 Electrochemical migration under AC and pulse DC conditions
2.4.3 Conductive anodic filament
2.4.3.1 Models describing CAF failures
2.4.4 Galvanic corrosion
2.4.5 Fretting corrosion
2.4.6 Creep corrosion
2.4.7 Gaseous corrosion
2.4.7.1 Gaseous environment formed due to polymer degassing
2.4.8 Polymer degradation due to moisture absorption
2.4.8.1 Fickian diffusion process
2.4.8.2 Non-Fickian diffusion process
2.4.8.3 Consequences of moisture absorption into the electronic materials
References
3 Factors determining water film buildup on surfaces and relevance to corrosion in electronics
3.1 Characteristics of moisture
3.1.1 Water
3.1.2 Water vapor pressure
3.1.3 Raoult’s law
3.1.4 Absolute humidity
3.1.5 Relative humidity
3.1.6 Dew point
3.1.7 Water activity
3.1.8 Mollier diagram
3.2 Interaction of humidity with electronics
3.2.1 Water layer formation on the clean PCBA surface
3.2.1.1 Adsorption
3.2.1.2 Capillary condensation
3.2.2 Importance of PCBA surface topography
3.2.3 Critical level of water film thickness for transient failures on a PCBA
3.2.4 Water layer formation on the contaminated PCBA surface
3.2.4.1 Interaction of moisture with hygroscopic contamination
3.2.4.2 Deliquescence
3.2.4.3 Deliquescence lowering
3.2.4.4 Efflorescence
3.2.5 Effect of exposure climatic conditions
References
4 Importance of PCBA cleanliness in humidity interaction with electronics
4.1 Nature of the residue and humidity interaction
4.1.1 Ionic residues
4.1.2 Nonionic residues
4.2 Contamination originating from the PCBA manufacturing process
4.2.1 Contamination arising from PCBA materials and components
4.2.2 Contamination resulting from the PCB fabrication process
4.2.3 Contamination resulting from the PCBA assembling process
4.2.3.1 Solder flux composition and use of “no-clean” flux
4.2.3.2 Residue formation versus soldering process
Wave and selective soldering processes and corrosion reliability
Reflow soldering process and corrosion reliability
Hand soldering process and corrosion reliability
4.2.4 Solder flux chemistry and humidity interaction
4.2.4.1 Solvents
4.2.4.2 Vehicles
4.2.4.3 Activators
Activator residue and interaction with humidity
Moisture sorption by flux chemistry and resulting water layer thickness
Effect of moisture sorption by flux chemistry on electrical behavior
Other activators and humidity interaction
4.2.5 Better PCBA surface cleanliness from the manufacturing process: perspectives
4.3 Contamination related to operator handling
4.4 Contamination originating from the user environment (service life)
4.4.1 Particulate contamination and corrosion reliability of electronics
4.4.2 Gaseous contamination and effect on electronics
4.4.3 Corrosion caused by contamination from external sources
4.4.4 Classification of atmosphere corrosivity and corrosion classes
References
5 Materials and processes for electronic devices and components: how they contribute to corrosion reliability?
5.1 Manufacturing process of bare printed circuit board
5.1.1 Laminate as a basic substrate
5.1.2 PCB surface finish involving metallic combinations
5.1.3 Bare PCB and its contribution to corrosion reliability issues
5.1.3.1 Bare PCB as a source of contamination: leftovers from processing and degassing from materials
5.1.3.2 Laminate as a medium for moisture transport and storage
5.1.3.3 Corrosion of multimetallic surface finish layers on a bare PCB
5.1.3.4 Solder mask under-curing and coverage issues
5.2 Component assembling, mounting, and soldering processes
5.2.1 Component assembling process
5.2.1.1 Through-hole technology and wave soldering process
5.2.1.2 Surface mount technology and reflow soldering process
5.2.1.3 Hand soldering process
5.2.2 Specific aspects of PCBA manufacturing process influencing corrosion reliability
5.2.2.1 Soldering process and PCBA cleanliness
5.2.2.2 Solder alloys and corrosion reliability issues
5.2.3 Other PCBA design aspects affecting corrosion reliability
5.3 Electronic components
5.3.1 Passive components
5.3.1.1 Chip capacitors
5.3.1.1.1 Structure, characteristics, and material makeup
5.3.1.1.2 Capacitors and their role in corrosion reliability issues
5.3.1.2 Resistors
5.3.1.2.1 Structure, characteristics, and material makeup
5.3.1.2.2 Resistors and their role in corrosion reliability issues
5.3.1.3 Switches, connectors, and contacts
5.3.1.3.1 Involved materials and processes
5.3.1.3.2 Switches, connectors, contacts, and their role in corrosion reliability issues
5.3.2 Active components
5.3.2.1 Integrated circuit
5.3.2.1.1 Material makeup and design
5.3.2.1.2 Integrated circuits and their role in corrosion reliability issues
5.3.3 Other electronic components and corrosion
References
6 Examples of corrosion failures in electronics: summary of case studies
6.1 Failures due to process-related cleanliness issues as a main factor
6.1.1 Corrosion of a tactile switch due to flux residue originating from hand soldering
6.1.2 Contamination on PCBA as a precursor for fire in a device exposed to humidity
6.1.3 Failures due to PCBA design and cleanliness issues
6.2 Failures caused by corrosive gases and humidity
6.2.1 Failure of a hybrid power module due to sulfur gas exposure
6.2.2 Creep corrosion of electronics used for telecommunication equipment
6.2.2.1 Failure of palladium pre-plated lead frames used in the telecom industry
6.2.2.2 Failure of printed circuit board of a telephone system due to creep corrosion
6.2.3 Chip resistor failures due to sulfur gases
6.3 Failures due to material combinations and environment
6.3.1 Corrosion of microcircuits wire bonding under different exposure conditions
6.3.2 Corrosion of connectors and contact materials used in electronics
6.4 Failures of high-power electronic systems due to environmental conditions
6.4.1 Failures due to formation of conductive anodic filament: effect of conductor spacing
6.4.2 Corrosion issues related to high-power semiconductor devices
References
7 Preventive measures for corrosion in electronics: intrinsic and extrinsic strategies
7.1 Intrinsic methods for corrosion mitigation in electronics
7.1.1 PCBA materials and component selection
7.1.2 PCBA layout design and component placement
7.1.3 Ensuring PCBA cleanliness at various stages of the manufacturing process
7.1.3.1 Base PCB manufacturing process
7.1.3.2 Automated soldering process (reflow, wave, selective) and hand soldering
7.2 Extrinsic methods for enhancing the humidity robustness
7.2.1 Barrier against the humidity access and contact with liquid water: conformal coating and potting
7.2.1.1 Conformal coating
General information
Types of conformal coatings and their properties
Acrylic conformal coating
Silicone conformal coating
Epoxy conformal coating
Polyurethane conformal coating
Fluoropolymer conformal coating
Parylene conformal coating
Other vapor-phase and thin coatings
Application of conformal coating onto a PCBA surface
7.2.1.2 Potting
7.2.1.3 Factors determining the performance of the conformal coating and potting on a PCBA
Moisture transport capability of the conformal coating/potting material
Adhesion of coating/potting on the heterogeneous PCBA surface
Conformity of coating/potting on the PCBA architecture
7.2.1.4 Conformal coating/potting and protection against other pollutants from the environment
7.2.2 Climate-compatible design of electronic enclosure
7.2.2.1 Passive humidity control using optimized enclosure design
Intentional or unintentional openings
Importance of enclosure material
Effect of internal arrangement of PCBA and other aspects
Use of water-absorbing desiccants
7.2.2.2 Active humidity control using moisture removal methods
Humidity control by reduction of RH: heating for controlling humidity
Humidity control by reduction of AH: active cooling solution
References
8 Corrosion reliability testing, standards, and failure analysis
8.1 Testing of PCBA cleanliness and contamination level
8.1.1 Overall cleanliness determination by measurement of electrical property: ROSE
8.1.2 Localized cleanliness assessment by extraction methods
8.1.2.1 Critical Cleanliness Control (C3)
8.1.2.2 Quantiflux
8.1.3 Bag extraction
8.1.4 Chemical analysis of the extract: ion chromatography
8.1.5 Qualitative residue visualization methods
8.1.5.1 Residues RAT™
8.1.5.2 ZESTRON® Flux Test and Resin Test
8.2 Tests investigating the cleanliness and corrosion effects
8.2.1 Electrochemical impedance spectroscopy (EIS)
8.2.1.1 General overview of the technique
8.2.1.2 Impedance for corrosion testing of metals/alloys: three-electrode setup
8.2.1.3 Impedance for corrosion testing of PCBA surfaces: two-electrode setup
8.2.1.4 Usefulness of impedance testing for evaluation of the humidity effects on electronics
8.2.2 DC electrochemical testing methods
8.2.2.1 General overview of the technique
8.2.2.2 DC methods for corrosion testing of metals/alloys: three-electrode setup
8.2.2.3 SIR testing of PCBA surfaces: two-electrode setup
8.2.2.4 Comparison of DC and AC electrochemical methods
8.2.2.5 Instrumentation for EIS and DC electrochemical testing methods
8.2.3 Other exposure tests for studying the corrosion effects
8.2.3.1 Bono test
8.2.3.2 Salt spray test
8.2.3.3 Multigas corrosion testing
8.3 Methodology of accelerated testing of electronic equipment
8.4 Spectroscopic and microscopic methods
8.4.1 Fourier Transform Infrared spectroscopy
8.4.1.1 General overview of the technique
8.4.1.2 Application in electronics reliability studies
8.4.2 Microscopy methods
References
Index
Back Cover