Electronics Guide

Manufacturing Contamination

Production processes introduce various contaminants:

• Flux residues: Activators, organic acids, and halides remaining after soldering are among the most common contamination sources. Even "no-clean" fluxes leave residues that can cause problems in certain environments.
• Solder paste residues: Incomplete reflow or improper paste storage can leave active residues.
• Cleaning chemical residues: Improperly rinsed cleaning solutions leave ionic contamination.
• Handling contamination: Skin oils, fingerprints, lotions, and other handling residues introduce organic and ionic contaminants.
• Component contamination: Residues from component manufacturing or storage.
• Plating bath contamination: Drag-out from plating baths or inadequate rinsing.

Environmental Contamination

Operating environments expose assemblies to various contaminants:

• Atmospheric pollutants: Sulfur compounds, chlorine, and other airborne reactive species.
• Moisture: Water vapor provides the electrolyte necessary for electrochemical corrosion.
• Salt spray: Marine and coastal environments introduce chloride ions.
• Particulates: Dust and debris that may be hygroscopic or contain reactive species.
• Off-gassing: Volatile compounds released from packaging materials, enclosures, or nearby components.

Contamination from Adjacent Materials

Materials in contact with or near electronic assemblies can contribute contamination:

• Packaging materials: Sulfur-containing cardboard or paper, halogenated plastics, or materials treated with flame retardants.
• Adhesives and sealants: Outgassing of corrosive compounds, particularly from silicone materials releasing acetic acid.
• Labels and coatings: Adhesive residues or coating degradation products.
• Thermal interface materials: Some materials may release corrosive compounds when heated.

Electrochemical Corrosion Fundamentals

Basic corrosion requires:

• Anode: A metal surface undergoing oxidation (metal atoms lose electrons).
• Cathode: A surface where reduction occurs (electrons are consumed).
• Electrolyte: An ionic conduction path between anode and cathode, typically provided by moisture absorbed by hygroscopic contaminants.
• Electrical connection: A path for electron flow between anode and cathode.

The corrosion rate depends on the electrochemical potential difference between metals, electrolyte conductivity, temperature, and other factors. Removing any of the four requirements stops corrosion.

Galvanic Corrosion

When dissimilar metals are in contact with an electrolyte, the more active metal corrodes preferentially. In electronics, common galvanic couples include:

• Copper and tin: Solder joints on copper traces with flux residue electrolyte.
• Aluminum and copper: Wire bond or pad interfaces.
• Nickel and gold: Plated finishes with exposed nickel.

The galvanic series ranks metals by their electrochemical potential. Metals far apart in the series experience more severe galvanic corrosion when coupled.

Electrochemical Migration (ECM)

Electrochemical migration occurs when metal ions dissolve from an anodic conductor, migrate through an electrolyte film under an applied electric field, and deposit on or toward a cathodic conductor. This can form conductive dendrites that bridge insulating gaps and cause short circuits.

ECM requirements include:

• Bias voltage: DC potential difference between conductors.
• Moisture: Liquid water or high humidity to dissolve metal ions.
• Contamination: Ionic species that increase electrolyte conductivity.
• Susceptible metals: Silver is most prone, but copper, tin, lead, and other metals can migrate.

Silver migration is particularly problematic due to silver's high mobility and the stability of silver ions in solution. Even trace silver contamination from solder or component metallizations can cause failures.

Conductive Anodic Filament (CAF) Formation

CAF is a special form of electrochemical migration that occurs within PCB laminates. Copper ions migrate along the glass fiber/epoxy interface from anode to cathode, forming a conductive path through the board.

CAF formation is promoted by:

• Poor fiber-resin adhesion: Weak interfaces provide paths for ion transport.
• Moisture absorption: Water penetrating along interfaces enables ion transport.
• Contamination: Processing residues or absorbed contaminants lower electrolyte resistance.
• High via density: More opportunities for CAF initiation.
• Thin dielectrics: Shorter paths between conductors.

CAF can cause sudden shorts between vias, between vias and planes, or along other paths through the laminate. Once formed, CAF paths may be invisible from the board surface.

Creep Corrosion

Creep corrosion involves the growth of corrosion products across insulating surfaces from a corroding metal source. The corrosion products may be conductive enough to cause leakage or shorts.

Common in environments with sulfur contamination, creep corrosion affects:

• Copper: Forms copper sulfide (Cu2S) corrosion products.
• Silver: Tarnishes readily to silver sulfide (Ag2S).
• Immersion silver finishes: Particularly susceptible.

Creep corrosion is accelerated by sulfur-containing atmospheres, high humidity, and elevated temperatures. Data centers with free air cooling in polluted environments have experienced significant creep corrosion problems.

Whisker Growth Mechanisms

Tin whiskers grow to relieve compressive stress in tin plating. Stress sources include:

• Intermetallic formation: Copper-tin intermetallics growing at the interface create compressive stress in the tin.
• Coefficient of thermal expansion mismatch: Different expansion rates between tin and substrate.
• Mechanical stress: Bending, clamping, or connector insertion forces.
• Corrosion: Oxide formation on tin surfaces can promote whisker growth.

Whiskers can grow to several millimeters in length, easily bridging conductor spacings on modern electronics. They can cause shorts, intermittent connections, and arc-induced failures.

Whisker Mitigation

Strategies to reduce tin whisker risk include:

• Alloying: Adding lead, bismuth, or other elements to tin reduces whisker propensity. This is why lead-free electronics require special attention to whisker risk.
• Nickel underlayer: A nickel barrier between copper and tin slows intermetallic formation and reduces stress.
• Matte tin: Larger grain sizes in matte tin (versus bright tin) reduce whisker growth.
• Annealing: Heat treatment after plating can relieve stress and stabilize the tin structure.
• Conformal coating: Coatings can constrain whisker growth and prevent shorts even if whiskers form.
• Design spacing: Maintaining adequate clearance between tin-plated surfaces.

Ionic Contamination Testing

Several methods quantify ionic contamination:

• ROSE (Resistivity of Solvent Extract): Measures bulk ionic contamination by extracting residues into isopropyl alcohol/water solution and measuring resistivity change. Results expressed in micrograms NaCl equivalent per square centimeter. IPC standards specify acceptable limits.
• Ion chromatography (IC): Identifies and quantifies specific ionic species. More informative than ROSE testing as it reveals the type of contamination, enabling root cause identification.
• C3 (Cleanliness, Contamination Control): Localized extraction testing using a heated cell pressed against the board surface. Identifies contamination location.

Surface Analysis Techniques

Advanced surface analysis characterizes contamination composition and distribution:

• FTIR (Fourier Transform Infrared Spectroscopy): Identifies organic compounds including flux residues, oils, and polymeric contamination.
• XPS (X-ray Photoelectron Spectroscopy): Surface elemental composition and chemical state information.
• ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry): Extremely sensitive surface analysis identifying organic and inorganic species.
• SEM-EDS (Scanning Electron Microscopy with Energy Dispersive Spectroscopy): Imaging with elemental mapping of contamination deposits.

Environmental Testing

Tests that expose assemblies to controlled environments reveal susceptibility to contamination-related failures:

• Temperature-humidity-bias (THB): Extended exposure to elevated temperature and humidity with voltage applied reveals electrochemical migration susceptibility.
• HAST (Highly Accelerated Stress Test): High temperature and humidity under pressure for accelerated testing.
• Mixed flowing gas (MFG): Exposure to corrosive gases like H2S, Cl2, and NOx to evaluate corrosion resistance.
• Salt spray: Exposure to salt-laden mist per ASTM B117 for evaluating marine environment performance.

Visual Examination

Initial visual inspection often reveals telltale signs:

• Discoloration: Color changes on metal surfaces indicating oxidation or corrosion product formation.
• Dendrites: Branching metallic growths between conductors.
• White residues: May indicate flux residue, corrosion products, or other contamination.
• Green or blue deposits: Copper corrosion products.
• Black tarnish: Silver sulfide formation.

Stereomicroscopy and optical microscopy at higher magnifications reveal finer details of corrosion patterns.

SEM Analysis

Scanning electron microscopy provides high-resolution imaging of corrosion features:

• Dendrite morphology: Crystal structure and growth patterns.
• Corrosion product identification: Composition via EDS analysis.
• Pit morphology: Shape and depth of corrosion pits.
• Tin whisker observation: Whisker length, density, and growth patterns.

Cross-Section Analysis

Metallographic cross-sectioning reveals subsurface corrosion:

• CAF paths: Internal corrosion channels through laminate.
• Underfilm corrosion: Corrosion beneath conformal coatings or solder mask.
• Via barrel corrosion: Internal corrosion of plated through-holes.
• Intermetallic growth: Characterizing interface conditions.

Chemical Analysis

Identifying contaminant species aids root cause determination:

• Ion chromatography: Quantifies specific ions like chloride, bromide, sulfate, and organic acids.
• FTIR: Identifies organic contaminants.
• XRF: Elemental screening of deposits.
• Micro-extraction: Localized sampling for analysis of specific residues.

Coating Failure Modes

• Delamination: Loss of adhesion between coating and substrate allowing moisture ingress.
• Cracking: Brittle fracture from thermal stress, mechanical shock, or coating aging.
• Incomplete coverage: Areas left uncoated due to process problems or shadowing.
• Moisture permeation: All coatings have finite moisture permeability; some applications exceed coating capability.
• Contamination trapping: Coating applied over contamination seals the problem in place.

Coating Analysis

Evaluating conformal coating performance:

• Thickness measurement: Verifying adequate coating thickness.
• Coverage inspection: UV fluorescence for fluorescent coatings; visual inspection for others.
• Adhesion testing: Tape pull tests or cross-hatch testing.
• FTIR analysis: Identifying coating material type and degradation.

Design Considerations

• Material selection: Choosing compatible metallizations, avoiding problematic galvanic couples.
• Adequate spacing: Increased conductor spacing reduces electrochemical migration risk.
• Conformal coating specification: Appropriate coating selection for the application environment.
• Environmental protection: Enclosures, sealing, and environmental control for harsh environments.

Manufacturing Controls

• Cleanliness specifications: Defining and verifying acceptable contamination levels.
• Process controls: Monitoring flux activity, cleaning effectiveness, and handling procedures.
• Material controls: Shelf life management, proper storage, and incoming inspection.
• Clean manufacturing environment: Air filtration, humidity control, and contamination-free materials.

Testing and Qualification

• Ionic contamination testing: Routine verification of assembly cleanliness.
• Environmental stress testing: Temperature-humidity-bias testing to validate robustness.
• Accelerated corrosion testing: Mixed flowing gas or salt spray for high-reliability applications.
• Field return analysis: Monitoring field failures for corrosion-related problems.