A Step-by-Step Guide to PCB Assembly

 

Introduction to PCB Assembly

Printed Circuit Board (PCB) assembly is the process of mounting electronic components onto a bare printed circuit board to create a functional electronic device. This comprehensive guide walks through every stage of the PCB assembly process, from initial design considerations to final quality control testing. Whether you're a hobbyist embarking on your first PCB project or an engineer looking to refine your assembly process, this guide provides the knowledge and techniques needed for successful PCB assembly.

PCB assembly combines precision engineering, materials science, and electronic design principles. The quality of assembly directly impacts the reliability, performance, and lifespan of electronic devices. By understanding each step in the assembly process, you can avoid common pitfalls, optimize your workflow, and produce high-quality circuit 

Understanding PCB Types

Before diving into the assembly process, it's important to understand the different types of PCBs available:

PCB Type
Description
Common Applications
Single-sided
Components mounted on one side, traces on opposite side
Simple electronic devices, calculators, radios
Double-sided
Components and traces on both sides
Consumer electronics, industrial controls
Multi-layer
Multiple layers of conductive material separated by insulating layers
Smartphones, computers, complex electronic systems
Rigid
Standard inflexible PCB
Most electronic devices
Flexible
Can bend and flex without damage
Wearable electronics, folding devices
Rigid-flex
Combination of rigid and flexible sections
Medical devices, aerospace applications
High-frequency
Special materials for RF applications
Communication equipment, radar systems
Aluminum-backed
Metal core for heat dissipation
LED lighting, power electronics

Assembly Methods Overview

There are two primary methods for mounting components on PCBs:

1. Surface Mount Technology (SMT): Components are mounted directly onto the surface of the PCB.
   • Advantages: Smaller components, higher component density, better performance at high frequencies
   • Process: Solder paste application, component placement, reflow soldering
2. Through-Hole Technology (THT): Component leads are inserted through holes in the PCB and soldered.
   • Advantages: Stronger mechanical bonds, better for high-power applications
   • Process: Component insertion, wave soldering or manual soldering

Many modern PCBs use a mixed technology approach that incorporates both SMT and THT components to leverage the advantages of each method.

The PCB Assembly Process Flow

The overall PCB assembly process follows these general steps:

1. Design and prepare PCB files
2. Procure components and bare PCBs
3. Prepare for assembly (stencil creation, equipment setup)
4. Apply solder paste (for SMT)
5. Place SMT components
6. Perform reflow soldering
7. Insert through-hole components (if applicable)
8. Perform wave soldering or manual soldering for through-hole components
9. Clean the assembled PCB
10. Inspect for defects
11. Test functionality
12. Apply protective coatings (if required)
13. Final inspection and packaging

Pre-Assembly Preparation

PCB Design Considerations for Assembly

The assembly process begins with a well-designed PCB. Several design factors directly impact assembly success:

• Component spacing: Adequate clearance between components prevents solder bridges and facilitates rework.
• Thermal relief: Proper thermal relief connections help with soldering to large copper areas.
• Testability: Including test points improves testing capabilities.
• Fiducial markers: These reference points assist automated pick-and-place machines with alignment.
• Panelization: Multiple PCBs can be fabricated as a single panel to improve manufacturing efficiency.
• Design for Manufacturing (DFM): Following DFM guidelines reduces manufacturing issues.

Design File Preparation

Before assembly, your PCB design files need proper preparation:

1. Generate manufacturing files: Create Gerber files, drill files, and pick-and-place files from your PCB design software.
2. Create Bill of Materials (BOM): Document all components, their values, packages, and quantities.
3. Prepare assembly drawings: Create detailed documentation showing component placement and orientation.

PCB Fabrication Considerations

When ordering bare PCBs for assembly, consider these specifications:

Specification
Description
Impact on Assembly
Surface finish
HASL, ENIG, OSP, immersion silver, etc.
Affects solderability and component assembly
Solder mask
Color and material
Impacts automatic optical inspection
Copper weight
Thickness of copper layers
Affects heat dissipation during soldering
Board thickness
Overall PCB thickness
Impacts handling and fixture requirements
Controlled impedance
For high-frequency designs
Critical for signal integrity
Minimum trace/space
Smallest trace width and spacing
Affects component density
Via technology
Through, blind, buried
Influences assembly process complexity

Material and Component Compatibility

Ensure compatibility between:

• PCB substrate material and soldering temperatures
• Component package types and pad designs
• Surface finishes and solder paste materials

Component Procurement and Management

BOM Development and Verification

A comprehensive and accurate Bill of Materials (BOM) is critical to successful PCB assembly:

1. Component details: Include manufacturer part numbers, package types, values, and tolerances.
2. Alternative parts: List acceptable substitute components in case of availability issues.
3. BOM verification: Check for obsolete components and availability issues.
4. Quantity calculation: Include extras (typically 5-10%) to account for losses during assembly.

Component Sourcing Strategies

Sourcing Strategy
Advantages
Disadvantages
Authorized distributors
Guaranteed authentic parts, full warranty
Higher prices, longer lead times
Independent distributors
Better availability, lower prices
Potential for counterfeit parts
Direct from manufacturer
Technical support, genuine parts
Minimum order quantities, longer lead times
Brokers
Good for obsolete or hard-to-find parts
Higher risk of counterfeit parts
Electronic component marketplaces
Price comparison, wide selection
Variable quality control

Component Storage and Handling

Proper component storage is essential to prevent damage:

1. Moisture-sensitive components: Follow moisture sensitivity level (MSL) guidelines for storage and baking.
2. ESD protection: Use anti-static packaging, wrist straps, and ESD mats when handling components.
3. Temperature control: Store components within manufacturer-recommended temperature ranges.
4. Inventory tracking: Implement first-in, first-out (FIFO) inventory management.

Component Preparation for Assembly

Before assembly, components need proper preparation:

1. Moisture-sensitive device (MSD) handling: Bake components if they've exceeded their floor life.
2. Component reels and feeders: Set up component reels in the correct feeders for pick-and-place machines.
3. Component verification: Verify component values, polarity, and package types.
4. Kitting: Organize all necessary components for a specific assembly run.

Surface Mount Technology (SMT) Assembly

SMT Process Overview

SMT assembly involves these key steps:

1. Solder paste application
2. Component placement
3. Reflow soldering
4. Inspection
5. Cleaning (if required)

Solder Paste Application

Stencil Preparation

Stencils are used to apply solder paste precisely to the PCB pads:

1. Stencil design: The stencil design should match the PCB's pad layout.
2. Stencil thickness: Typical thicknesses range from 0.004" to 0.008" (0.1mm to 0.2mm), depending on component requirements.
3. Aperture design: Aperture size and shape affect the amount of solder paste deposited.

Solder Paste Types and Selection

Solder Paste Type
Lead Content
Melting Temperature
Applications
Sn63/Pb37
37% lead
183°C
Non-RoHS applications
SAC305 (Sn96.5/Ag3.0/Cu0.5)
Lead-free
217-220°C
RoHS-compliant electronics
SAC405 (Sn95.5/Ag4.0/Cu0.5)
Lead-free
217-220°C
Higher reliability applications
SN100C (Sn/Cu/Ni/Ge)
Lead-free
227°C
Cost-effective RoHS-compliant solution
Bi58/Sn42
Lead-free
138°C
Low-temperature assembly

Paste Application Methods

1. Screen printing/stencil printing: The most common method for production runs
   • Place stencil over PCB
   • Apply solder paste on stencil
   • Sweep with squeegee to force paste through apertures
   • Remove stencil to reveal paste deposits on pads
2. Manual dispensing: Used for prototypes or rework
   • Use syringe to dispense paste on individual pads
   • Not suitable for high-volume production
3. Jet printing: Advanced method for complex assemblies
   • Non-contact method using specialized equipment
   • Can adjust paste volume pad-by-pad
   • Higher cost but greater precision

Component Placement

Manual Placement

For prototypes or low-volume production:

1. Use tweezers or vacuum pickup tools
2. Place components according to assembly drawing
3. Verify polarity and orientation
4. Ensure proper alignment with pads

Automated Placement

For higher volume production:

1. Programming: Create pick-and-place program using PCB design files
2. Feeder setup: Load component reels into feeders
3. Machine calibration: Calibrate machine using fiducial marks
4. Placement sequence optimization: Optimize for efficiency and component requirements
5. Placement operation: Machine picks and places components automatically

Component Placement Considerations

• Component order: Generally place from smallest to largest
• Polarity: Ensure correct orientation of polarized components
• Placement accuracy: Typical tolerances range from ±0.05mm to ±0.1mm
• Placement force: Adjust for different component types to avoid damage

Reflow Soldering

Reflow Profile Development

A proper reflow profile includes these phases:

1. Preheat: Gradual temperature rise to activate flux and reduce thermal shock
2. Soak: Temperature plateau to allow thermal equilibrium across the PCB
3. Reflow: Peak temperature above solder melting point to form proper joints
4. Cooling: Controlled cooling to form strong crystalline structure

Reflow Phase
Temperature Range
Duration
Purpose
Preheat
25°C to 150°C
60-120 sec
Activate flux, reduce thermal shock
Soak
150°C to 180°C
60-120 sec
Thermal equilibrium, flux activation
Reflow
210°C to 250°C
30-90 sec
Solder melting and wetting
Cooling
250°C to 25°C
90-180 sec
Solder solidification

Reflow Equipment Types

1. Infrared (IR) reflow ovens: Heat transfer through infrared radiation
   • Advantages: Lower cost, good for simple boards
   • Disadvantages: Uneven heating for components with varying colors
2. Convection reflow ovens: Heat transfer through forced hot air
   • Advantages: More uniform heating, better for complex assemblies
   • Disadvantages: Higher cost, more complex control systems
3. Vapor phase reflow: Components heated by condensation of inert fluid vapor
   • Advantages: Very uniform heating, precise temperature control
   • Disadvantages: Higher cost, specialized process

Reflow Process Monitoring

• Thermal profiling: Use thermocouples attached to PCB to monitor temperatures
• Profile verification: Compare actual temperatures to target profile
• Process window index (PWI): Quantifies how well the process stays within specifications

Post-Reflow Inspection

Visual Inspection

Manual or automated visual inspection to check for:

• Solder bridges
• Misaligned components
• Missing components
• Insufficient solder
• Component damage

Automated Optical Inspection (AOI)

AOI systems use cameras and software to automatically detect:

• Component presence and placement
• Polarity issues
• Solder quality
• Manufacturing defects

Through-Hole Technology (THT) Assembly

THT Assembly Process Overview

1. Component insertion
2. Soldering (wave, selective, or manual)
3. Lead trimming
4. Cleaning
5. Inspection

Component Insertion

Manual Insertion

1. Identify component placement from assembly drawings
2. Insert component leads through correct holes
3. Bend leads slightly to hold component in place
4. Verify correct polarity and orientation

Automated Insertion

For high-volume production:

1. Program automated insertion equipment
2. Verify component sequencing
3. Machine inserts components into PCB
4. Verify insertion quality

Wave Soldering

Wave Soldering Process

1. Flux application: Apply flux to PCB bottom side
2. Preheating: Heat PCB to activate flux and reduce thermal shock
3. Wave contact: PCB passes over molten solder wave
4. Cooling: Controlled cooling for proper joint formation

Wave Soldering Parameters

Parameter
Typical Range
Impact
Solder temperature
245-260°C (lead), 260-270°C (lead-free)
Joint formation quality
Conveyor speed
3-7 feet/minute
Solder contact time
Wave height
1/2 to 2/3 board thickness
Coverage of connections
Preheat temperature
90-120°C
Flux activation, thermal shock prevention
Contact angle
3-7 degrees
Solder flow characteristics

Wave Soldering Challenges

• Shadow effect: Components blocking solder flow to nearby connections
• Solder bridges: Unwanted connections between adjacent pads
• Insufficient filling: Incomplete filling of plated through-holes
• Component exposure: Heat-sensitive components may be damaged

Selective Soldering

Process Overview

1. Flux application to specific areas
2. Preheat PCB
3. Selective application of solder to targeted joints
4. Cooling and cleaning

Selective Soldering Methods

1. Mini-wave: Small, focused solder wave targets specific areas
2. Laser soldering: Precise laser energy melts solder at specific points
3. Robot soldering: Automated soldering iron targets individual joints

Applications

• Mixed technology boards with heat-sensitive components
• Densely populated boards where wave soldering is problematic
• Boards with components on both sides

Manual Soldering

Equipment and Tools

• Soldering iron with temperature control
• Appropriate soldering tips
• Solder wire (leaded or lead-free)
• Flux
• Cleaning supplies
• Magnification aids

Manual Soldering Technique

1. Heat the pad and component lead simultaneously
2. Apply solder to the heated joint, not the iron tip
3. Use enough solder to form proper fillet
4. Remove heat and allow joint to cool without movement
5. Inspect joint quality

Quality Control for Manual Soldering

• Consistent heating time
• Proper iron temperature
• Correct solder amount
• Clean soldering tip
• Appropriate flux usage

Mixed Technology Assembly

Process Sequence Options

SMT First Approach

1. Apply solder paste to top side
2. Place and reflow SMT components on top side
3. Apply adhesive for bottom side SMT components (if applicable)
4. Place SMT components on bottom side
5. Cure adhesive (if used)
6. Reflow bottom side (if applicable)
7. Insert through-hole components
8. Wave or selective solder through-hole components

Through-Hole First Approach

1. Insert and solder through-hole components
2. Apply solder paste for SMT components
3. Place and reflow SMT components

Special Considerations for Mixed Technology

• Temperature exposure: Track cumulative heat exposure
• Component masking: Protect sensitive components during secondary processes
• Design layout: Position components to minimize shadowing and processing conflicts
• Thermal management: Account for different thermal requirements of component types

Typical Challenges in Mixed Technology Assembly

Challenge
Description
Solution
Component shadowing
Tall components block solder wave access
Design with shadow effects in mind
Thermal sensitivity
Some components may not withstand multiple heating cycles
Process sequencing optimization
Flux compatibility
Different processes may require different flux types
Select compatible materials
Component placement constraints
Component placement influenced by soldering method
Design with process requirements in mind
Cleaning complexity
Mixed technologies may require different cleaning approaches
Select compatible cleaning processes

Soldering Techniques and Best Practices

Solder Joint Quality Factors

Solder Joint Formation Physics

1. Wetting: Molten solder spreads across properly fluxed and heated surfaces
2. Intermetallic compound formation: Chemical bonds form between solder and metal surfaces
3. Solidification: Controlled cooling forms proper crystalline structure

Common Solder Defects

Defect
Appearance
Causes
Prevention
Cold solder joint
Dull, grainy appearance
Insufficient heating, movement during cooling
Proper heating, no movement during cooling
Solder bridge
Unwanted connection between adjacent pads
Excessive solder, improper pad design
Proper paste amount, adequate pad spacing
Insufficient solder
Incomplete fillet, visible pad
Too little solder paste, poor wetting
Correct paste volume, proper surface preparation
Tombstoning
Component stands on one end
Uneven heating, pad design issues
Balanced thermal design, proper pad geometry
Voiding
Voids/bubbles in solder joint
Trapped gases, improper profile
Optimized reflow profile, proper paste selection

Flux Types and Selection

Flux Type
Composition
Cleaning Requirement
Applications
Rosin (R)
Natural rosin
Cleaning recommended
General purpose
Rosin Mildly Activated (RMA)
Rosin with mild activators
Cleaning recommended
Electronics with moderate cleaning needs
Rosin Activated (RA)
Rosin with strong activators
Cleaning required
Difficult-to-solder surfaces
Water Soluble (OA)
Organic acids
Cleaning required
High reliability applications
No-Clean (NC)
Minimal residue formulation
No cleaning required
Consumer electronics
Low-Residue (LR)
Very minimal residue
Optional cleaning
High-density applications

Lead vs. Lead-Free Soldering

Comparison of Solder Types

Characteristic
Lead-Based Solder
Lead-Free Solder
Typical composition
Sn63/Pb37
SAC305 (Sn96.5/Ag3.0/Cu0.5)
Melting point
183°C
217-220°C
Wetting properties
Excellent
Good, but requires better flux
Process window
Wider
Narrower
Joint appearance
Shiny, smooth
Less shiny, rougher
Cost
Lower
Higher
Environmental impact
Contains hazardous lead
Reduced environmental impact
Reliability concerns
Tin whiskers rare
Potential for tin whiskers

Process Adjustments for Lead-Free Soldering

1. Higher temperatures: Typically 30-40°C higher than lead-based
2. Enhanced flux activity: Compensate for poorer wetting
3. Tighter process control: Narrower process window
4. Equipment considerations: May require upgraded equipment
5. Material compatibility: Ensure all materials can withstand higher temperatures

Specialized Soldering Situations

High-Reliability Applications

1. Component pre-treatment: Baking, cleaning, and inspection
2. Process control: Tighter parameters and monitoring
3. Materials selection: Higher-grade materials and components
4. Testing: Enhanced inspection and reliability testing
5. Documentation: Complete traceability and process recording

Heat-Sensitive Components

1. Thermal shielding: Protect sensitive components with heat sinks or shields
2. Process sequencing: Install heat-sensitive components last
3. Localized soldering: Use selective soldering techniques
4. Temperature profiling: Develop specific profiles for sensitive areas
5. Alternative joining methods: Consider conductive adhesives for extremely sensitive components

Cleaning and Inspection

PCB Cleaning Methods

Types of Contaminants

1. Flux residues: Remaining flux after soldering
2. Ionic contaminants: Salts and acids that can cause corrosion
3. Particulate matter: Dust, fibers, and other particles
4. Organic residues: Oils and fingerprints
5. Processing chemicals: Residual chemicals from manufacturing

Cleaning Processes

Cleaning Method
Process
Advantages
Disadvantages
Applications
Aqueous cleaning
Water-based with detergents
Environmentally friendly, effective
Requires drying, water disposal
General purpose
Semi-aqueous
Solvent emulsified in water
Effective for heavy residues
More complex process
Heavy flux residues
Solvent cleaning
Pure solvent washing
Very effective for difficult residues
Environmental concerns, cost
Specialized applications
No-clean
Skip cleaning (with no-clean flux)
Cost-effective, simplified process
Residues remain on board
Consumer electronics
Ultrasonic cleaning
Any cleaning medium with ultrasonic agitation
Excellent for tight spaces
Can damage sensitive components
Complex geometries
Vapor degreasing
Condensing solvent vapor
Excellent cleaning, self-drying
Higher cost, environmental concerns
Precision electronics

Cleanliness Testing

1. Visual inspection: Examine for visible residues
2. Ionic contamination testing: Measures extractable ionic compounds
3. Water break test: Tests for hydrophobic contaminants
4. Contact angle measurement: Quantifies surface energy changes due to contamination
5. Solvent extract resistivity: Measures ionic residues via solvent extraction

Inspection Techniques

Visual Inspection Methods

1. Manual visual inspection: Using naked eye or simple magnification
   • Advantages: Low cost, no special equipment
   • Limitations: Operator fatigue, subjective, limited magnification
2. Microscope inspection: Using stereo microscopes
   • Advantages: Better magnification, good depth perception
   • Limitations: Slow, still subjective
3. Video magnification: Using camera systems with monitors
   • Advantages: Comfortable viewing, digital zoom, image capture
   • Limitations: 2D view may miss some defects

Automated Optical Inspection (AOI)

1. 2D AOI: Camera-based systems comparing images to reference standards
   • Process: PCB scanned, images compared to "golden board" or CAD data
   • Detection capabilities: Missing/misaligned components, polarity, solder defects
2. 3D AOI: Uses multiple cameras or laser technology for 3D imaging
   • Advantages: Height measurement, better solder joint analysis
   • Applications: Complex assemblies, fine-pitch components

X-ray Inspection

1. 2D X-ray: Single-angle X-ray imaging
   • Applications: BGA connections, internal connections
   • Limitations: Overlapping features can be difficult to interpret
2. 3D X-ray/Computed Tomography: Multi-angle X-ray imaging
   • Advantages: Layer-by-layer inspection, complete internal visualization
   • Applications: Complex multilayer assemblies, package-on-package

Automated Test Equipment (ATE)

1. In-Circuit Testing (ICT): Tests individual components while installed
   • Process: Test probes contact test points on PCB
   • Capabilities: Component value verification, open/short detection
2. Flying Probe Testing: Moving probes test without custom fixtures
   • Advantages: No custom fixture needed, good for low volume
   • Limitations: Slower than ICT, less comprehensive

Testing and Troubleshooting

Electrical Testing Methods

Continuity and Short Circuit Testing

1. Basic continuity testing: Verify electrical connections between points
2. Short circuit detection: Identify unwanted connections
3. Resistance measurement: Verify resistance values within tolerance

In-Circuit Testing (ICT)

1. Fixture-based ICT: Custom fixture with pins contacting test points
   • Tests: Component presence, orientation, value, basic functionality
   • Advantages: Fast, comprehensive, repeatable
   • Limitations: Expensive fixtures, requires test point access
2. Flying probe testing: Movable probes test without custom fixture
   • Tests: Similar to ICT but without dedicated fixture
   • Advantages: No fixture cost, flexible for revisions
   • Limitations: Slower than ICT

Functional Testing

1. Powered testing: Apply power and verify circuit operation
2. Signal simulation: Inject test signals and measure responses
3. Environmental testing: Test under temperature, humidity, vibration
4. Burn-in testing: Extended operation to identify early failures

Common PCB Assembly Defects

Component-Related Defects

Defect
Description
Detection Method
Prevention
Missing component
Component not placed
AOI, visual inspection
Process control, verification
Misalignment
Component not properly aligned with pads
AOI, visual inspection
Proper placement machine calibration
Polarity error
Component installed backward
AOI, functional test
Clear polarity marking, verification
Wrong component
Incorrect component installed
AOI, ICT, functional test
Component verification, proper kitting
Damaged component
Physical damage during assembly
Visual inspection, functional test
Proper handling procedures

Solder Joint Defects

Defect
Description
Detection Method
Prevention
Solder bridge
Unwanted connection between adjacent pads
Visual inspection, AOI, ICT
Proper paste amount, adequate spacing
Insufficient solder
Not enough solder for proper connection
Visual inspection, AOI, X-ray
Correct paste volume, stencil design
Cold solder joint
Poor metallurgical bond due to insufficient heat
Visual inspection, ICT
Proper heating profile
Voiding
Gas bubbles trapped in solder
X-ray inspection
Optimized reflow profile
Tombstoning
Component standing on one end
Visual inspection, AOI
Balanced pad design, proper paste deposit

Troubleshooting Methodology

Systematic Approach

1. Symptom identification: Clearly define the problem
2. Visual inspection: Look for obvious defects
3. Non-powered testing: Continuity, resistance measurements
4. Powered testing: Voltage and signal measurements
5. Environmental factors: Test under different conditions
6. Root cause analysis: Identify underlying issues
7. Corrective action: Fix immediate problem and prevent recurrence

Debugging Tools and Equipment

Tool
Application
Capabilities
Digital multimeter
Basic measurements
Voltage, current, resistance measurement
Oscilloscope
Signal analysis
Waveform visualization, timing analysis
Logic analyzer
Digital circuit analysis
Multiple signal capture, protocol decoding
Thermal camera
Heat distribution analysis
Identify hot spots, thermal issues
Signal generator
Input simulation
Create test signals for circuit verification
Power supply
Controlled power
Provide stable, adjustable power for testing

Conformal Coating and Protection

Types of Conformal Coatings

Coating Type
Base Material
Protection Level
Application Method
Removal Difficulty
Applications
Acrylic
Acrylic resin
Good
Spray, dip, brush
Easy
Consumer electronics
Urethane
Polyurethane
Very good
Spray, dip
Difficult
Industrial equipment
Silicone
Silicone resin
Excellent
Spray, dip, automated
Moderate
High temperature, vibration environments
Epoxy
Epoxy resin
Excellent
Spray, automated
Very difficult
Harsh environments
Parylene
Poly-para-xylylene
Excellent
Vapor deposition
Very difficult
Medical devices, aerospace

Coating Application Methods

1. Manual spray: Using aerosol or spray gun
   • Advantages: Low setup cost, flexible
   • Limitations: Operator dependent, less consistent
2. Dip coating: Immersing PCB in coating solution
   • Advantages: Complete coverage, simple process
   • Limitations: Difficult to control thickness, pool contamination
3. Selective coating: Automated application to specific areas
   • Advantages: Protects critical areas, leaves connectors clean
   • Limitations: Equipment cost, programming required
4. Automated spray: Robotic spray systems
   • Advantages: Consistent application, good control
   • Limitations: Higher equipment cost
5. Vapor deposition (Parylene): Chemical vapor deposition process
   • Advantages: Ultra-thin, pinhole-free coverage
   • Limitations: Specialized equipment, higher cost

Coating Inspection and Quality Control

1. Visual inspection: Check coverage and uniformity
   • Methods: Naked eye, UV inspection (with fluorescent additives)
2. Thickness measurement: Verify coating thickness
   • Methods: Wet film gauge, eddy current, microscopic cross-section
3. Adhesion testing: Verify proper adhesion to PCB
   • Methods: Cross-cut test, tape test
4. Functionality verification: Ensure coating doesn't affect function
   • Methods: Full functional testing after coating

Assembly Documentation

Documentation Types

Manufacturing Documentation

1. Assembly drawings: Show component placement and orientation
2. Work instructions: Step-by-step assembly procedures
3. Bill of Materials (BOM): Complete component list with specifications
4. Pick-and-place files: Machine programming information
5. Stencil designs: Specifications for solder paste stencils

Quality Documentation

1. Inspection criteria: Specific quality standards for the assembly
2. Test procedures: Detailed testing methods and acceptance criteria
3. Defect catalogs: Reference images of common defects
4. Quality reports: Results of quality checks and inspections

Traceability Documentation

1. Component traceability: Tracking component sources and lots
2. Process parameters: Record of assembly process settings
3. Test results: Results of all testing performed
4. Rework records: Documentation of any rework performed

Creating Effective Assembly Instructions

1. Clear visual aids: Images marking critical details
2. Step-by-step format: Numbered steps in logical sequence
3. Critical parameters: Highlight important settings and measurements
4. Common errors: Note potential mistakes and how to avoid them
5. Verification points: Include checkpoints throughout process

Record-Keeping for Quality Management

1. Component traceability: Track from supplier to finished product
2. Process parameters: Record key parameters for each production run
3. Test results: Document all test outcomes
4. Non-conformance reports: Document and track defects
5. Corrective actions: Record actions taken to address issues

PCB Assembly Equipment

SMT Equipment

Screen Printers and Paste Dispensers

1. Manual stencil printers: Simple frame and stencil systems
   • Applications: Prototyping, low-volume production
   • Features: Manual alignment, squeegee operation
2. Semi-automatic printers: Assisted printing with some automation
   • Applications: Medium-volume production
   • Features: Camera alignment, automatic squeegee
3. Fully automatic printers: Complete printing automation
   • Applications: High-volume production
   • Features: Automatic alignment, paste dispensing, cleaning
4. Jet printers: Non-contact paste application
   • Applications: Complex, high-mix assemblies
   • Features: