Designing PCBs with Surface Mount Components

 

Introduction to Surface Mount Technology

Surface Mount Technology (SMT) has revolutionized the electronics manufacturing industry since its widespread adoption in the 1980s. Compared to traditional through-hole components, surface mount devices (SMDs) offer significant advantages in terms of miniaturization, automated assembly capabilities, and enhanced electrical performance. As electronic devices continue to shrink in size while increasing in functionality, mastering the design of printed circuit boards (PCBs) with surface mount components has become an essential skill for electronics engineers and hobbyists alike.

This comprehensive guide will walk you through the entire process of designing PCBs with surface mount components—from understanding the fundamental principles of SMT to implementing advanced techniques for high-density, high-performance designs. Whether you're a seasoned professional looking to refine your skills or a newcomer eager to embrace modern electronics design practices, this article provides the knowledge and practical insights you need.

Understanding Surface Mount Technology Basics

What Are Surface Mount Components?

Surface mount components are electronic parts designed to be mounted directly onto the surface of a PCB, as opposed to through-hole components which require holes drilled through the board for their leads. SMDs connect to the PCB via pads rather than through plated holes, allowing components to be placed on both sides of the board and enabling higher component densities.

SMT vs. Through-Hole Technology

Before diving deeper into SMT design principles, it's important to understand how it differs from traditional through-hole technology:

Feature
Surface Mount Technology
Through-Hole Technology
Component Size
Typically smaller (up to 10x smaller)
Larger
Assembly Method
Reflow soldering, wave soldering
Manual soldering, wave soldering
Component Density
High (more components per unit area)
Low
Automated Assembly
Excellent compatibility
Limited compatibility
Mechanical Strength
Lower (but sufficient for most applications)
Higher
High-Frequency Performance
Better (shorter leads = less parasitic inductance)
Worse
Prototyping Ease
More challenging without specialized equipment
Easier for manual assembly
Cost for Mass Production
Lower
Higher

Common Types of Surface Mount Components

Passive Components

Surface mount passive components include resistors, capacitors, and inductors, which are categorized by their package sizes:

Package Code
Dimensions (mm)
Common Uses
0201
0.6 × 0.3
Ultra-compact mobile devices
0402
1.0 × 0.5
Mobile phones, wearables
0603
1.6 × 0.8
General electronics, IoT devices
0805
2.0 × 1.25
General purpose, power circuits
1206
3.2 × 1.6
Power applications, higher voltage rating
1210
3.2 × 2.5
High power applications

Integrated Circuits

Surface mount ICs come in various package types:

1. Small Outline Integrated Circuit (SOIC): Common for op-amps, microcontrollers, and memory ICs
2. Quad Flat Pack (QFP): Used for microcontrollers, DSPs, and other complex ICs
3. Ball Grid Array (BGA): Used for high pin count processors, FPGAs
4. Quad Flat No-Lead (QFN): Common for power management ICs and RF components
5. Small Outline Transistor (SOT): Used for transistors, voltage regulators

Specialized Components

1. LEDs: Available in various sizes and colors
2. Crystals and Oscillators: For timing references
3. Connectors: Surface mount versions of various connector types
4. Transformers and Inductors: For power and RF applications

Design Considerations for SMT PCBs

Design Rules and Constraints

Minimum Trace Width and Spacing

When designing with SMT components, especially fine-pitch devices, maintaining appropriate trace widths and spacing is crucial:

Board Type
Minimum Trace Width
Minimum Spacing
Notes
Standard PCB
6 mil (0.152mm)
6 mil (0.152mm)
Most economical option
Advanced PCB
4 mil (0.102mm)
4 mil (0.102mm)
Higher cost, better for dense designs
High-Density PCB
3 mil (0.076mm)
3 mil (0.076mm)
Premium cost, for very dense designs

Component Placement Guidelines

1. Component Orientation: Orient similar components in the same direction to facilitate automated assembly
2. Thermal Considerations: Keep heat-generating components separated or provide adequate thermal relief
3. Component Density: Balance between compact design and manufacturability
4. Edge Clearances: Maintain at least 5mm clearance from board edges for components

Footprint Design Best Practices

Creating proper footprints for surface mount components is critical for successful assembly:

Pad Dimensioning

Pad dimensions should follow IPC standards, typically classified into three density levels:

Density Level
Description
Suitable For
Level A
Most conservative, largest pads
High reliability, harsh environments
Level B
Moderate, balanced approach
General purpose commercial electronics
Level C
Smallest pads, highest density
Space-constrained consumer electronics

Thermal Relief for Power and Ground Connections

When connecting pads to large copper planes:

1. Use thermal relief connections (spoke-like connections)
2. This prevents heat sinking during soldering
3. Ensures proper solder joints formation

Fiducial Markers

For automated assembly, include:

1. Global fiducials: At least three non-collinear markers on the board
2. Local fiducials: Near fine-pitch components
3. Standard fiducial size: 1mm copper pad with 2mm solder mask clearance

PCB Layout Techniques for SMT Designs

Component Placement Strategy

Critical Component Placement

1. Analog vs. Digital: Separate analog and digital circuits to minimize interference
2. High-Speed Components: Place close to related components to minimize trace lengths
3. Power Components: Place near power input and ensure adequate thermal management
4. Sensitive Components: Shield from noise sources and heat-generating elements

Optimizing for Different Assembly Methods

Based on your assembly method, consider these placement strategies:

Assembly Method
Placement Considerations
Reflow Soldering (single-sided)
Place all SMT components on top side
Reflow Soldering (double-sided)
Place larger/heavier components on bottom side
Wave Soldering
Place SM and TH components on top side, only SM on bottom
Mixed Technology
Place SMT components away from tall through-hole components

Routing Techniques for SMT Boards

Dealing with High-Density Components

1. Escape Routing: For BGAs and fine-pitch components, use:
   • Microvias or buried vias under component
   • "Dog bone" patterns for simpler designs
   • Fan-out patterns for efficient connection to inner layers
2. Via-in-Pad Technique:
   • Allows direct connection from pad to inner/bottom layers
   • Requires filled and plated-over vias to prevent solder wicking
   • Increases manufacturing cost but enables higher densities

Signal Integrity Considerations

1. Controlled Impedance Routing:
   • Essential for high-speed signals
   • Maintain consistent trace width and reference plane distance
   • Use differential pairs for sensitive signals
2. Ground and Power Planes:
   • Use solid planes when possible
   • Minimize splits in reference planes
   • Place bypass capacitors close to IC power pins

Layer Stackup Planning

The layer stackup significantly impacts signal integrity, manufacturing cost, and thermal performance:

Layer Count
Typical Stackup
Best For
2 Layers
Signal - Ground/Power
Simple designs, low cost
4 Layers
Signal - Ground - Power - Signal
General purpose, good balance
6 Layers
Signal - Ground - Signal - Power - Ground - Signal
Medium complexity, better signal integrity
8+ Layers
Multiple signal and power/ground layers
Complex designs, high-speed signals

When designing stackups:

1. Place signal layers adjacent to continuous reference planes
2. Keep high-speed signals on outer layers for controlled impedance
3. Consider power integrity requirements when placing power planes

Design for Manufacturing and Assembly

SMT-Specific DFM Guidelines

Pad and Stencil Design

Proper pad design ensures reliable solder joints:

1. Solder Paste Aperture Design:
   • Typically 1:1 with copper pad for most components
   • Reduced for large pads (e.g., 80-90% for QFN thermal pads)
   • Split into grid pattern for large thermal pads to prevent solder bridging
2. Solder Mask Considerations:
   • Solder mask defined (SMD) vs. non-solder mask defined (NSMD) pads
   • NSMD generally preferred for better solder joint reliability
   • Typical solder mask clearance: 0.05-0.1mm from pad edge

Component Spacing Requirements

Component Type
Minimum Spacing
Recommended Spacing
Chip Components (0603, etc.)
0.3mm
0.5mm
SOIC/TSSOP
0.5mm
0.8mm
QFP/QFN
0.5mm
1.0mm
BGA
1.0mm
1.5mm
Tall Components
Height-dependent
1.5× component height

Design for Automated Assembly

Pick and Place Considerations

1. Component Polarity Marking:
   • Clear polarity indicators on the silkscreen
   • Consistent orientation of similar components
2. Clearance for Pick and Place Heads:
   • Avoid placing components too close to tall objects
   • Account for nozzle geometry in dense designs

Test Point Planning

1. Accessibility:
   • Place test points on same side as components when possible
   • Maintain minimum 1.5mm spacing between test points
2. Design for In-Circuit Testing:
   • Test point diameter: typically 1.0-1.5mm
   • Test point grid pattern: typically 2.54mm (100mil) or 1.27mm (50mil)

Advanced SMT Design Techniques

Designing with Fine-Pitch Components

BGA Design Guidelines

Ball Grid Arrays require special attention:

1. Fanout Strategies:
   • Dog bone: via adjacent to pad, connected by short trace
   • Via-in-pad: via directly in the pad (requires filled vias)
   • Via-near-pad: via close to pad but not within solder mask opening
2. Layer Requirements:
   • Fine-pitch BGAs (<0.5mm pitch) typically require 6+ layers
   • Ultra-fine-pitch BGAs (<0.4mm pitch) may require 8+ layers

BGA Pitch
Minimum Via Size
Minimum Trace Width
Typical Layer Count
1.0mm
0.3mm
0.15mm
4-6
0.8mm
0.25mm
0.125mm
6-8
0.5mm
0.2mm
0.1mm
8-10
0.4mm
0.15mm
0.075mm
10+

Designing with QFN/DFN Packages

Quad Flat No-Lead packages present unique challenges:

1. Thermal Pad Design:
   • Divide large thermal pads into grid pattern on stencil
   • Add vias in thermal pad for heat dissipation
   • Vias should be tented or filled to prevent solder wicking
2. Edge Connection Considerations:
   • Extend pads beyond package outline (0.2-0.3mm recommended)
   • Provides visual inspection capability for solder joints

Mixed-Technology Design Techniques

Combining SMT with Through-Hole Components

When designs require both technologies:

1. Component Placement Strategy:
   • Place SMT components away from wave soldering areas if possible
   • Protect SMT components from excessive heat during wave soldering
2. Assembly Process Planning:
   • Determine optimal assembly sequence (typically SMT first)
   • Consider shadow effects of tall components

Hybrid Reflow Techniques

For mixed-technology boards using only reflow soldering:

1. Pin-in-Paste Method:
   • Through-hole component leads placed in solder paste-filled holes
   • Reflowed simultaneously with SMT components
   • Requires specific hole and paste mask design
2. Requirements for Successful Pin-in-Paste:
   • Hole diameter = component lead diameter + 0.2-0.4mm
   • Solder paste volume: typically 2-3× the hole volume
   • Maximum lead diameter: typically 1.0mm for reliable results

Thermal Management for SMT Designs

Thermal Considerations in Component Selection

Heat dissipation is critical for many surface mount components:

Component Type
Thermal Resistance Range (°C/W)
Cooling Requirements
Small SOT packages
100-400
Minimal (natural convection)
SOIC packages
50-150
Natural convection
QFN packages
15-50
Enhanced pads, possibly thermal vias
Power QFN/PQFN
5-20
Thermal vias, possibly heatsink
BGA processors
10-30
Thermal vias, often active cooling

PCB Layout for Optimal Thermal Performance

Thermal Via Patterns

Thermal vias increase heat transfer from component to inner layers and/or bottom side:

1. Via Parameters:
   • Typical diameter: 0.3-0.5mm
   • Typical grid spacing: 1.0-1.27mm
   • Number of vias: depends on power dissipation
2. Thermal Via Design Guidelines:
   • For every watt dissipated, provide approximately 10-20 thermal vias
   • Connect thermal vias to internal ground planes when possible
   • For highest performance, fill and plate over vias

Copper Spreading Techniques

1. Thicker Copper:
   • Standard: 1oz (35μm)
   • Heavy thermal loads: 2oz (70μm) or higher
2. Internal Copper Planes:
   • Maximize copper connection to thermal areas
   • Minimize splits in planes under high-power components

High-Reliability SMT Design

Designing for Harsh Environments

Vibration Resistance

1. Component Orientation:
   • Orient rectangular components perpendicular to primary vibration axis
   • Maintain minimum distance from board edges (10mm recommended)
2. Additional Support Methods:
   • Use adhesives or underfill for critical components
   • Consider mechanical supports for large components

Temperature Extreme Considerations

For designs operating in extreme temperatures:

Temperature Range
Design Considerations
Commercial (0°C to 70°C)
Standard design practices
Industrial (-40°C to 85°C)
Enhanced pad designs, controlled CTE materials
Military (-55°C to 125°C)
Specialized materials, underfill for BGAs
Extreme (beyond -55°C to 125°C)
Custom solutions, extensive testing required

Conformal Coating Considerations

Conformal coatings protect SMT assemblies from moisture, dust, and chemical exposure:

1. Coating Selection Guide:

Coating Type
Protection Level
Application Method
Reworkability
Acrylic
Good
Spray, dip
Excellent
Urethane
Very good
Spray, dip
Poor
Silicone
Excellent
Spray, dip
Good
Epoxy
Excellent
Spray
Poor
Parylene
Outstanding
Vapor deposition
Poor

2. Design for Coating Application:
   • Provide keep-out areas for connectors and test points
   • Consider component spacing for coating flow
   • Avoid trapping air in tight spaces

SMT Design for Special Applications

RF and Microwave PCB Design

Surface mount technology is widely used in RF designs but requires special considerations:

1. Critical RF Design Factors:
   • Controlled impedance throughout signal path
   • Minimized vias on RF traces
   • Ground via fencing for isolation
2. SMT Component Selection for RF:
   • Use components rated for target frequency
   • Consider parasitic effects in layout
   • Follow manufacturer guidelines for pad designs

Grounding Techniques for SMT RF Designs

Proper grounding is essential for RF performance:

1. Via Stitching Guidelines:
   • Maximum distance between ground vias: λ/10 or less
   • Via diameter: typically 0.3-0.5mm
   • Ensure low-inductance paths to ground planes
2. Ground Pour Considerations:
   • Avoid isolated ground islands
   • Use ground fills with multiple connection points
   • Maintain consistent ground reference under transmission lines

High-Speed Digital Design

Impedance Control with SMT Components

1. Differential Pair Routing:
   • Keep traces symmetric and parallel
   • Maintain consistent spacing throughout route
   • Typical differential impedances: 85Ω, 90Ω, or 100Ω
2. Length Matching Techniques:
   • Use serpentine traces for length matching
   • Avoid right angles in high-speed traces
   • Match lengths to within 5-10 mil for most applications

Signal Integrity Optimization

1. Bypass Capacitor Placement:
   • Place as close as possible to IC power pins
   • Use multiple capacitor values for broadband decoupling
   • Typical arrangement: 0.1μF + 0.01μF per power pin
2. Recommended Capacitor Placement Distances:

Speed
Maximum Distance
Capacitor Size
<50MHz
25mm
0603 or larger
50-200MHz
12mm
0402 or 0603
>200MHz
5mm
0201 or 0402

Tools and Software for SMT PCB Design

CAD Software Features for SMT Design

Essential CAD Capabilities

Modern PCB design for SMT requires software with specific capabilities:

1. Component Libraries and Management:
   • IPC-compliant footprint generation
   • Library management for consistent footprints
   • 3D component models for mechanical verification
2. Advanced Routing Features:
   • Differential pair routing
   • Length matching
   • Via fanout automation
3. Design Rule Checking:
   • Manufacturability checks
   • Assembly clearance verification
   • Electrical rule verification

Popular PCB Design Tools Comparison

Software
Strengths
Limitations
Price Range
Altium Designer
Comprehensive, powerful
Steep learning curve, expensive
$$$$
Eagle
Good for beginners, wide community
Limited layer count in lower tiers
$-$$
KiCad
Free, open-source, improving rapidly
Less automation than commercial tools
Free
OrCAD/Allegro
Industry standard, powerful
Complex, expensive
$$$$
Fusion 360 Electronics
Integration with mechanical design
Newer to the PCB market
$$

Design Verification Tools

Signal Integrity Simulation

1. Pre-Layout Analysis:
   • Establish impedance requirements
   • Determine stackup requirements
   • Define routing constraints
2. Post-Layout Analysis:
   • Verify signal integrity on critical nets
   • Check for crosstalk issues
   • Validate power delivery network

Thermal Analysis for SMT Designs

1. Junction Temperature Estimation:
   • Calculate expected junction temperatures
   • Identify potential hotspots
   • Validate thermal design
2. Simulation Tools:
   • Integrated thermal analysis in CAD tools
   • Specialized thermal simulation software
   • CFD (Computational Fluid Dynamics) for complex systems

Assembly and Soldering Processes

SMT Assembly Process Overview

The typical SMT assembly process follows these steps:

1. Solder Paste Application:
   • Screen printing through metal stencil
   • Typical stencil thickness: 4-5mil (0.1-0.125mm)
   • Aperture design crucial for quality
2. Component Placement:
   • Automated pick-and-place machine positioning
   • Placement accuracy: typically ±0.05mm
   • Component recognition via vision systems
3. Reflow Soldering:
   • Controlled temperature profile
   • Typical zones: preheat, soak, reflow, cooling
   • Maximum temperature: typically 235-245°C
4. Inspection:
   • Automated Optical Inspection (AOI)
   • X-ray inspection for hidden joints (BGAs)
   • Manual visual inspection

Hand Soldering SMT Components

For prototyping or repairs, hand soldering techniques include:

1. Equipment Requirements:
   • Temperature-controlled soldering iron (preferably with fine tip)
   • Flux pen or gel
   • Tweezers
   • Magnification aid
2. Technique for Passive Components:
   • Apply small amount of solder to one pad
   • Place component while reflowing this solder
   • Solder the second pad
   • Reflow first joint if needed for adjustment
3. Technique for Multi-Pin Components:
   • Align component
   • Tack opposite corners
   • Verify alignment
   • Solder remaining pins

Testing and Debugging SMT Boards

Test Methods for SMT Assemblies

In-Circuit Testing

ICT provides thorough electrical testing but requires planning:

1. Test Point Design Guidelines:
   • Minimum diameter: 1.0mm (40mil)
   • Preferred test point shape: round
   • Test point spacing: minimum 2.54mm (100mil)
2. Test Coverage Considerations:
   • Typically 70-90% coverage achievable
   • Coverage limited by access to nets
   • Balance between coverage and design impact

Boundary Scan Testing

For complex digital boards:

1. JTAG Implementation Requirements:
   • Components must support boundary scan
   • JTAG chain properly connected
   • Test access port (TAP) connector provided
2. Coverage and Limitations:
   • Good for testing interconnects
   • Limited for analog testing
   • Cannot test passive components directly

Rework Techniques for SMT Components

Component Removal Procedures

1. For Small Passive Components:
   • Apply flux
   • Heat both ends simultaneously or use hot air
   • Remove with tweezers
2. For Multi-Pin Components:
   • Use hot air rework station
   • Apply flux around component
   • Control temperature to minimize board damage

Component Replacement Best Practices

1. Pad Preparation:
   • Clean pads with solder wick
   • Apply fresh flux
   • Ensure planar surface
2. Placement Techniques:
   • Use paste or tacky flux to hold component
   • Carefully align before soldering
   • Consider stencils for complex components

Future Trends in SMT Design

Emerging Component Technologies

Component Miniaturization Trends

The industry continues to shrink component sizes:

1. Ultra-Miniature Passive Components:
   • 01005 (0.4mm × 0.2mm) now in production
   • 008004 (0.25mm × 0.125mm) emerging
   • Requires specialized equipment for placement
2. Advanced IC Packaging:
   • System-in-Package (SiP)
   • Package-on-Package (PoP)
   • Wafer-Level Chip-Scale Packaging (WLCSP)

Embedded Component Technology

Embedding components within PCB layers offers advantages:

1. Benefits of Embedded Components:
   • Reduced overall size
   • Improved signal integrity
   • Enhanced thermal performance
   • Better protection from environment
2. Design Considerations for Embedding:
   • Specialized design tools required
   • Limited rework capability
   • Higher manufacturing costs

Sustainable and Green Design Approaches

Lead-Free Design Considerations

Since the RoHS directive, lead-free design has become standard:

1. Material Selection Impacts:
   • Higher reflow temperatures (peak ~245°C vs ~220°C)
   • Component and board materials must withstand higher temps
   • Different pad designs for optimal wetting
2. Reliability Considerations:
   • Different failure mechanisms
   • More sensitive to vibration
   • Tin whisker mitigation strategies needed

Design for Recyclability

Sustainable design practices include:

1. Material Selection Guidelines:
   • Avoid mixed materials when possible
   • Label plastics for recycling
   • Use standardized components
2. End-of-Life Considerations:
   • Design for disassembly
   • Minimize use of adhesives
   • Document material content

Case Studies and Practical Examples

Simple SMT Design Example

Single-Sided SMT Board

A basic design example for beginners:

1. Component Selection:
   • 0603 or larger passive components
   • SOIC or TSSOP ICs
   • Minimum 0.5mm pitch QFP if needed
2. Layout Approach:
   • 2-layer board with ground plane
   • 8/8mil trace/space minimum
   • Hand-solderable component spacing

Assembly and Testing

1. Manual Assembly Process:
   • Stencil application of paste
   • Hand placement of components
   • Reflow in oven or with hot air
2. Basic Testing Approach:
   • Visual inspection
   • Power-up testing
   • Functional verification

Complex High-Density Design Example

Multi-Layer High-Density Board

For advanced designs:

1. Component Selection Strategy:
   • Mix of package types based on function
   • Balance between density and manufacturability
   • Consider thermal requirements
2. Layer Stackup Planning:
   • 8+ layers with multiple ground/power planes
   • Controlled impedance for high-speed signals
   • Proper isolation between analog and digital

Advanced Assembly Considerations

1. Manufacturing Partner Selection:
   • Verify capability for fine-pitch components
   • Review DFM guidelines and constraints
   • Discuss test strategy early in process
2. Comprehensive Test Strategy:
   • Combination of ICT, functional, and boundary scan
   • Custom test fixtures
   • Automated optical inspection

Frequently Asked Questions

Q1: What is the minimum equipment needed to work with SMT components for hobbyists?

A: For hobbyists starting with SMT, you'll need:

• A temperature-controlled soldering iron with fine tips (preferably with temperature control)
• Tweezers (high-quality, anti-magnetic, fine-tip)
• Magnification (illuminated magnifier or digital microscope)
• Flux (no-clean flux pen or gel)
• Fine solder (0.5mm or 0.3mm diameter)
• Solder wick for cleanup
• Optional but helpful: hot air rework station, solder paste, and a small reflow oven or hot plate

With practice, components down to 0603 size can be hand-soldered reliably. For finer-pitch ICs, consider using breakout boards initially.

Q2: How do I choose between different SMT component sizes for my design?

A: When selecting component sizes, consider:

• Manufacturing method: For hand assembly, use 0603 or larger components. For automated assembly, 0402 or smaller is fine.
• Available space: Choose smaller components when board space is limited.
• Electrical requirements: Larger packages often handle higher power/voltage.
• Reliability needs: Larger packages generally provide better mechanical stability.
• Cost factors: Mid-sized packages (0603, 0805) often offer the best value.

For beginners, start with 0805 or 0603 components and work your way down as you gain experience. For professional designs, 0402 has become the most common size for general purposes.

Q3: What are the most common SMT soldering defects and how can I prevent them?

A: Common SMT soldering defects include:

• Solder bridging: Caused by excessive solder or insufficient spacing Prevention: Proper stencil design, adequate component spacing, controlled paste volume
• Tombstoning: Component stands on one end due to uneven heating or surface tension Prevention: Balanced pad design, simultaneous heating of both ends
• Cold joints: Insufficient heat or poor wetting Prevention: Proper temperature profile, clean pads, adequate flux
• Component misalignment: Shifting during reflow Prevention: Proper paste amount, controlled reflow profile, adequate pad design
• Voids: Air bubbles trapped in solder Prevention: Proper reflow profile with adequate soak time, quality solder paste

Q4: How do I design PCBs for both automated and manual assembly?

A: To design boards compatible with both assembly methods:

1. Use component sizes that work for both methods (0603 or larger)
2. Provide adequate spacing between components (minimum 0.5mm)
3. Include fiducial marks for automated placement
4. Design with consistent component orientation
5. Avoid components on the bottom side if possible
6. Include test points accessible after assembly
7. Use standard footprints from IPC libraries
8. Provide clear polarity and orientation markings
9. Consider thermal relief pads for easier manual soldering

This approach ensures your design can be manufactured in both low and high volumes without redesign.

Q5: What special considerations are needed for mixed SMT and through-hole designs?

A: For mixed-technology designs:

1. Assembly sequence planning:
   • SMT components are typically assembled first
   • Through-hole components added later (wave soldering or manual)
2. Component placement strategy:
   • Keep SMT components away from wave soldering areas when possible
   • Ensure adequate clearance around through-hole components for assembly tools
3. Thermal considerations:
   • SMT components may be exposed to multiple heating cycles
   • Use components rated for multiple reflow cycles
4. Alternative approaches:
   • Pin-in-paste technology for through-hole components
   • Selective wave soldering
   • Hand soldering of through-hole after SMT reflow
5. Design documentation:
   • Clearly indicate assembly sequence and methods
   • Provide separate pick-and-place files for different assembly steps