Introduction

Surface mount technology (SMT) refers to the method of mounting electronic components directly onto the surface of printed circuit boards (PCBs) without through-hole connectors. Originally developed in the 1960s, SMT has now become the standard method of assembling modern PCBs across consumer, industrial, automotive, telecom, and military electronics.

This comprehensive guide covers everything related to SMT including:

• The history of surface mount assembly
• Benefits compared to through-hole mounting
• Description of common SMT components
• SMT soldering and attachment methods
• Typical SMT assembly process steps
• SMT equipment for fabrication, printing, placement, reflow
• Quality control and yield best practices
• Environmental considerations for lead-free assembly
• Latest innovations advancing SMT technology

Let’s take a detailed look at what surface mount technology entails and how it has revolutionized electronics manufacturing.

History of Surface Mount Technology

Surface mount technology evolved from earlier hybrid assembly approaches combining surface mounted and through-hole components:

1960s – Components glue-bonded onto PCB surface

1970s – Tape automated bonding (TAB) for ICs

1980s – Adoption of standard gull-wing SMT components

1990s – High speed SMT pick-and-place and reflow equipment

2000s – Lead-free manufacturing and micro-SMT advances

2010s – 01005 passives, microBGA, PoP, flip-chip, high-density SMT

Thanks to continuing innovations, SMT now supports ultra-miniature components for highly automated assembly, driving electronics product performance, size, and cost improvements.

Benefits of SMT

Compared to through-hole PCB assembly, surface mounting offers numerous advantages:

• Smaller components and closer spacing enables miniaturization
• Higher component density allows more functionality per PCB area
• Simpler board layouts reduce layers and vias
• Easier automation with faster component placement
• Eliminates tedious lead clinching and trimming
• Cost-effective volume manufacturing
• Lead-free assembly improves environmental friendliness
• Better electrical performance from reduced parasitics

Thanks to these benefits, SMT has largely replaced through-hole mounting as the standard assembly technique for high-volume production. Next we’ll examine common component packages designed for SMT.

Surface Mount Component Packages

A range of standardized component packages have been developed optimized for surface mount assembly. Some popular examples include:

Quad Flat Packages (QFP)

• Square ICs with leads extending from four sides
• Pins underneath spaced on 0.5mm grid typically
• Available in various sizes from 20 pins to over 200 pins
• Allowing effective washing during assembly

Ball Grid Arrays (BGA)

• ICs with input/output terminations in grid array of solder balls on bottom
• Allows high density component mounting without perimeter leads
• Larger pitch balls used for mechanical support; finer pitch for I/O
• Advanced variants like PBGA, CBGA, EBGA, etc.

Chip Scale Packaging (CSP)

• IC packages size nearly identical to silicon chip dimensions
• Very compact with fine lead spacing
• Allows maximizing PCB area utilization
• PoP, fan-out wafer level, and other CSP configurations

Land Grid Arrays (LGA)

• Input/output pads in grid pattern on underside of component
• Mated with corresponding pads on PCB through compression
• Enables contacts across entire package underside for very high I/O density
• Common for large FPGAs, GPUs, and other complex devices

Passives

• Rectangular chip resistors, capacitors, inductors, etc.
• Available in sizes including 0201, 0402, 0603, 0805, 1206, etc.
• Metric codes denote size (e.g. 1201 is 0.6mm x 0.3mm approx)

Continuing technology improvements allow steadily shrinking component sizes while increasing available I/O density. This drives electronics innovation and new product capabilities.

SMT Attachment Methods

With no leads to insert, surface mount components are attached to pads on the PCB through methods including:

Soldering

• By far the most common attachment approach
• Provides both electrical connection and mechanical bond
• Reflow or wave soldering used depending on package style

Conductive Adhesives

• Silver epoxy glues conduct electricity for connection
• Provides higher resistance and lower mechanical strength than solder
• Limitations on temperature during curing

Press-Fit Connections

• Spring-loaded pins compressed into plated through holes
• Gas-tight, reworkable connection without solder
• Requires precise hole machining and pad oxide cleaning

Sockets

• Intermediate socket soldered to board mates with pins of package
• Allows easy replacement of components like CPUs for upgrades
• Higher cost, contact resistance, and poorer thermal performance

Among these options, soldering remains the optimal choice for most components due to superior electrical, thermal, and mechanical characteristics.

SMT Soldering Processes

Attachment of surface mount components is typically achieved using either of two primary soldering processes:

Reflow Soldering

• Solder paste applied to pads via stencil/screen printing
• Components placed onto paste deposits
• Entire assembly heated in oven profile above solder melting point
• Solder reflows wetting pads to form solder joints

Wave Soldering

• Bottom side passed over flowing wave of molten solder
• Solder adheres to exposed pads to form solder joints
• Top already assembled separately with adhesive or reflow
• Limited to durable components able to withstand wave exposure

The assembly side used depends on the package style. Sensitive small components use reflow while connectors and some passives are wave soldered.

SMT Assembly Process Overview

A typical complete surface mount assembly sequence consists of the following steps:

1. Solder Paste Printing

Solder paste containing powdered solder and flux is applied to pads through a stencil.

2. SMT Component Placement

Components are precisely picked from feeders and positioned onto pads using automated pick-and-place machines.

3. Reflow Soldering

A controlled thermal reflow profile melts the solder paste deposits to wet pads and form solder joints.

4. Through-Hole Component Insertion

Any through-hole components are inserted either manually or with automated inserters.

5. Wave Soldering

The bottom side goes through a solder wave to attach remaining components.

6. Cleaning/Finishing

Excess fluxes are removed using cleaning processes. Final finishes or coats are applied.

7. Inspection/Test

Automated optical inspection and electrical testing validate assembly quality and function.

8. Conformal Coating

A protective conformal coating may be applied to shield components.

Highly optimized implementation of these process steps allows efficient, high-yield assembly of quality boards. Next let’s look closer at some of the key surface mount technologies and equipment used.

Solder Paste Printing Equipment

Solder paste application tools include:

Stencils

• Laser cut metal foils 0.1-0.15mm thick
• Stencil apertures align with PCB pad positions
• Allows rapid dispensing of precise paste volumes

Solder Screens

• Steel or polyester fine mesh screen stretches on frame
• Best for high-volume production with repeating designs
• Allows higher height tolerance

Printers

• Automated machines align stencil and print solder paste
• Prints can print ~15 cm^2/sec at 150 μm feature size
• Provides consistent, high-speed, high-precision paste dispensing

With high quality stencils, precision printers, and well-tuned processes, printing delivers paste deposits with tight process margins supporting advanced assembly.

SMT Component Placement Systems

Automated pick-and-place systems position components for assembly. Key characteristics include:

• Feeders – Tape, sticks, trays, etc. to supply components
• Pick Heads – Nozzles for vacuum pick-up of components
• Cameras – Pattern recognition for precise alignments
• Work Holder – Moving PCB platform and clamping
• Software – Programming component locations and sequences

High-end chip shooters provide fast, flexible, precision placement with advanced features like:

• Feeder racks with hundreds of components
• Dual gantry heads for simultaneous pickup/placement
• Component flipping/rotation capabilities
• Modular heads supporting various components
• On-head barcode scanning for traceability

State-of-the-art pick-and-place systems can accurately assemble over 150,000 components per hour, enabling extremely high-volume production.

SMT Reflow Oven Options

Reflow ovens pass assembled boards through a controlled thermal profile to reflow (melt) solder paste and form solder joints. Types include:

Convection Ovens

• Use fans to circulate hot forced air
• Provides good heat transfer efficiency
• Challenging to evenly heat large boards

Vapor Phase Ovens

• Use evaporating solvent to uniformly heat PCB
• Excellent thermal uniformity
• Condensation/cleaning considerations

Infrared Ovens

• Use IR lamps or panels to heat boards
• Very fast response time
• Can be difficult to evenly heat all areas

Laser Soldering

• Directed laser energy to melt solder joints
• Minimal thermal impact on rest of assembly
• Typically used for small rework applications

Modern ovens with multiple controllable heating zones provide the thermal precision needed for lead-free solders and advanced packages.

SMT Inspection Systems

Automated inspection validates assembly quality:

2D AOI

• 2D machine vision inspection of joints, parts, defects
• High-speed image matching algorithms
• Limited ability to detect misplaced bottom-side parts

3D AOI

• Laser profiling and scanning to obtain 3D surface data
• Detects lifted leads, skewed parts, and solder defects
• Significantly higher sensitivity than 2D AOI

AXI

• Automated x-ray inspection
• Sees inside components and connections in 3D
• Detects defects like voids, lack of wetting, etc.
• Complementary technique to optical AOI

These automated optical and x-ray tools replace tedious manual inspection with comprehensive analysis for process feedback and quality control.

Lead-Free SMT Assembly

Growing environmental concerns led the industry shift from tin-lead solders to lead-free alloys like SAC 305 (SnAgCu). Key considerations:

• Higher soldering temperatures required
• More susceptible to defects like head-in-pillow
• Components and boards must withstand higher temps
• Solder baths converted to lead-free
• Process tuning for optimal wetting and joints

Despite challenges, rigorous process controls allow assembling lead-free electronics reliably.

Latest SMT Innovations

Ongoing technology developments continue advancing SMT capabilities:

• Micro SMT – Enabling 0201 passives, microBGAs, wafer level packages, and beyond
• 3D Packaging – Alternatives to larger chips like SiP and PoP
• ** additive processes** – Inkjet solder printing, aerosol jetting, ultimately pick-and-place at chip scale
• Smart Data – Analytics and machine learning for process optimization
• Direct Integration – Flip-chip and other direct on-substrate approaches
• Flexible Hybrid Electronics – Printing electronics onto flexible substrates

These innovations will allow packing greater functionality into smaller and smaller electronic assemblies.

Conclusion

This guide provided a comprehensive overview of surface mount technology, the transformational manufacturing approach that has enabled electronics to become ubiquitous across all areas of life today.

We traced SMT’s origins in the 1960s to present-day automated factories assembling miniature components at remarkable speeds and volumes. The benefits over through-hole assembly are clear in terms of size, performance, cost, and manufacturability.

Understanding SMT provides key insight into the production processes, components, equipment, inspection methods, and innovations that allow translating circuit board designs into functioning devices. Mastering SMT fundamentals is therefore critical knowledge for electrical engineers and professionals across the electronics industry.

The next wave of SMT breakthroughs in areas like additive manufacturing and 3D integration will continue driving electronics advancement into coming decades. By leveraging these manufacturing capabilities, engineers can turn visions into reality.

Frequently Asked Questions

What are common SMT assembly defects?

Typical SMT defects include missing components, skewed/misaligned parts, insufficient solder, excess solder, solder balls/bridges, thermal damage, tombstoning, cuts/nicks, and holes/voids in joints.

What types of solder alloys are used in SMT?

Common solder pastes are SAC 305 (3% silver, 0.5% copper) and SAC 105 (lower silver). Other alloys include SnCu, SnAg, and SnZn. Lead-free solders require higher melting temperatures than tin-lead.

How small can SMT components be made?

0201 chip components measuring just 0.25mm x 0.125mm are common. New techniques allow soldering chips down to 0.15mm size. The lower limit is still advancing through micro-assembly breakthroughs.

What causes tombstoning defects in SMT?

Insufficient solder volume and surface tension imbalances during reflow can lift smaller components up on one end like a tombstone. Adjusting paste deposits, profiles, and layout helps prevent.

How does SMT affect thermal management?

The move to SMT helps by eliminating through-hole lead thermal resistance. But high density SMT heightens the need for thermal vias, internal layers, and airflow to prevent excessive component temperatures.

Conclusion

From its origins decades ago, surface mount technology has revolutionized electronics manufacturing and enabled steady improvements in capabilities, size, functionality, speed, and cost. Mastering SMT principles provides invaluable insight into the foundation underpinning electronics assembly and innovation.