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What is Surface Mount Soldering?

Surface mount soldering (or SMD soldering) is the process of electrically and mechanically joining surface mount components (SMCs or SMT components) to printed circuit boards (PCBs) using solder. It enables automated assembly of miniature SMT components for electronics manufacturing. This article covers the key characteristics, processes, techniques and applications of surface mount soldering.

What is Surface Mount Technology (SMT)?

Surface mount technology (SMT) utilizes components that have terminations or “lands” that solder directly to matching pads on the surface of PCBs, as opposed to inserting leads into holes. Some benefits of SMT components include:

  • Smaller size – More compact, portable products
  • Faster automated assembly – Reduced manufacturing costs
  • Higher density – Complex circuitry fits into smaller spaces
  • Enhanced performance – Shorter connections, less noise and parasitics
  • Improved reliability and repeatability – Machined soldering vs. manual

SMT helped enable the electronics miniaturization and performance revolutions of recent decades. But it requires specialized soldering techniques tailored for small surface mount devices (SMDs).

What is Surface Mount Soldering?


Surface mount soldering describes the methods used to solder SMT component terminations onto matching conductive pads on a PCB surface utilizing specialized solder alloys and precisely controlled automated equipment. This creates both electrical connections and mechanical joints securing components.

Some defining characteristics of surface mount soldering include:

  • Typically performed by pick-and-place machines and reflow ovens
  • Requires bespoke pastes and precisely formed solder deposits
  • Adapted for leadless tiny device packages
  • Mandates tightly controlled thermal profiles
  • Mixes processes for array and discrete packages

The core objective of surface mount soldering is to rapidly produce high volumes of reliable solder joints on SMT boards. Next, we’ll look closely at the SMT soldering process steps.

Surface Mount Soldering Process Overview

A typical professional surface mount soldering process consists of five primary steps:

  1. Solder Paste Deposition – A solder alloy paste is precisely printed or dispensed onto pads on the PCB.
  2. Component Placement – Robotic pick-and-place machines position SMC components onto the solder paste deposits.
  3. Reflow – The board passes through a reflow oven melting the solder to attach components.
  4. Inspection – Automated optical inspection (AOI) validates joint quality.
  5. Rework – Any defective joints are repaired by reheating and reapplying solder.

Let’s explore each stage of the surface mount soldering process in more depth.

Solder Paste Application

Solder paste consists of a mixture of fine solder alloy particles and flux suspended in a thick medium. Solder paste must be applied in accurate locations with precise volumes and orientations. Two primary methods used include:

Printing – Screens or stencils with etched apertures align over boards. Solder paste forced through the openings prints exact deposits.

Jet Dispensing – Programmable valves directly jet paste droplets only where needed. Lower volumes but more flexibility.

Both printing and dispensing precisely deposit the small amounts of solder paste required for SMT components prior to placement.

SMT Component Placement

Electronic surface mount components are precisely positioned onto the applied solder paste using automated pick-and-place machines:

  • High speed robotic placement arms fetch components from feeders
  • Cameras visually identify part locations and alignment
  • Nozzles pick, orient and place components on target pads
  • Some devices require additional fluxes or adhesives
  • Different size nozzles or heads accommodate diverse components

Accurately placing a range of tiny SMCs is a sophisticated robotic process with tight tolerance requirements.

Solder Reflow Methods

Reflow soldering melts the deposited solder paste to wet component terminations and PCB pads forming solder joints:

  • Oven – Board conveyed through heated tunnel on conveyor
  • Hot Plate – Board heated on programmable hot plate
  • Laser – Directed beam targeting joints individually
  • Vapor Phase – Saturated vapor condenses only on board briefly

Most SMT production utilizes industrial convection reflow ovens to uniformly heat the assembly and reliably form millions of precise soldered connections.

Soldering Thermal Profiles

Reflow ovens follow optimized thermal profiles tailored to the board, components and solder paste:

  • Preheat – Gradually heats to avoid thermal shock
  • Soak – Dwell time allowing uniform temperature stabilization
  • Reflow – Above liquidus temperature to fully melt solder
  • Cool down – Controlled rate avoids disturbing joints

Profiles are precisely tuned to produce flawless solder joints across the populated PCB assembly.

Automated Inspection

Once soldering is complete, automated optical inspection (AOI) examines each joint:

  • High resolution cameras or lasers scan joints
  • Software compares to ideal profiles
  • Flags defects like shorts, opens, voids
  • Can integrate with rework station

Immediately identifying any insufficient joints enables quick reworking while the process is still hot.

Solder Joint Rework

Defective solder connections detected during AOI must be reworked:

  • Remove old solder first with solder wick if needed
  • Carefully heat joint with hot air tool
  • Use flux dispenser if necessary
  • Apply fresh solder paste and reflow
  • Clean any residues
  • Verify joints meet criteria

Proper rework corrects issues to restore high solder joint yield.

This overview of the surface mount soldering steps provides context on producing SMT assemblies in high volume production environments. Next, we’ll focus on the critical soldering operations.

Key Aspects of Surface Mount Soldering

Several aspects of surface mount soldering require tight process control and oversight:

Solder Paste Mix

  • Powder particle size distribution
  • Powder shape – spherical preferred
  • Flux chemistry and activity
  • Viscosity and rheological behavior

Stencil Design

  • Aperture shapes and alignment
  • Stencil thickness and material
  • Print speed, pressure, separation

Component Placement

  • Accuracy within 0.05mm typically
  • Consistent pressure and orientation
  • Minimal rotation/skew
  • Avoiding tombstoning

Thermal Profile

  • Ramp rates, dwell times, peaks
  • Accounting for materials and geometries
  • Minimizing ΔT across assembly

Wetting and Microstructure

  • Pad and termination metallurgy
  • Ensuring dissolution and intermetallic formation
  • Rounded smooth fillets versus pointed peaks

Optimizing each step and interaction between processes ensures reliable solder joints.

Solder Paste Types

Specialized solder pastes have been developed for surface mount soldering applications:

No-Clean Solder Paste

  • Most common variety
  • Designed to not require cleaning after reflow
  • Reduces costs and processing steps

Water-Soluble Solder Paste

  • Allows easy paste removal after soldering
  • Ideal for rework or less common alloys

No-Slump Solder Paste

  • Thixotropic rheology prevents slumping
  • Useful for non-horizontal surfaces

Halogen-Free Solder Paste

  • Eliminates corrosive halogens like chlorine
  • Meets environmental regulations

Modern solder pastes are highly engineered materials tuned for the increasing demands of surface mount soldering.

Solder Paste Printing

Printing solder paste requires optimized stencil design and tightly controlled processes:

Laser Cut Stencils

  • Precisely cut apertures etched to match pads
  • Allow very fine pitch prints down to 01005 components
  • Clean laser cut edges prevent paste retention

CNC Cut Stencils

  • Economical method for prototyping
  • Limited on fine features below 0402 size

3D Printed Stencils

  • Enables high mix, fast turnaround
  • Challenging getting adequate aperture accuracy

Step Stencils

  • Separate stencils for pastes requiring different volumes

Nanocoated Stencils

  • Low surface energy coating prevents paste sticking
  • Allows easier print deposit alignment

With robust stencil design and printing processes, paste can be deposited accurately even for microscopic components.

Surface Mount Components

Billions of different specialized surface mount components are manufactured for electronics assembly. Some major categories include:

Passives – Resistors, capacitors, inductors and transformers. Common package sizes down to 0201 or smaller.

Actives – ICs, transistors, diodes, LEDs, etc. Wide variety of package types from large BGAs to tiny QFNs.

Connectors – High density board-to-board connectors including mezzanine and edge mount.

Electromechanical – SMT switches, relays, buttons, sensors, crystals, clocks etc.

Interposers – Adapters to integrate non-surface mount components.

Continued miniaturization and expanding package options enables placing more functionality into each square millimeter.

Solder Paste Inspection

Solder paste stencil frame

After printing but before component placement, the applied solder paste deposits are typically inspected:

2D Paste Inspection

  • Color cameras compare print outcomes to ideal
  • Verify positioning, offsets, rotations
  • Check for bridging, insufficient volumes

3D Solder Paste Inspection

  • Laser or photogrammetry scanning
  • Generates detailed 3D paste volume profile
  • Measures paste heights across entire area

Paste inspection helps confirm the print process is dialed in before committing components.

Pick-and-Place Machines

High speed pick-and-place (PnP) machines precisely populate printed circuit boards:

  • Utilize multiple placement heads for productivity
  • Cameras identify part locations and orientations
  • Vacuum nozzles pick components from feeders
  • Robotic arms rapidly place parts onto pads
  • Advanced models incorporate artificial intelligence

High end PnP machines can place over 120,000 components per hour with accuracy down to 0.030mm. This enables automated assembly of SMT boards containing thousands of unique parts.

Reflow Soldering Methods

In addition to thermal profiling, different reflow techniques suit certain applications:

Infrared Reflow

  • IR heaters or lasers solder small assemblies
  • Limited by slower process speed

Vapor Phase Reflow

  • Condensation uniformly heats small boards
  • Minimal overheating or thermal shock

Laser Soldering

  • Focused laser on each joint
  • Great for repairs or selective soldering

Induction Soldering

  • Magnetic field induced eddy currents melt solder
  • Contactless, localized heating

There are many options to deliver tightly controlled thermal input and form high quality soldered interconnections.

Solder Joint Inspection

Beyond visual inspection during assembly, automated optical inspection (AOI) is routinely performed:


  • Color cameras image entire assemblies
  • Checks for missing, misaligned or faulty components
  • Flags collapsed, bridging or shorted joints


  • Laser or photogrammetry scanning
  • Generates detailed 3D surface map
  • Measures volumes, standoff heights and coplanarity

AOI immediately identifies any insufficient joints requiring rework.

Lead-Free Soldering Challenges

Switching to lead-free solders introduced new processing challenges:

  • Higher melting temperatures stress components
  • Poorer wetting increases difficulty forming reliable joints
  • Oxidation and intermetallic growth impact reliability
  • Reduced fatigue resistance risks future failures
  • Tin whiskering can cause electrical shorts
  • Narrower process windows mandate tight control

Through experience and research over the past two decades, the industry has largely mastered lead-free soldering to achieve comparable longevity to leaded solder processes.

Summary of Surface Mount Soldering Attributes

In summary, core attributes of surface mount soldering:

  • Enables automated manufacturing of electronics assemblies with SMT components
  • Requires specialized solder paste materials and deposition processes
  • Leverages advanced robotic technology for precision component placement
  • Controlled thermal profiling ensures melting and wetting to create joints
  • Automated inspection identifies any defects needing rework
  • Process tightly controlled to ensure small components are reliably soldered

Continuous improvement in SMT soldering has helped enable ongoing electronics miniaturization and performance gains.

Applications of Surface Mount Soldering

Surface mount soldering is utilized across virtually all electronics sectors:

Consumer Electronics – Cellphones, laptops, home appliances, gaming systems, etc.

Telecommunications5G infrastructure, network switches, servers.

Automotive – Engine control units, infotainment, driver assistance.

Medical – Patient monitors, imaging systems, prosthetics.

Aerospace/Defense – Avionics, guidance systems, communications.

Industrial – Programmable automation controllers and robotics.

Any application where small, lightweight, high performance electronics are advantageous leverages the capabilities enabled by surface mount soldering.

Frequently Asked Questions

What are some key differences between surface mount soldering and through-hole soldering?

Key differences between SMT soldering and through-hole soldering include:

  • SMT is automated while through-hole is manual
  • SMT uses precisely applied paste while through-hole dips or waves
  • SMT requires ovens for reflow while through-hole uses irons
  • SMT requires specially formulated solder while through-hole uses wire
  • SMT allows miniature components vs. through-hole’s larger sizes

The automated precision of SMT enables modern miniature electronics assemblies.

What defects commonly occur with surface mount soldering?

Common SMT soldering defects include:

  • Insufficient solder or dry joints
  • Excessive voiding in solder joints
  • Cold or fractured solder joints
  • Bridging between adjacent joints
  • Solder balls or splatter
  • Overheated, burnt or lifted pads
  • Tombstoning or drawbriding of components

Tight process control during pasting, placement and reflow minimizes defects.

What are some key tips for hand soldering SMT components?

Tips for manually hand soldering SMT parts:

  • Use a fine tip suitable for the component size
  • Carefully control soldering iron temperature
  • Use miniature solder wire or premixed paste
  • Apply flux to enable good wetting
  • Avoid overheating parts or lifting pads
  • Visual inspect joints for acceptable fill and fillets

Though challenging, with proper tools and technique SMT components can be hand assembled successfully.




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