SMT – PCB Assembly Cutting-edge Technology



SMT stands for Surface Mount Technology, which is a widely used method for assembling electronic components onto printed circuit boards (PCBs). It has largely replaced traditional through-hole technology due to its numerous advantages, including a smaller PCB footprint, higher component density, and improved manufacturing efficiency. SMT involves mounting components directly onto the surface of the PCB, rather than inserting them through holes and soldering on the other side.

Surface Mount Technology (SMT) is a cutting-edge and widely adopted method for assembling electronic components onto printed circuit boards (PCBs). It has transformed the landscape of electronics manufacturing, supplanting traditional through-hole technology with its numerous advantages and versatility. With SMT, components are mounted directly onto the surface of the PCB, eliminating the need for through-hole vias and allowing for a higher component density on smaller board footprints. The SMT process involves applying solder paste to the PCB pads, followed by automated placement of components using pick-and-place machines. During reflow soldering, the solder paste is melted, creating strong and reliable electrical and mechanical connections between the components and the PCB. SMT offers a host of benefits, including reduced manufacturing costs, faster assembly times, improved electrical performance, and the ability to accommodate complex and compact designs. As electronic devices continue to shrink in size and demand for high-performance products rises, Surface Mount Technology remains at the forefront of PCB assembly, enabling the development of innovative and sophisticated electronic products across various industries.


The SMT Process

The Surface Mount Technology (SMT) process is a sophisticated method for assembling electronic components onto printed circuit boards (PCBs). It involves several sequential steps to ensure precise and efficient component placement, soldering, and quality control. The SMT process can be summarized in the following steps:

PCB SMT Pick & Place Machine
PCB SMT Pick & Place Machine


Material Examination of Preparation

Material examination and preparation are critical stages in the Surface Mount Technology (SMT) process, setting the foundation for successful PCB assembly. The first step involves a thorough examination of the Bill of Materials (BOM), a comprehensive list of components required for the PCB assembly. Our MorePCB Procurement teams meticulously source components from trusted suppliers to ensure their authenticity and quality. Once acquired, components are carefully inspected to verify their specifications, package types, and quantities, avoiding potential shortages or mismatches during assembly. SMT components are often supplied in reels or tape packaging, necessitating material handlers to set up feeders and align components correctly for the pick-and-place process. Solder paste, a crucial component in the SMT process, undergoes careful handling and storage to maintain its consistency. Furthermore, the stencil, used to apply solder paste onto PCB pads accurately, must be prepared and aligned precisely for seamless paste deposition. 

Bill of material ( BOM)
Bill of material ( BOM)
SMD Components
SMD Components


Preparation of Stencil

Stencil preparation is a critical aspect of the Surface Mount Technology (SMT) process, significantly influencing the quality and accuracy of solder paste application during PCB assembly. The stencil is a thin metal sheet, usually made of stainless steel, with precisely laser-cut apertures that correspond to the PCB’s solder pads. These apertures allow the solder paste to be deposited onto the PCB during the soldering process. Stencil preparation begins with the design of the stencil layout, which is based on the PCB’s solder pad pattern and component footprint data. The stencil design is created using specialized software, ensuring precise alignment and optimal solder paste volume for each solder pad.

After the stencil design is finalized, the physical stencil is manufactured using a laser-cutting process or other advanced techniques. The stencil thickness is carefully selected based on the application and solder paste requirements. Thicker stencils are suitable for higher-volume production, while thinner stencils are preferred for smaller, more intricate PCBs.

Before the stencil is ready for use, it undergoes a thorough cleaning process to remove any residual solder paste from previous use. This cleaning is crucial to prevent contamination and ensure consistent paste deposition.

STM Stencil
SMT Stencil

Solder Paste Printing

Solder paste printing is critical in the Surface Mount Technology (SMT) assembly process. It involves accurately depositing solder paste onto the pads of a printed circuit board (PCB) before placing surface-mount components. The goal is to ensure precise and consistent application of solder paste to create reliable solder joints during the subsequent reflow process.

Solder paste printing on a PCB
Solder paste printing on PCB


Here’s an overview of the solder paste printing process:

Stencil Setup

The first step is to prepare the stencil. The stencil is a thin sheet of material (usually stainless steel) with openings (apertures) that correspond to the pads on the PCB. The stencil is aligned with the PCB using alignment guides, and a stencil printer is commonly used for this purpose.

Solder Paste Selection

The appropriate solder paste is chosen based on the PCB design, component requirements, and the reflow process parameters. Factors like lead-free or leaded solder, particle size, and flux content are taken into account during the selection.

Stencil Cleaning

Before the printing process, the stencil is cleaned to remove any solder residues or contaminants from previous use. This ensures that only the desired amount of solder paste is deposited on the pads.

Solder Paste Application

The solder paste is applied to the stencil, and a squeegee or a stencil printer is used to spread the paste across the stencil openings. The squeegee is typically made of rubber or metal, and it moves across the stencil, forcing the solder paste through the apertures and onto the PCB pads.

Inspection and Quality Control

After the solder paste application, the PCB is often visually inspected or undergoes an automated inspection to check for any defects, such as insufficient or excess solder paste.

Surface Mount Technology component placement

It is a critical stage in electronic circuit board manufacturing, where surface-mount components are accurately positioned and soldered onto the PCB. SMT has largely replaced through-hole technology due to its advantages in miniaturization, better electrical performance, and automated assembly capabilities.

Component Feeding

The first step in SMT placement is to prepare the components. Surface-mount components are typically supplied in reels, tubes, trays, or tape-and-reel packaging for automated assembly. For manual assembly or prototyping, loose components or small reels might be used.

Pick-and-Place Machine Setup 

A pick-and-place machine (also called a placement machine or a mounter) is used for automated component placement. Before starting, the operator sets up the pick-and-place machine by loading the appropriate component reels or trays onto the machine’s feeders.

Component Placement on a PCB
Component Placement on PCB


Program the Machine

The pick-and-place machine requires a programming file that contains information about the PCB design, the component types, orientations, coordinates, and other parameters. This file is usually generated by converting the PCB design data (such as Gerber files) into a format compatible with the pick-and-place machine.

Vision System Alignment

Many modern pick-and-place machines include a vision system that aligns the PCB and components. The vision system identifies fiducial marks on the PCB to ensure precise alignment and compensates for any slight variations in the PCB’s position.

Component Pickup

The pick-and-place machine’s robotic heads use vacuum nozzles to pick up individual components from their reels or trays. The vacuum is activated, and the nozzle collects the component based on the programmed coordinates and orientation.

Placement on PCB

Once a component is picked up, the pick-and-place machine moves the robotic head to the designated location on the PCB. The vision system assists in aligning the component with the correct pads on the PCB.

Placement and Inspection

The machine places the component onto the PCB with precise accuracy. After placement, the vision system or additional sensors may be used to inspect the component’s alignment and positioning for quality control purposes.

Adhesive or Glue Dispensing (Optional) 

In some cases, before component placement, a pick-and-place machine can also dispense a small amount of adhesive or glue to hold specific components in place temporarily. This is especially common for components that might move during the reflow process, like heavy or tall components.

Repeat the Process

The pick-and-place machine continues this process, picking up various components and placing them on the PCB until all components are accurately positioned.

Reflow Soldering

PCB Reflow Soldering
PCB Reflow Soldering


There are several stages involved in this process. A detailed illustration is given below: 

Preheating Stage

In this stage, the PCB with solder paste and components enters the reflow oven. The temperature is gradually raised to a pre-defined value to remove any residual moisture from the solder paste and the components. This prevents solder defects caused by the sudden release of trapped moisture during the reflow process. Preheating also prevents thermal shock to the components by gradually raising their temperature.

Soaking Stage

Once the preheating stage is complete, the temperature is increased further to a soaking temperature. The PCB is held at this temperature for a specific period, typically around 60 to 120 seconds. Soaking allows the solder paste to reach a uniform temperature throughout the assembly, ensuring proper flux activation and evaporation of any remaining solvents.

Reflow Stage

After the soaking stage, the temperature is rapidly raised to the reflow temperature, which is above the melting point of the solder paste. As the temperature increases, the solder particles in the paste begin to melt, turning into a liquid state. The molten solder forms a metallurgical bond between the component leads/pads and the PCB pads, creating solder joints.

Peak Temperature Stage

At the reflow temperature, the assembly reaches the peak temperature for a specific duration, usually between 200°C to 260°C (392°F to 500°F). The peak temperature ensures that the solder completely liquefies, allowing for proper wetting and intermetallic bonding between the solder and the metallization on the component leads and PCB pads. The duration of the peak temperature stage is critical as it affects the time available for the solder to form strong and reliable joints.

Cooling Stage

After the peak temperature stage, the assembly is rapidly cooled down to solidify the solder joints. The cooling rate is controlled to prevent thermal stress on the components, as sudden temperature changes can lead to component damage or solder joint fractures. The cooling stage is also essential for preventing the warping of the PCB due to temperature differentials. Throughout the reflow soldering process, the temperature profile is carefully controlled by the reflow oven’s heating elements. A well-defined temperature profile ensures that the solder paste reaches the appropriate temperatures for flux activation, solder melting, and forming reliable solder joints.

Reflow Oven Types

Convection Reflow Oven

Uses heated air circulated within the oven to transfer heat to the PCB and components.

Infrared Reflow Oven

Utilizes infrared radiation to directly heat the PCB and components, allowing for faster heating rates.

Vapor Phase Reflow Oven

Utilizes a vapor medium with a constant boiling temperature to achieve precise and controlled reflow temperatures.

Inspection and Cleaning 

Inspection and cleaning are critical steps in the Surface Mount Technology (SMT) assembly process to ensure the quality and reliability of the final electronic circuit boards. Both processes are essential for detecting defects, removing contaminants, and verifying the integrity of the solder joints and components. Let’s explore each step in detail:

Inspection in SMT

The inspection involves the examination of the PCB and its components at various stages of the assembly process to identify any defects or inconsistencies that may affect the functionality and reliability of the finished product. Several inspection methods are used in SMT assembly:

Visual Inspection

Operators or automated systems visually inspect the PCB, solder paste application, and component placement for misalignment, missing components, solder bridges, tombstoning (when a component stands on one end due to imbalanced solder reflow), and other visible defects.

Automated Optical Inspection (AOI)

AOI machines use cameras and image recognition algorithms to inspect the PCB for defects quickly and accurately. They can detect issues such as component polarity, incorrect values, and solder joint quality.

X-ray Inspection

X-ray machines are used to inspect hidden solder joints and identify defects, such as solder voids and insufficient solder, which may not be visible through other inspection methods.

In-Circuit Testing (ICT)

ICT machines are used to verify the electrical functionality of the assembled PCB. They can detect faulty components, incorrect component values, and opens/shorts in the circuit.

Cleaning in SMT

Cleaning is performed to remove any residues, flux, solder balls, or other contaminants that may remain on the PCB after the soldering process. Proper cleaning is crucial, especially when no-clean solder paste is used, as some flux residues may still be present.

No-Clean Solder Paste

In recent years, “no-clean” solder pastes have become popular. They are designed to leave minimal flux residue after reflow, reducing the need for cleaning. In many cases, no-clean solder paste is sufficient, but it is essential to verify its suitability for specific applications.

Cleaning Methods 

When cleaning is required, it can be done through various methods, including ultrasonic cleaning, spray cleaning, or batch cleaning systems. The choice of cleaning method depends on factors such as production volume, type of contaminants, and the level of cleanliness required.



Surface Mount Technology (SMT) assembly is a highly efficient and widely adopted process in the electronics industry for manufacturing printed circuit boards (PCBs). It offers numerous advantages, including miniaturization, improved electrical performance, and automated production capabilities. Here’s a comprehensive conclusion for SMT assembly:

Miniaturization and Component Density

SMT assembly enables the placement of smaller and more compact components on PCBs, leading to higher component density and reduced board size.

Improved Electrical Performance

SMT components have shorter lead lengths and smaller parasitic capacitance and inductance, resulting in improved high-frequency performance and signal integrity. This makes SMT ideal for high-speed digital and RF applications.

Automated Production

SMT assembly is highly amenable to automated manufacturing processes, leading to increased efficiency, consistency, and reduced production costs. Pick-and-place machines, stencil printers, and reflow ovens play key roles in achieving high levels of automation.


While initial setup costs for SMT assembly can be higher, the overall production costs are lower due to reduced labor requirements, faster assembly times, and better yields. The scalability of SMT production makes it cost-effective for both high-volume manufacturing and prototyping.

High Reliability

The precise and controlled nature of SMT assembly results in reliable solder joints and consistent performance. Additionally, solder paste and reflow soldering ensure robust connections that can withstand thermal and mechanical stresses.

Versatility and Flexibility

SMT technology accommodates various component types, from simple passive components to complex microprocessors and integrated circuits. This versatility allows SMT assembly to cater to diverse applications and industries.

Quality Assurance

Inspection and testing procedures, such as visual inspection, AOI, X-ray, and in-circuit testing, ensure the quality and reliability of SMT-assembled PCBs. These measures detect defects early in the production process, reducing the likelihood of faulty products reaching the end users.

Industry Standard

SMT assembly has become the industry standard for electronic circuit board manufacturing due to its numerous benefits and ability to meet the demands of modern electronic devices.

In conclusion, Surface Mount Technology (SMT) assembly has revolutionized the electronics industry by enabling miniaturization, improving electrical performance, and providing efficient and cost-effective manufacturing processes. Its high reliability, versatility, and environmental friendliness make it the preferred choice for producing a wide range of electronic products that power our modern world. As technology continues to advance, SMT assembly will likely remain at the forefront of electronic circuit board manufacturing for years to come.

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