SMT Manufacturing: Everything You Need to Know

Surface mount technology (SMT) is an assembly and production method that applies electronic components directly onto the surface of a printed circuit board (PCB).

 

Introduction

 

Surface mount technology (SMT) is a facet of electronic assembly wherein electronic components, referred to as surface mount devices (SMD), are directly affixed to the surface of a printed circuit board (PCB). Due to its cost-effectiveness and efficiency in ensuring quality, SMT has garnered significant demand in the industry.

 

What is Surface Mount Technology (SMT)?

 

Surface mount technology (SMT) is an assembly and production method that applies electronic components directly onto the surface of a printed circuit board (PCB).

SMT Production Process

 

The SMT manufacturing process comprises three main stages: solder paste printing, component placement, and reflow soldering. However, to address the intricacies of the SMT production process, the following outline provides a more detailed analysis of each stage:

 

1. SMC and PCB Preparation

 

This initial stage involves the selection of Surface Mount Components (SMCs) and the design of the Printed Circuit Board (PCB). The PCB typically features flat, commonly silver, tin-lead, or gold-plated copper pads without holes, known as solder pads. These solder pads provide support for the pins of components such as transistors and chips.

 

A crucial tool in this stage is the stencil, which establishes a fixed position for the subsequent phase of the process (solder paste printing). The stencil aligns with the predetermined positions of solder pads on the PCB. All materials used in the manufacturing process, including SMCs and the stencil, undergo thorough examination to detect any flaws.

 

2. Solder Paste Printing

 

This phase is pivotal in the SMT production process. Here, a printer applies solder paste using the prepared stencil and squeegee (a tool for cleaning in printing) at an angle ranging from 45° to 60°. Solder paste is a putty-like mixture comprising powdered metal solder and sticky flux. The flux acts as a temporary adhesive, holding the surface mount components in place while also cleansing the soldering surfaces of impurities and oxidation.

 

The solder paste is utilized to establish the connection between the Surface Mount Components (SMC) and the solder pads on the PCB. It is crucial to ensure that each pad receives the correct quantity of paste; otherwise, insufficient paste could prevent a proper connection when the solder is melted in the reflow oven. In the electronics manufacturing industry, a reflow oven is an electronic heating device used in Surface Mount Technology (SMT) to position electronic components on printed circuit boards (PCBs).

 

3. Components Placement

 

In this stage, pick-and-place machines come into play to affix components onto the PCB. Utilizing a vacuum or gripper nozzle, each component is extracted from its packaging, and the placement machine positions it in its designated location. The PCB advances on a conveyor belt, with the swift and precise machines depositing electronic components onto it; some machines boast the capability to place up to 80,000 individual components per hour.

 

Precision is paramount in this process, as any inaccuracies in component placement that are soldered into position can incur significant costs and require time-consuming rework.

 

4. Reflow Soldering

 

Once the Surface Mount Components (SMCs) are positioned, the PCB is conveyed into the reflow soldering oven, where it traverses through the following zones for the soldering process:

 

a. Preheat zone: This initial zone raises the temperature of the board and attached components simultaneously and gradually. The temperature increases at a rate of 1.0℃-2.0℃ per second until reaching 140℃-160℃.

 

b. Soak zone: In this phase, the board is maintained at a temperature between 140℃ and 160℃ for 60-90 seconds.

 

c. Reflow zone: The boards then enter a zone where the temperature is ramped up at 1.0℃-2.0℃ per second, reaching a maximum of 210℃-230℃. This melting point allows the tin in the solder paste to weld the component leads to the pads on the PCB. Throughout this process, the molten solder's surface tension keeps the components in place.

 

d. Cooling zone: This final section ensures that the solder freezes upon exiting the heating zone, preventing joint defects.

 

For double-sided printed circuit boards, these processes may be repeated using either solder paste or glue to secure the Surface Mount Components (SMCs) in place.

 

6. Cleaning and Inspection

 

Following the soldering process, the board undergoes cleaning and meticulous inspection for any defects. In case flaws are identified, necessary repairs are conducted before the product is moved to storage. Various methods are employed for Surface Mount Technology (SMT) inspection, including magnifying lenses, Automated Optical Inspection (AOI), flying probe testers, X-ray inspection, and more. Utilizing machines instead of relying on the naked eye ensures swift and accurate results.

 

SMT: Pros and Cons

 

Surface Mount Technology (SMT) offers several advantages in PCB assembly (PCBA), PCB manufacturing, and electronics production:

 

Pros:

 

1. Allows for the use of smaller components.

2. Encourages increased automation throughout the assembly process.

3. Provides maximum flexibility in constructing PCBs.

4. Enhances reliability and overall performance.

5. Reduces the need for manual intervention in component placement.

6. Enables the creation of smaller and lighter boards.

7. Facilitates ease of PCB assembly, utilizing both sides of the board without the hole limitations of conventional methods.

8. Can coexist with through-hole components on the same board.

9. Increases component density, accommodating more Surface Mount Devices (SMD) in the same space or maintaining the same number of components within a smaller frame.

10. Offers a cost-effective approach with lower material expenses.

11. Simplifies the SMT production process, reducing overall production costs.

 

Cons:

 

1. Suited for small volume production.

2. Components may be more fragile and prone to breakage.

3. Demands high standards in soldering technology.

4. Components can be easily dropped or damaged during installation.

5. Visual inspection is challenging, making testing difficult.

6. The miniaturization and various solder joint types complicate both the SMT manufacturing process and inspection.

7. Requires a significant investment in equipment, such as SMT production machines.

8. Technical complexity necessitates high training and learning costs.

9. Rapid technological developments mandate continuous follow-up and adaptation.

 

Conclusion

 

Smaller size, quicker production, and reduced weight are the major allures of SMT, leading to much easier electronic circuitry design and production, especially crucial in complex circuits. This higher level of automation has saved time and resources throughout the electronics manufacturing industry. As such, even though there is always a chance of developing new technology, SMT has undoubtedly secured its relevance.