Surface Mount Assembly Process Step by Step

Summary

This transcript explains the process of applying solder paste to printed circuit boards (PCBs) for surface mount technology (SMT) assembly. The process involves stencil printing the paste, placing components, and reflowing the solder to form electrical bonds. It covers various soldering techniques like BGA placement, double-sided reflow, and wave soldering for conventional components. Additionally, it highlights cleaning, inspection, and rework stages, as well as the importance of in-circuit testing for electrical integrity. The overall procedure aims to ensure high-quality PCB assembly with precise component placement and soldering.

Takeaways

• 😀 Solder paste is applied to a printed circuit board using dispensing or stencil printing, with a metal stencil aligned to surface mount pads.
• 😀 After applying solder paste, components are placed into it, where its tacky nature helps hold them in place during assembly.
• 😀 Ball Grid Array (BGA) devices and fine-pitch parts are placed accurately using optical alignment systems, checking for missing ball terminations.
• 😀 The solder paste is heated to 215-225°C to reflow and form a bond between the circuit pads and component terminations.
• 😀 Different reflow methods, such as convection, infrared, and vapor phase reflow, are used for soldering.
• 😀 High-temperature solder balls or columns may be used in some cases and will not collapse during reflow.
• 😀 Cleaning processes, including solvent, aqueous, or semi-aqueous, may be used to remove flux residues from solder paste after reflow.
• 😀 Double-sided reflow assembly involves printing solder paste onto both sides of the board, allowing for the assembly of surface-mount components on both sides.
• 😀 Through-hole components are inserted during reflow, allowing for soldering of both surface-mount and through-hole leads in one operation.
• 😀 Wave soldering, including flux application, preheating, and solder wave contact, is used for soldering conventional components and through-hole connections.

Q & A

What is the purpose of applying solder paste to a printed circuit board (PCB)?

Solder paste is applied to a PCB to create a temporary bond that holds components in place during assembly. The paste is applied using dispensing or stencil printing and helps form electrical connections when heated.

How is solder paste applied to the PCB in the stencil printing method?

In stencil printing, a metal stencil is aligned with the PCB’s surface mount pads. Solder paste is then forced through the stencil’s apertures using a squeegee blade, which is selected based on the application.

Why is the solder paste tacky, and how does this help in component placement?

The solder paste is tacky, which helps it hold the components in place during assembly. This tackiness ensures that the components stay securely positioned on the PCB until they are soldered during the reflow process.

What role does optical inspection play during the component placement process?

Optical inspection is used to ensure the accurate placement of components, especially fine-pitch parts like Ball Grid Array (BGA) devices. It checks for alignment of the component balls with the corresponding pad locations and detects any missing ball terminations.

At what temperature is the solder paste reflowed, and why is this process important?

The solder paste is reflowed between 215 to 225 degrees Celsius. This process is crucial because it melts the paste, forming the bond between the circuit pads and the component terminations, ensuring a reliable electrical connection.

What are the different methods of reflow soldering mentioned in the script?

The methods of reflow soldering mentioned in the script include convection, infrared, and vapor phase reflow. Each method applies heat to melt the solder paste and create the necessary bonds between the components and the PCB.

How does the introduction of pin-in-hole reflow improve assembly processes?

Pin-in-hole reflow allows both surface-mount and through-hole components to be soldered in one operation, eliminating the need for hand and wave soldering. This enhances efficiency and simplifies the assembly process, especially for double-sided PCBs.

What is the role of adhesive in assembling conventional components, and how is it applied?

Adhesive is used to hold conventional components in place on the PCB. It can be dispensed, printed, or pin-transferred onto the board before the components are placed. The adhesive is then cured at temperatures between 110 to 125 degrees Celsius to secure the components.

What is the function of wave soldering in the assembly process?

Wave soldering is used to solder both conventional and bottom-side surface-mount components. It involves flux application, preheating, and passing the PCB through a wave of molten solder, which creates reliable solder joints for the components.

What happens after wave soldering, and why is inspection important?

After wave soldering, the PCB may go through a cleaning process to remove any residual flux. Inspection is important to ensure the quality of the solder joints and to identify any defects. It is recommended to perform inspection at each stage of the assembly process to maintain quality standards.