PCB Assembly Process

Assembly and Components
,
1

SierraConnect PCB Assembly Process
SierraConnect PCB Assembly Process1700Ă—1250 278 KB

PCB assemby is the process of mounting electronic components on a bare circuit board. Below is the step-by-step procedure:

1. DFA (design for assembly) analysis

It is the first stage of PCBA. During this phase, DFA engineers verify the design data present in the Gerber/ODB++/IPC-2581 files and BOM (bill of materials).

DFA ensures that the design is optimized for assembly. Engineers check for potential issues like incompatible components, insufficient clearances, and layout choices that could complicate the assembly process or reduce yield. Catching these problems early helps minimize costly rework, reduce lead times, and improve overall product reliability.

2. Check for non-washable components

If your design has non-washable components (surface mount and through-hole devices), they will be kept aside. These parts will be assembled at a later stage to avoid part damage during the cleaning process.

3. SMT assembly

This stage involves solder paste screening/inspection, SMT component placement, FAI (first article inspection), reflow soldering, FPT (flying probe testing), AOI (automated optical inspection), and AXI (automated X-ray inspection).

  • Solder paste screening/inspection: The circuit board is coated with solder paste, a mixture of solder alloy and flux, using a stencil. This ensures solder paste is applied only to the areas where components will be placed. After application, the paste is inspected—either manually or using automated systems—to verify it is correctly positioned and in the right quantity. This step is critical to ensure proper adhesion during soldering.
  • SMT component placement: Components are taken from reels or trays and placed onto the PCB using a pick-and-place machine. This device uses advanced vision systems to align components accurately with the solder paste, ensuring precision. This automation reduces human error and speeds up production, especially for high-density boards.
  • FAI (first article inspection): A preliminary X-ray inspection (AXI) is performed to identify any defects, such as misaligned or wrong component placement.
  • Reflow soldering: The PCB, with components, is passed through a reflow oven. The oven has a controlled temperature profile that melts the solder paste, forming a permanent bond between the components and the board. The solder hardens as the printed board cools, creating strong electrical connections.
  • FPT/AOI/AXI inspection: After reflow soldering, the board undergoes a series of inspections to ensure quality. They include:

  1. FPT (flying probe testing): This testing method uses movable probes to make contact with test points on a PCB. It checks for electrical connectivity, shorts, and open circuits without requiring a custom fixture.

  2. AOI (automated optical inspection): Cameras scan the board to detect visual flaws, such as misplaced components, poor solder joints, or bridges.

  3. AXI (automated X-ray inspection): This test procedure is used to identify internal issues like voids in solder joints.

4. Through-hole assembly

Through-hole components are inserted into drilled holes, either manually or using automated insertion machines. Precision is key for ensuring leads align properly, particularly on densely populated boards.

  • Soldering THT components

  • Depending on the board’s design and production volume, THT components can be soldered using the following methods:

  • Wave soldering: The PCB is passed over a wave of molten solder, which coats the leads and solders them to the board.

  • Selective soldering: Used for targeted soldering, especially on boards with both SMT and THT components.

  • Hand soldering: Manual soldering is employed for low-volume production, prototypes, or when automated methods are not possible.

After soldering, FAI and AXI are conducted to verify solder joint integrity and identify defects.

5. Cleaning/washing

The assembled PCB is cleaned using deionized water or solvents to remove flux residue. This prevents corrosion and potential electrical issues.

This step is essential because leftover flux can cause corrosion and electrical leakage over time, especially in high-density or high-voltage designs. Proper cleaning ensures long-term reliability, improves insulation resistance, and prevents potential field failures, particularly in mission-critical or harsh-environment applications.

6. Soldering non-washable components

Non-washable parts identified earlier are now soldered using no-clean flux, which does not require post-solder cleaning. After assembly, the board again undergoes AOI, AXI, and FPT to ensure quality.

7. Conformal coating

A protective coating (acrylic, polyurethane, silicone, or epoxy) is applied to protect the PCB from environmental factors such as moisture, dust, chemicals, and temperature extremes.

8. Final inspection

Before packaging, the PCB goes through a final inspection to ensure compliance with electrical, mechanical, and quality standards. This includes checking for visual defects and verifying overall functionality.

9. Packaging and delivery

PCBs are packed with antistatic protection, clear labeling, and cushioning in sturdy boxes to ensure safe transit to the customer.

1 Like