Basic Thing You Should Know About PCB Assembly Process

The world is advancing rapidly in terms of modern technology, which significantly influences our daily lives. Our lifestyles have drastically changed due to these technological advancements. Ten years ago, many of the advanced equipment we now take for granted were merely visions of the future. At the core of this equipment lies electronics engineering, with Printed Circuit Boards (PCBs) at its center.

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A PCB is typically a green, rigid board that supports various electronic components. These components are soldered onto the PCB through a process known as PCB Assembly (PCBA). The structure of the PCB consists of a substrate made of fiberglass, electronic components, a copper layer that creates traces, holes for components, and both inner and outer layers. At RayPCB, we offer multilayer PCB prototypes with up to 36 layers and mass production boards with up to 10 layers. Single-sided and double-sided PCBs have only outer layers without inner layers.

The PCB substrate and components are insulated with a solder mask, secured with epoxy resin. Common colors for solder masks include green, blue, and red. This layer prevents short circuits between components and traces.

Copper traces facilitate the transfer of electronic signals across the PCB. These signals may include high-speed digital signals or discrete analog signals. Traces can be thickened to supply power to various components.

For PCBs designed to carry high voltage or current, a dedicated ground plane is usually incorporated. The electronic components on the top layer connect to inner ground and signal planes using vias.

Assembling components on the PCB is crucial for its functionality. Even a minor mistake, such as a misplaced SMT resistor or a cut trace from the manufacturer, can lead to PCB failure. Therefore, it's essential that components are installed accurately, resulting in the term PCBA for the assembled PCB.

The complexity of PCBs varies based on specifications provided by clients or users, and their sizes differ according to requirements.

The PCB assembly process encompasses both automated and manual methods, which will be discussed further.

PCB Layers and Design

As previously mentioned, multiple signal layers exist between the outer layers. We will now review the types of outer layers and their functions.

1. Substrate: The rigid board is made of FR-4 material, which provides the PCB's rigidity.

2. Copper Layer: Thin copper foil is applied on the top and bottom layers to create copper traces.

3. Solder Mask: This layer is applied to the top and bottom of the PCB, creating non-conductive regions that isolate copper traces to prevent short circuits. The solder mask also ensures solder only adheres to designated areas, like holes or pads, for THT and SMT component installation.

4. Silkscreen: The white labeling visible on PCB fabrication shows component designators, such as R1 or C1, and includes logos and descriptions. This layer is essential for providing crucial information about the PCB.

PCBs can be categorized based on substrate materials:

1. Rigid PCBs: These are the most common type, made from fiberglass (FR-4), known for its fire-retardant properties. Rigid PCBs are used in power amplifiers and electronic devices like calculators and keyboards.

2. Flexible PCBs: Made from materials like Kapton, these can bend easily and are used in applications such as wearable electronics and keyboard connectors.

3. Metal Core PCBs: These utilize aluminum or other metals that effectively dissipate heat and are used in applications involving heat-sensitive components.

Types of Mounting Technology:

SMT: Surface Mount Technology (SMT) is suitable for small components and requires specialized automated pick-and-place robots for placement.

THT: Through-Hole Technology (THT) involves components with leads that pass through the PCB. Manual soldering is common for THT components, simplifying the assembly process.

Assembly Process Prerequisites:

Prior to fabrication and assembly, a thorough Design for Manufacturing (DFM) assessment is conducted to identify any flaws that could cause issues during operation.

1. Component Layout Consideration: The polarity of through-hole components must be verified, as well as space for heat sinks.

2. Hole and Vias Spacing: Ensure proper spacing to prevent overlapping between solder pads and via holes.

3. Copper Pads, Thickness, and Trace Width: Careful consideration of these parameters can significantly reduce manufacturing costs and improve turnaround times.

At RayPCB, we provide DFM and Design for Test (DFT) checks during assembly and prototyping. Our services include PCB OEM, wave soldering, PCB card testing, and SMT assembly.

PCB Assembly (PCBA) Process:

Step 1: Applying Solder Paste Using Stencil

The initial step involves applying solder paste to the areas of the PCB where components will be placed. A stainless steel stencil facilitates this, allowing precise application of the solder paste.

Step 2: Automated Placement of Components:

SMT components are placed on the PCB using pick-and-place robots based on pre-programmed locations that ensure accuracy in placement.

Step 3: Reflow Soldering

In this step, the PCBs with components pass through an oven where the solder melts and creates solid joints.

Step 4: QC and Inspection

A comprehensive inspection follows to identify any misalignments or defects.

Step 5: THT Component Fixation and Soldering

THT components are inserted and soldered, with methods varying between manual soldering and wave soldering.

Step 6: Final Inspection and Functional Test

Functionality tests evaluate electrical signals and parameters based on design requirements.

Step 7: Final Cleaning, Finishing, and Shipment:

The PCB undergoes cleaning to remove any residual substances before final packaging.

Conclusion

Understanding the PCB assembly process is essential for ensuring quality and functionality in electronic devices. From design considerations to rigorous testing, each step plays a vital role in the successful production of PCBs. For those looking for high-quality PCB assembly services, visit PCB And PCBA Assembly Service for expert solutions.