How Circuit Card Assembly (CCA) Works: Components, Processes, and Challenges

Circuit card assemblies (CCAs) are essential to modern electronics, enabling the production of compact, high-performance devices. This type of manufacturing requires skilled professionals to layout the design and then assemble it. One major advantage of CCAs is their ability to support more complex designs compared to other manufacturing methods. Additionally, CCAs offer a cost-effective solution, as their faster production process reduces both time and expense. This article will cover some common methodologies, design procedures, and challenges related to circuit card assembly.

Essential Components of a Circuit Card Assembly (CCA)

A circuit card assembly has several aspects that enable proper circuit functionality. These can be differentiated into:

● Printed Circuit Board (PCB)

● Electronic Components

● Onboard Connectors

1. Printed Circuit Board (PCB):

Circuit assembly features in all electronic devices, and the PCB provides the basic platform for the assembly and wiring of electronic components. The essential basic components of a PCB are:

● Substrate: It is the foundation of a circuit assembly that holds all electrical components in place. Substrate materials vary based on PCBA type, such as flexible, rigid, or metal core boards. Mostly, fiberglass is used in rigid PCBs while Polyimide is a common material for flexible PCBs.

● Copper Traces: Copper traces provide conductive paths to connect different PCB components. Circuit card assembly substrate and copper foil are laminated together using heat. Depending on PCB layers, multiple copper layers may be etched according to the circuit schematic.

● Solder Mask: It is a thin protective layer that covers the conductive parts of the circuit to prevent corrosion and reduce the risk of solder shorts. It also helps the manufacturers accurately place the components on the PCB.

● Silkscreen: It is the topmost PCB layer featuring the essential symbols, values, and text. The Silkscreen makes it easier for the user to understand the placement and functionality of the PCB.

2. Electronic Components:

Electronic components serve as the driving force in a circuit card assembly (CCA). Their placement is crucial for achieving optimal design and performance. While a CCA may contain numerous components, the following are commonly found in nearly every assembly.

Electronic circuits are designed using components such as resistors, capacitors, inductors, and integrated circuits. The behavior of these circuits can be analyzed and predicted using circuit theory and mathematical models. Here is a detailed guide about commonly used components and their applications.

● Resistors: Resistors are components that limit the flow of electrical current. They are essential for controlling current levels and dividing voltage within a circuit. Resistor works based on the principle of Ohm' s law which states that “voltage applied across the terminals of a resistor is directly proportional to the current flowing through it”.

● Capacitors: Capacitors store and release electrical energy.  It stores electrical energy in the form of an electric field. A capacitor blocks the DC signals and allows the AC signals and is also used with a resistor in a timing circuit. They are also used for filtering, smoothing, and timing applications in various circuits.

● Inductors: An inductor is also referred to as an AC resistor which stores electrical energy in the form of magnetic energy. It resists the changes in the current and the standard unit of inductance is Henry which stores energy in a magnetic field when current flows through them. They are used in filtering, tuning, and energy storage applications.

● Transistors: A transistor is a three-terminal semiconductor device. Mostly it is used as a switching device and also as an amplifier. This switching device can be voltage or current-controlled. By controlling the voltage applied to the one terminal controls the current flow through the other two terminals.

● Diodes: Diodes allow current to flow in one direction only, making them crucial for rectification and signal demodulation. It has two terminals, anode and cathode terminals. These are mostly used in converting circuits like AC to DC circuits.

● Integrated Circuits (ICs): ICs are miniaturized electronic circuits that contain multiple electronic components, such as transistors, resistors, and capacitors, fabricated on a single semiconductor chip. These are the building blocks of current electronic devices like cell phones, computers, etc. These can be analog or digital integrated circuits.

3. Onboard Connectors:

Connectors are a critical part of circuit cards. They ensure connectivity between the card and external components, allowing signals to flow smoothly to and from the circuit.  The connectors interface the circuit card assembly and external devices like PCBs, sensors, or other components. Other than connectivity, these components help to achieve a modular approach in electronic circuits. Based on their use, there are several types of connectors such as:

Board-to-Board Connectors - Connect two PCBs. These are often needed in systems where a circuit board needs to communicate with two or more PCBs. For instance, embedded systems connect with various sensor modules via dedicated connector pins.

I/O Connectors - Input Output (I/O) connectors interface the CCA with external components. They are used for data or power transmission. For instance, USB, HDMI, and Ethernet connectors are some examples of I/O connectors.

Wire-to-Board Connectors - These connectors provide connectivity between wires from sensors, actuators, and the PCB. These connectors are designed to handle the flexibility of wires without signal disruption.

High-Frequency Connectors - Used for connecting radio frequency and microwave signals. The connectors ensure minimal signal loss, thanks to various elements like dielectrics, tight tolerance, and advanced shielding.

FPC/FFC Connectors - FPC (Flexible Printed Circuit) and FFC (Flexible Flat Cable) connectors cater to flexible electronic components. FPC connectors link thin, flexible PCBs, while FFC connectors connect ribbon-like, extremely thin, and flexible flat cables.

Types of Circuit Card Assembly

1) Box Build Assembly

Box build assemblies are also called systems integration. It refers to a basic PCBA or CCA housed in an enclosure. So, a typical box-build assembly includes both electronic and electromechanical parts such as Connectors, power sources, Custom cable assemblies, Heatsinks, and other thermal attachments. Since the box is part of the system design, its enclosure is also specially designed according to the PCBA.

2) Surface Mount Technology (SMT) Assembly

Surface Mount Technology (SMT) refers to applying electronic components directly onto the surface of the PCB. The process enables compact PCB creation and promotes automation. The components don' t need to be inserted into through holes, as they are soldered directly on the metal pads on the PCB surface. Therefore, SMT can produce more complex and compact circuits leading to the production of high-density PCBs.

3) Through-Hole Assembly

Through-hole assembly refers to PCBs that contain drilled holes to allow the component leads to pass through. The leads are then soldered on the other side of the PCB. Through-hole assembly evolved from single-sided to double-sided, and finally to multi-layer boards. Through-hole assembly is still widely used in applications where components don' t support SMT assembly. For instance, transformers, and electrolytic capacitors require through-hole assembly.

What is the Circuit Card Assembly Process?

The circuit card assembly process typically involves several steps, including designing, fabrication of PCB, applying solder paste, placement of components, reflow soldering, and inspection. Here are the detailed assembly steps with instructions:

1) Designing of PCB: This step involves designing the PCB using CAD software. The design specifies the location and orientation of the components, as well as the electrical connections between them. Factors like impedance matching, EMI/EMC reduction, and power dissipation are kept into consideration while designing the PCB.

2) Fabrication of PCB: The design is then printed onto a copper-coated board using a PCB fabrication, to know all the PCB fabrication steps visit our step-by-step guide on PCB manufacturing.

3) Applying Solder Paste: Solder paste is applied to the board using a stencil, which ensures that the paste is applied in the correct locations with equal density.

4) Placing Components: Automated machines or technicians manually place the SMD and TH components onto the board.

5) Reflow Soldering: The board is then heated in a reflow oven, which melts the solder paste and fuses the components to the board. Different soldering techniques can be used as per board specifications.

6) Inspection: The board is inspected to ensure that all components are properly soldered and that there are no defects. Usually, AOI, X-Ray, and Flying probe machines are used for inspection purposes.

7) Conformal Coating and Packaging: Depending on the application, the CCA may receive a protective conformal coating or be encapsulated in resin for added protection against environmental factors and mechanical stress. The CCA is then mounted into an enclosure, connected to other system components, and packaged for shipment to the end-user.

Soldering Techniques Employed in SMT:

There are two main soldering techniques used in Surface mount assembly soldering:

Reflow Soldering:

 1. Apply solder paste to PCB.

 2. Place components using pick-and-place machines.

 3. Heat in a reflow oven through stages: preheat, soak, reflow, and cooling.

It is precise, automated and suitable for complex SMDs. However, there is a risk of thermal stress, and potential for voids in solder joints.

Wave Soldering:

 1. Apply flux to PCB.

 2. Preheat the PCB.

 3. Pass the PCB over a wave of molten solder.

 4. Cool to solidify solder joints.

It is very efficient for through-hole and some SMDs, good for high-volume production. When production volume is high, and there are many through-hole components, wave-soldering is the right choice. The process uses a pool of liquid solder to stick metal parts to the bottom of the board. A solder mask prevents the solder from sticking to areas where soldering is not required.  

However, due to less precision for fine-pitch components, there is a risk of solder bridges and defects. Each technique is selected based on the type of components, production volume, and specific PCB requirements.

Common Circuit Card Assembly Issues and Solutions

There are several issues that can arise during the circuit card assembly process. Let's take a look at some common issues and their solutions:

1) Soldering Issues: Soldering issues can include incomplete solder joints, dry solder joints, and excessive solder. These issues can be caused by incorrect solder paste application and by incorrect temperature settings. Solutions include adjusting the temperature settings as listed on the solder paste provided by the manufacturer.

2) Component Placement Issues: Component placement issues can include misplacement and misalignment of components. These issues can be caused by incorrect component orientation or incorrect machine settings. Solutions include manually adjusting the component position or adjusting the machine settings.

3) Design Issues: Design issues may arise from inaccurate design specifications or incorrect component footprints. These problems can stem from errors in the design files or inaccurate component data. Solutions involve updating the design files or rectifying the component data.

4) Thermal Management: As component density and power consumption increase, the heat generated within the assembly rises. Which led to component overheating, reduced performance, and decreased reliability. The solution involves designing the power components away from each other and reserving more area for them.

Advancements in CCA Technology

CCA technology has come a long way since its inception. Over the years, the following advancements have revolutionized the design concepts for CCAs. Here are some of the main points:

● High-Density Interconnect (HDI) PCBs

● Increased component density

● Improved 3D Packaging Techniques

● Improved Thermal management

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Conclusion

Circuit Card Assembly (CCA) is not ideal for mass production due to its complexity compared to other manufacturing methods, making it challenging to produce large quantities of circuit cards efficiently. Another disadvantage of CCA is that it can be difficult to replicate a design. This is because the design is created using a computer program. If the design is lost, it can be very difficult to recreate it.

However, CCA technology has revolutionized electronics by enabling the creation of compact, high-performance devices across various industries. CCAs are essential for systems from consumer electronics to aerospace, offering exceptional functionality and reliability within smaller form factors. Innovations like high-density interconnect (HDI) PCBs and 3D packaging techniques continue to push the boundaries of design possibilities and improve device performance.