• Understanding PCB Assembly: Components, Process, and Best Practices

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    Introduction

    In the modern era, technology is growing at a very fast pace and its impact can easily be seen in daily life. The human lifestyle has also greatly enhanced. Advancements in technology have introduced state-of-the-art equipment throughout the world that was not imaginable almost a decade ago. The core ingredient of technological innovation and advancements is electronics specifically PCBs or Printed Circuit Boards. PCBs traditionally in green color are rigid structures holding different electronic components on to it. The components are attached using solder, in a process known as PCB assembly. PCBs are composed of substrates that are manufactured from fiber glass and have various layers of copper metal making traces for conducting the low voltage electrical signals to and from the components. PCBs can either be single or multi-layered depending on the requirements. Epoxy resin is utilized for holding the components and these are insulated with a solder mask. The soldering mask can be of any color e.g. green, violet, blue, red, and orange etc. The solder mask’s purpose is avoiding any kind of short-circuit between the components.

    The traces of copper are for conducting the electric current. The current is transferred between the components as discreteanalog o r high-speed digital signals. A specific plane for ground conduction is given in PCBs carrying higher current or voltages. Vias (holes drilled in the laminate) are dedicated for connection of the components of upper layer to the inner layer i.e. GND. PCB Assembly process is responsible for making the components work in accordance with the product design. The PCB will malfunction even if a small resistor is not placed in accordance to the design or if a small track or copper layer is missing or damaged. Functioning of PCBs can either be simple or complex based on described specifications by users or clients and the sizes of PCBs also differs with respect to those requirements.

    The Design and Layers of Printed Circuit Boards

    1. Substrate: This is the rigid structure that is composed of FR-4 material. All of the components are assembled on to it with solder as the interconnection material. The purpose is to give the PCB a rigid structure.

    2. Traces or Copper Layer: Copper layers are usually very thin and are applied both on bottom and top of PCB for creation of traces for carrying the electric current.

    3. Solder Mask: Solder mask is a layer also applied to both bottom and top of PCB with purpose of creating a non-conductive region, isolating traces of copper, to limit chances of short-circuit.
    4. Silkscreen: The layer of silkscreen is utilized for white labelling that is often seen on PCBs. The labelling is used for company logos or for labelling or naming components that are on PCBs.

    PCB Types Based on Substrate Material

    1. Traditional PCBs: The traditional PCBs are solid in nature and its thickness vary depending on the application. Predominantly, such PCBs are manufactured from FR-4 material or fiber glass. FR-4 refers to fire retardant material. FR-4 material is having self-extinguishing characteristic making it favorable for its use in industrial electronics.

    2. Flexible PCBs: These PCBs arecomposed of substrate material like Kapton withstanding higher temperatures and its thickness is very low, usually 0.005 of an inch. These PCBs are easily bendable and moldable and are utilized in wearable electronics, medical equipment, and consumer electronics.

    3. Metal Core: Such PCBs are composed of material such as Aluminumhaving effi cient heat dispersing capabilities. These PCBs are utilized in applications where heat sensitivity is a problem such as high-power LEDs etc.

    Mounting Technology Types

    1. Through Hole Technology (THT): With THT, the components are inserted on to one side of PCB and then pulled through the other side. THT technology can be assembled using pick and place machines, but is often used when the soldering process is performed manually.
    2. Surface Mount Technology (SMT): With SMT, the components are ssembled onto the surface of the PCB in an automated process using pick and place machines. The components mounted on to PCBs with SMT are usually very small which cannot be handled easily by human operators.

    Prerequisites of PCB Design and Assembly

    Before starting the PCB Assembly Process, the manufacturer is required to ensure some basic checks. The manufacturer must check flaws for any defect on the PCB that may result in its malfunction.

    Following are major steps of the process.

    1. Consideration for Components Layout: All of the through-hole components having polarity should be checked. For example, the polarity of an electrolytic capacitor is to be ensured, cathode and anode terminals of diodes are to be checked, and the direction of ICs head etc. Heat sinks should be placed on components having its requirements.

    2. Vias and Hole Spacing: The spacing between traces and holes must be verified. The spacing among vias and solder pads must also be checked.

    What is PCB Assembly?

    The answer to the question what is PCB assembly is straightforward and the process of PCB Assembly is described in following major steps.

    1. Application of Soldering Paste: The first step is application of soldering paste all through the area of circuit board where components are to be placed. A stencil is used for the printing of the soldering paste. The soldering paste is melted to create a rigid joint after heat is applied.

    2. Placement of Components: A pick and place machine is utilized for the purpose of placing components on the circuit board. The designer is responsible for creating a file that is fed to the automated machine for placing the components. The file contains relevant coordinates for placing the components in the right locations on the board.

    3. Reflow Soldering: Reflow soldering is a mechanism in which the circuit board with components in the right places is put onto a conveyer belt taking it into a reflow oven. The conveyer belt then moves the PCB through preheat zones an oven until it reaches a peak temperature of around 250 degree Celsius. This high temperature melts the solder completely applied in Step 1. Solder joints are created on the PCB fixing the components in place, permanently. Afterwards, the PCB is moved to a cooling zone which solidifies the joints. It is worth noticing that in the case of multi-layered PCBs the side having fewer components is processed first followed byother la yers in a similar fashion.

    4. Inspection and Quality Control: After the completion of PCB Design and Assembly process, it is time for inspection and quality control to check for errors, if any. A thorough check is made to ensure all components are placed in the right places and there is no danger of a short-cir-cuit. The inspection can be both automatic and manual explained below in detail.

    -Manual Inspection: A PCB ismanually checked extensively for any malfunction because of misplaced component, misalignment, faults, or short-circuit etc. It has been discussed that SMT components are very small; therefore, a manual inspection may result in eye strains or fatigue. Therefore, such an inspection method is not feasible for SMT based PCBs but can be useful in THT based PCBs.

    -Optical Inspection: Optical inspection is very useful for checking a batch of PCBs. The process involves an Automatic Optical Inspection (AOI) machine with high resolution cameras installed at different locations and angles for viewing the solder joints. This method of inspection is speedy and completes inspection within a short time.

    -X-Ray Inspection: An X-raymachine is capable of allowing the technicians to view the PCBs to its inner layers and check its defects. It is also used to inspect Bottom-Terminated Components (BTCs) such as BGAs. X-Ray inspection is expensive, but is critical for inspecting the increasing number of BTCs. This method is only utilized in inspection of advanced and complex application based PCBs.

    5. THT Component Soldering and Fixing: THT components are also commonly called as Plated Through Hole or PTH components. Such components are having leads that are passing through PCB’s holes. Theholes are  interconnected with vias. There are two major types of THT soldering.

    -Manual Soldering: In manual soldering a technician is dilligently working on insertion of the specific component on to the board e.g., resistor, capacitor, and inductor etc. After that the board
    is moved to another technician for insertion of other component. The board is passed on until all components are placed on to it. This can make the entire PCB Board Assembly process very lengthy and tedious.

    -Wave Soldering: Wave Soldering is the updated version of manual soldering but with automation. When the components are all stuffed on to the PCB is moved to a conveyer belt under an oven. Molten solder wave is splashed all over the circuit board on to which the leads of the components are present. In this way, all the components are soldered in one pass, making the PCB Board Assemblyprocess fast a nd convenient. However, this process only works for single-sided PCBs ,not for double-sided PCBs.

    6. Functional Testing: Next step is final inspection of the PCBs and then testing it is functioning. Electrical signals are applied through supplying power to the PCB at its dedicated power pins and its working is monitored at specified points. This testing requires dedicated laboratory equipment and instrumentssuch as fun ction generator, digital multi-meter, and oscilloscope etc. In case if any of the parameters of PCB is not up to the mark or are unacceptable, the PCB is then discarded directly, or reworked as per the policies of the company. It can be said that testing is very important phase in determining the failure or success of the Assembly PCB Board.

    7. Cleaning and Shipment: After the functional testing phase, when PCB gets clear with all aspects and testing, the time is to clean them and remove all unwanted flux residues, stains, oil and fingerprints. An automated washing tool with high pressure jets is utilized for this purpose. It uses deionized water and a small amount of cleaning chemistry for the purpose of cleaning. After drying the PCBs are then packed and shipped.

    Difference Between Types of PCB Assemblies

    1. Low-Volume PCB Assembly: It has a batch size ranging from 1 to 100 units of PCBs. It encompass mostly of engineered-to-order, pilot builds, and protype PCBs. The process of assembly utilized is most manual. The cost is much higher and production is low.

    2. Medium-Volume PCB Assembly: The batch size range from 100 to 10000 units. It encompass mostly of pre-production verification and intermediate run PCBs. The mode of automation of assembly process is moderate. It has affordable costs and moderate production.

    3. High-Volume Assembly: It has batch sizes of over 10000 units. It is done on post-launch and fullscale production basis. The level of automation of assembly process is high-tech, state-of-the-art and SMT. The manufacturing cost is low and higher production.

    Steps of PCB Assembly Process

    PCB Assembly Companies are referred to as Original Equipment Manufacturers (OEMs), Contract Electronic Manufacturers (CEMs), Electronic Manufacturing Services (EMS) or Original Design Manufacturers (ODMs). They are competent companies providing electronic assembly processes products for a variety of applications and markets, including aerospace, military, consumer, industrial automation, telecoms, medical etc.. The PCB Assembly manufacturing process comprises of procurement of material, processing of PCBs and SMT chips, testing, finishing products, and
    shipment. The following are the main steps involved.

    1. Determining the cooperation and then signing a contract after negotiation of two parties for manufacturing of PCBs.

    2. Next step is orders from the customers and delivering information related to processing. The customer initiates the placement of the order and delivers the Bill Of Materials (BOM), all relevant PCB and Gerber files, and testing plan to start processing the product.

    3. Procurement of material is also very important. The circuit boards, fixtures, and steel mesh are procured in accordance with the orders placed by customers.
    4. Inspection and processing is done afterwards when materials are delivered to the manufacturing floor and are then delivered to PMC forproduc tion planned.

    5. Testing is accomplished after manufacturing the desired PCBAs. All tests are undergone on entire PCBA batches as requested by customers.
    6. As mentioned earlier, the final step is cleaning the PCBs for removing all unwanted residuals. Afterwards, packaging of PCBs is done and then delivered to the customer.

    Conclusion

    It can be summarized that there are a number of ways for the classification of the PCB Assembly and PCB Assembly Equipment. The classification is mainly grounded on the types of components, level of automation, and production volume. A number of medium and small companies manufacturing PCBs leverage outsourcing PCB Assembly Design for avoiding larger investments. Whereas, higher production volumes use automated SMT assembly, THT is utilized for PCBs with large size of components, such as power devices and heatsinks.

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