The process of Printed Circuit Board PCB Assembly: Introduction to PCBA

The printed circuit board is the heart of any electronic device ranging from cell phones to vehicles. A layman can recognize it as a green board with lines over it and electrical components connected to it with solder. The green color is the characteristics of the solder mask. Every PCB has a component connected to it and is called PCB assembly and the process is called the PCB assembly process. In an assembled PCB, components are connected to each other by copper lines called traces for completing a specific circuit.  The process of PCB assembly is simple and can be performed manually and/or automatically. Here is an example of a PCB assembly company that is located in the USA. PCB companies range in size and industries. There are also OEMs (original equipment manufacturers) that assemble their own PCBAs, depending on the technology.

Parts of PCB

Layers

PCB consists of one to several layers and every layer plays an important role in the completion of the circuit.

Substrate

A substrate is a rigid material that makes a PCB tough and provides housing for components.

Copper

A layer of copper is where all the circuit is completed. The copper layer allows the current to flow from one component to another using copper foils with specific lines called traces. Based on the availability of copper foil PCB may be regarded as single-sided PCB or double-sided PCB.

Solder Mask

Solder Mask avoids unintentional short circuit above the copper layer. The mask gives the PCB a green color and is an insulating material.

Silk Screen

The silkscreen is used for labeling the PCB and symbolling to identify the components and function.

Types of PCB

There are three types of printed circuit boards.

Rigid PCB

These printed circuit boards are inflexible and rigid. These types of PCBs are commonly used. The substrate of the PCB is made from fiberglass, FR4, epoxies, or phenolics.

Flexible PCB

These are flexible circuit board and it is bendable. The substrate is made with high-temperature Kaplan.

Metal Core PCB

These are alternative to rigid PCB as the metal core dissipates heat easily and protect the expensive components and devices.

Technology

There are two mounting technologies in the Printed circuit board assembly.

Thru-Holes Technology

Thru-holes technology is used for the component with leads and wires. Components leads are plugged into the holes of the PCB and are soldered on the other side of the board with traces. This technology is used for large components like capacitors and inductors.

Surface Mount Technology

Small components like diodes and resistors are placed on the top of the board without any hole. Surface mount technology can place Surface Mount Device (SMD) to the PCB.

pcb assembly process

PCB Assembly Process

Solder Paste Stencilling

A stencil of solder paste is applied to the PCB board. The process only allows the solder to the place where the components are connected in the final PCB. The solder is a metal substance contains tin 96%, silver 3%, and copper 1%. A machinal fixture holds the stencil and PCB both in place, the applied solder paste is spread for even distribution and then the stencil is removed leaving the solder paste in specific locations.

Pick and Place

After removal of the solder stencil, components are placed and soldered on the board using robots. Traditionally the task was performed manually.

Reflow Soldering

The components are kept in place and the PCB is moved across a reflow oven using a conveyor belt.  The temperature of the oven is about 250 degrees Celsius.  This high temperature melts the solder paste and connects the components to the board. After the oven, the PCB boards are passed through coolers. Double-sided PCB is passed through a reflow oven twice.

Inspection

The PCB is now inspected and tested. As the reflow process can result in poor connection, missing components, or extra connection. The checking of the board may involve manual checking, automatic optical inspection, or X-ray inspection.

Thru-Hole Insertion

Depending upon the requirements of the board components should be inserted into the holes besides SMDs. PTH components require a more specific type of processes like manual soldering or wave soldering. In the manual process, the components are inserted and soldered by a technical person in multiple cycles. In wave soldering, PCB is passed through a conveyor belt?

Final Inspection and function testing

Finally, PCB boards in a test for its functionality by simulating normal circumstances where the PCB will operate later. Electrical signal run through the PCB and the electrical characteristics is observed. If a PCB board gives an unexpected response, the PCB is regarded as a fail based on company policy.

Cleansing

In the PCB Assembly process finally, PCB is cleaned for residue soldering paste and flux which can damage solder with the passage of time and can suffer customer satisfaction. High-pressure Deionized water is used for cleaning the PCB.

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