In this article, you will learn how to assemble printed circuit boards (PCBs) with surface mount technology (SMT) components in a production environment. Surface Mount Technology (SMT) became the technology of choice for Printed Circuit Board Assembly (
PCBA) in the early 1980’s as electronic products became smaller and demanded smarter, easier to use and more accurate technology.
Prior to the popularity of SMT, through-hole technology was the dominant technology where component leads (axial or radial style) were inserted into holes on a bare PCB and then soldered. One side of the board was referred to as the “component side” and the other side as the “solder side”, a term that is still used today.
Even today, both technologies are still in use and can even coexist on the same board, each with its own advantages. Because through-hole technology allows for a stronger physical connection, it is considered a better way to design highly reliable products for rugged or commercial applications that need to withstand environmental stress.
It is also a good choice for components that may be subjected to mechanical stress (i.e., vibration) due to mass (e.g., connectors) or for larger components (e.g., decoupling capacitors, transformers, etc.).
Surface mount technology, originally known as planar mount, is a method of mounting electronic components to the surface of a printed circuit board using flat leads or small metal pads. This technique is helpful when large quantities of PCBs are required or when miniaturization is needed to meet unique requirements.
Unlike through-hole technology, which allows components to be easily mounted on either side of the board within limits, SMT technology does not require through-holes to create connections between board layers.
Today’s multilayer SMT PCB stacks range from 2 to 32 layers, with 4 and 6 layers being the most common. In this case, the advantage of surface mount technology is to further reduce the size of the PCB by increasing the number of connections per square inch of PCB. To better understand the complexity, let’s take a look inside a PCB stack.
Note that the PCB in Figure 2 consists of several layers. This arrangement of materials is called a “PCB stack” and can vary depending on the PCB requirements.
The two key components of the stack are the copper-clad layers (for electrical conductivity) and the laminating layers (to provide structure and insulation).
The copper-clad layers are “etched” to create a specific wiring pattern and can vary in thickness: thicker layers allow more current to flow.
The standard copper thickness used is 1 ounce. Interestingly, copper thickness in PCB manufacturing is measured in ounces (oz.), which is a specific weight of copper spread over an area of 1 square foot (a thickness equal to 1.37 mils or 0.0348 mm).
In addition, 1.5 oz. and 2 oz. are also standard copper thicknesses, which will allow correspondingly higher currents to flow in a circuit. The laminates consist of fiber-reinforced epoxy resins. They are available in two basic types: “Cores”, with a pre-cured copper foil, and “PrePreg”, without a copper foil and without the epoxy fully cured.
The PrePreg layer acts as the glue that holds all the layers together as they are assembled and cured by pressure and heat.
The most common type of laminate used in PCB manufacturing is called “FR4”, but different laminates (BT Epoxy, Polyimide, Teflon, etc.) can be used for more demanding applications such as high frequency or high temperature. Also, there are different types of PCBs: rigid, flexible and rigid-flexible. Rigid PCBs are more traditional PCBs and, as the name suggests, are inherently rigid or inflexible.
Flexible PCBs are built by replacing the core or prepreg layer with a thin polyimide layer that can be bent or shaped to fit space constraint requirements. Flexible PCBs are becoming more mainstream today due to the miniaturization and popularity of wearable devices. On the other hand, rigid-flex PCBs are a combination of rigid and flexible PCBs, combining the versatility of flexible circuits with the stability of rigid PCBs. By eliminating the need for connectors and cables, the reliability of board-to-board connections is improved, resulting in a more reliable system. It is the most expensive type of PCB and is primarily used in aerospace or military applications.
In addition, laminates are available in a variety of thicknesses to accommodate a wide range of stacking configurations, PCB thicknesses, and PCB types. Standard options for total thickness of all laminates on a rigid PCB range from 0.2 mm to 3.2 mm (0.008 in. to 0.240 in.). The most common options include 0.8 mm (0.032 in.), 1.6 mm (0.063 in.), and 3.175 mm (0.125 in.).
While common boards fall within this range, backsheets tend to be thicker, approaching the upper end of the range for added mechanical strength. On the other hand, we have very thin (0.065 mm to 0.42 mm) and highly flexible PCBs. PCBs usually have their outer copper layer covered in a thin “soldermask” to protect the copper from corrosion and act as an insulator, exposing only the pads. In most cases, the soldermask is green, but it can also be red, blue, black, white, etc. On top of the soldermask, the PCB is covered with a thin layer of copper.
Over the soldermask, the PCB presents a layer of inscription called a silkscreen.
A silkscreen is a layer of ink traces used to mark text and symbols on the PCB to identify the location of components (based on reference indicators) and to specify the orientation of components.
A word of caution: It is important to avoid overlapping silkscreens on pads, as shown in Figure 6, as they can create problems during the soldering process, which can be disastrous in the long run.
The most common finishes for pads in use today are: Hot Air Solder Leveling (HASL), Lead Free HASL, Immersion Silver (Ag), Immersion Tin (Sn), Electroless Nickel Immersion Gold (ENIG) and Electroless Nickel Palladium Immersion Gold (ENEPIG). These technologies are listed from least to most expensive.
Simply put, a better surface finish provides a flatter surface for the pads, allowing for better soldering of the components on the pads.
ENIG is a proven industry standard that is easily implemented by most PCB manufacturers, and ENIG deposits provide a tighter, more uniform grain structure, maintain high solderability, and resist corrosion.
The most commonly used stack today is a 4-layer stack with ENIG coating and green soldermask, with two middle layers dedicated to “power” (VCC or a combination of VCC values) and “ground” (GND).
The remaining outer layers are used for connections between components.
Now that we have a general understanding of PCB technology, let’s review the various steps involved in the PCB assembly process.
Pre-PCB Assembly
Printed Circuit Board Design
Electronic designers translate functional requirements into “schematics” – symbolic recipes that show which electronic components will be used and how they will be connected to form a functional circuit.
What You Need to Know About PCB Assembly Manufacturing
