As electronics continue to shrink and the demand for miniaturized devices increases, surface-mount components have become integral in PCB assemblies due to their tiny size.
Unlike THT (Through-Hole Technology) components, surface-mount devices sit directly on the PCB surface because they have small lead frames. This frame design makes them occupy smaller real estate on the board.
Therefore, SMDs make it possible to have miniaturized electronics with sleeker designs and better performance. As an electronics enthusiast, designer, or professional engineer, it is essential to understand these components to be able to build such electronic devices.
We’ve covered everything you need to know below, so read on to learn more.
Table of Contents
- What are Surface Mount Components?
- Types of Surface Mount Components and Identification
- SMD Component Selection and Application
- The Mechanics Behind SMCs
- What Are the Benefits of Using SMCs
- Challenges and Considerations
- Surface Mount Components Sourcing and PCB Assembly One-Stop Solutions
- Surface Mount Components FAQs
What are Surface Mount Components?
Surface-mount components are electrical parts mounted directly on the surface of PCBs, specifically on solder pads, to make electrical contact with the circuit. To better understand these electrical parts, let’s compare them against the alternative, which are through-hole components.
Through-hole and surface-mount PCB Components
Through Hole vs. Surface Mount: A Comparative Analysis
| Characteristic | Surface-Mount Components | Through-Hole Components |
| Size and Weight | Tiny and lightweight | Large and heavy |
| Mounting on PCBs | On the surface (pads) | Through plated-through holes |
| Density on PCBs | High | Low |
| Placement on PCBs | One or both sides | One side |
| EMC Performance | Low radiation emissions | Higher radiation emissions |
| Resistance and Inductance | Fewer unwanted RF effects | Higher unwanted RF effects |
| Assembly Automation | Easier to automate | Challenging to automate |
| Mechanical Strength | Relatively weak solder joint | Strong solder joint |
| Assembly Cost | Low | High |
Types of Surface Mount Components and Identification
SMT components are available in different types, but the most common ones include the following.
Inductors
Inductors are wire-wound devices with a copper wire wound around a magnetic core. This structure converts electrical energy into magnetic energy, providing high inductance and current handling capabilities.
How To Identify Inductors
They store electrical energy in magnetic form and are primarily used in switched-mode power supply systems that produce direct current. Inductors keep the current flowing in the off cycle to produce smooth DC output.
This energy-storing capability is also critical in choking, blocking, filtering, smoothing, or attenuating high-frequency noise in electrical circuits.
You can identify these devices from their black round, square, or rectangular packages that house the wire-wound structure. SMD multilayer and ferrite-bead inductors are slightly different because they resemble resistors, with the metal terminals on either end covering the entire side.
Resistors
Resistors are the most common SMDs in PCB assemblies, and their purpose is to limit current flow in the circuit. They are available in several types, with thin-film and thick-film being the most prevalent.
Thin film resistors are highly precise and stable in varying current conditions, making them suitable for applications that require precision, such as in instrumentation.
How To Identify Resistors
On the other hand, thick-film resistors are affordable and more commonly used in general-purpose electronics that don’t need high precision. Their typical tolerance values range from 1% to 5%.
SMD chip resistors have a rectangular shape with terminals on the sides and the resistance value code printed above in 3-digit or 4-digit formats.
Capacitors
Like inductors, capacitors store electrical energy. However, capacitors store this energy as a charge between electrical plates, so they can store it for longer. Essentially, they act as temporary batteries.
This capability of instantly storing and releasing energy makes capacitors ideal for filtering noise, stabilizing voltage, and storing energy for later use in circuits.
How To Identify Capacitors
SMD capacitors come in different types, including ceramic, tantalum, electrolytic, and film capacitors. Each has specific properties that suit different applications.
Electrolytic capacitors are cylindrical, while the rest adopt a boxy shape, with ceramic capacitors resembling resistor designs, although thicker.
Diodes
Diodes are semiconductors that allow current to flow in one direction, primarily in rectification for AC to DC conversion. But these diodes are known as rectifier diodes and can withstand high reverse currents during operation.
Other types include Schottky diodes with low forward voltage drops and quick switching speeds. This makes them suitable for switching power supplies and RF circuits.
How To Identify Diodes
On the other hand, Zener diodes are ideal for voltage regulation because they have specific voltage limits beyond which they conduct in reverse, while SMD LEDs emit light when current flows through them.
These components have square or rectangular shapes with terminals on at least one side.
Transistors
Transistors are semiconductors, but they have three terminals instead of two (like diodes). They provide amplification or switching functions in circuits, making them suitable for power amplifiers and digital logic circuits.
These devices are available in different types, such as BJT (NPN or PNP), JFET, and MOSFET. BJTs are good at amplification, switching, and voltage regulation, while JFETs are suitable for low-noise, high-input impedance tasks.
How To Identify Transistors
MOSFETs are becoming more popular in modern electronics because they provide high switching speeds and high input impedance while consuming little power.
Visually, SMD transistors resemble diodes because they have black, plastic rectangular packages. However, they have three terminals on the longer sides, with two protruding from one side and one on the other. Some have all three terminals on one side.
Integrated Circuits
Integrated circuits are silicon dies that house multiple electronic components on a single chip, such as transistors, resistors, capacitors, and diodes.
These components in these circuits can be organized to perform various functions, resulting in ICs for tasks like power management, data processing, data storage (memory), signal processing, and analog-to-digital conversion.
SMD ICs are easy to spot because they have multiple contacts (lead or no lead) on two or all four sides of the square or rectangular plastic package
Connectors
Connectors simply join external wiring to the circuit temporarily or permanently. They feature several pins or sockets in a block format that attach to the external connector of the opposite gender.
Contact us for more information about surface mount components.
SMD Component Selection and Application
When selecting SMD components for specific applications, consider these three factors to select the most appropriate part.
- Electrical Characteristics: Check the datasheets or component markings to determine things like the resistance values, capacitance values, and switching speeds, which are critical when limiting current flow, smoothing power flow, filtering noise, detecting/mixing RF signals, etc.
- Compatibility With Other PCB Components: Compatibility covers electrical and mechanical aspects. With electrical compatibility, components in high-power applications must have high-power ratings to operate without getting fried. Mechanically, these components should have similar properties, such as temperature coefficients, to work seamlessly across a broad range of temperature-sensitive applications.
- Physical Dimensions: Each SMD must fit on the available space on the PCB. Although tiny, consider the dimensions of each part using their size codes like 0805 and 0402 to pick those that fit.
Let’s consider power supply, RF, and microcontroller-based circuit applications to see how these three factors affect component selection.
Power Supply Circuits
These circuits need various components, such as resistors, capacitors, diodes, inductors, and transistors, to regulate the current flow and filter the output voltage.
Therefore, you must consider each component’s electrical characteristics, including the input and output voltage, power efficiency, and maximum load current.
Radio Frequency Circuits
High-frequency circuits in the RF range need SMD inductors and capacitors to filter or block certain frequencies. The specific selection depends on the required filtering characteristics, which include attenuation, bandwidth, and frequency.
Schottky diodes are also important in these circuits because they provide high switching speeds.
Microcontroller-Based Circuits
A combination of resistors and capacitors can be used to create timing circuits. Resistors can provide pull-up or pull-down circuits for microcontrollers, while capacitors can also provide decoupling.
The ICs I/O requirements, timing constraints, and voltage requirements will determine the electrical characteristics to look for.
The Mechanics Behind SMCs
When mass-producing PCBs with surface-mount components, the assembly process involves these steps.
- Solder paste printing
- Solder paste inspection
- Component mounting (using pick-and-place machines)
- Visual inspection
- Reflow soldering
- Automated Optical Inspection (AOI)
- Testing
But let’s focus on the manual SMT assembly process for DIY projects and rework/repair tasks.
How To Manually Solder Surface Mount Components
You’ll need these components to get started.
- Solder wire or paste ( paste is recommended)
- Soldering iron (for solder iron use) or hot air soldering rework station (for solder paste use)
- Tweezers
- Flux
- Desoldering wick
- Isopropyl alcohol
- Surface-mount component
- PCB
An SMD held in place using tweezers
Step 1: Prepare the PCB
Secure the circuit board and check if the leads on the surface-mount device match the contacts on the pad. Then, clean the PCB to remove dust and debris.
Step 2: Turn On The Solder Iron or Station
Turn on the soldering iron or station and set it to 650°F to 700°F (343°C to 371°C) if using lead-free solder or 600°F to 650°F (316°C to 343°C) if using lead-based solder.
For Solder Paste
Step 3: Dispense Solder Paste
Dispense solder paste above the pad using a syringe and don’t worry about the paste that falls on the solder mask. This layer repels and pushes solder paste into the pad when heated.
Step 4: Position the SMT Component
Use the pair of tweezers to position the SMD above the solder paste on the copper pad.
Step 5: Heat the Board
Blow hot air using the rework station to activate the flux in the solder paste and melt the solder balls. Hold the PCB flat and steady until the solder cools.
For Solder Wire
Step 3: Solder One Corner/Side
Tin the iron tip, then contact it with the pin and pad on one side for 2 – 3 seconds to warm them. After that, apply some solder, which should melt on contact.
Step 4: Apply Flux
Once the component is secure above the pad, apply enough flux over the pins using an applicator.
Step 5: Solder
Touch the hot iron on the pin and pad of the corresponding contact points, then apply solder to join the remaining parts.
Step 6: Remove Excess Solder
After cooling, remove excess solder using the solder iron and a desoldering wick. Lightly press the iron against the wick on the solder bridges and other unwanted areas to melt and absorb the molten metal.
For Both Solder Wire and Paste
Final Step: Clean the Soldered Area
Clean the work area using isopropyl alcohol to remove excess flux.
SMD Component Sizes and Codes
SMDs have a standardized 4-digit code to denote their size, which can be something like 0402, 0805, or 0603. The first two indicate the length in a hundredths of an inch, while the last two show the width in a similar format.
The 0402 device, for example, measures 0.04 inches in length and 0.02 inches in width.
However, the code that indicates the SMD component’s electrical value is not standard; it depends on the specific component type.
Resistor color codes
What Are the Benefits of Using SMCs
- Space-saving advantages in electronic design to create miniaturized devices
- Enables high assembly densities
- Provides reliable performance and signal processing due to low unwanted RF effects
- Creates EMC-compliant circuit boards
- Suitable for mass production and makes the process cheaper than THT assembly
Challenges and Considerations
- Poor resistance to thermal & environmental stresses
- Unsuitable for small circuit testing
- More susceptible to damage during handling and soldering (SMDs are sensitive to ESD, mechanical stress, high temperature, and high humidity)
- Difficult to inspect after soldering
- Costlier to assemble in low-volume PCB batches
Surface Mount Components Sourcing and PCB Assembly One-Stop Solutions
Sourcing the required components for your project and assembling them can be challenging, especially if you want a large batch of PCBAs. But we can help you out.
At FlexiPCB, we provide a one-stop solution for sourcing and assembling components on your PCBs at reasonable costs.
Founded in 2007, we have over 15 years of experience in PCB assembly. Our ISO-certified assembly plants only produce the highest-quality PCBAs for our customers.
Additionally, we have built networks with SMT component manufacturers and only source the best quality parts for our customers and projects.
Feel free to send us your requirements to get a quote for surface mount components sourcing or assembly today!
Surface Mount Components FAQs
What Is the Difference Between SMT and SMD Components?
SMT refers to the technology used to attach surface-mount components on PCBs, while SMDs are the components themselves. SMD stands for surface-mount device.
Are SMD Components Cheaper?
It depends on the specific component. Some through-hole components are cheaper than their SMT counterparts. However, SMT is preferred because the assembly cost is lower, and the process is easier to automate than THT.
Is SMD Better Than Through-Hole?
SMDs are generally better than through-hole components because they are smaller, enable high-density assemblies, create reliable circuits, allow easier assembly automation, etc. However, they have downsides, the key among them being poor resistance to thermal and environmental stress and weaker mechanical bonds. So, THT is better in some applications.
