Comprehensive Guide on Surface Mount Technology (SMT)

Understanding Surface Mount Technology (SMT)

What is SMT?

Before printed circuit boards can become fully-fledged printed circuit board assemblies (PCBA), they must undergo a mounting process. This is where the manufacturer permanently attaches electronic components to the PCB.

Instead of drilling and plugging the component into the board, Surface Mount Technology (SMT) involves mounting electronic components directly on the surface of the PCB. We refer to these surface-mountable component packages as surface-mount devices (SMD).

SMT has allowed manufacturers to achieve high degrees of automation and precision in the assembly process. This has in turn enabled smaller assemblies with excellent repeatability.

The technology has proliferated throughout the industry, encouraging innovation in PCB design. Thanks to SMT, makers have been able to rapidly design and build small, lightweight, and compact PCBs.

Surface Mount Technology involves a less strenuous assembly process. It primarily employs a simple process of picking and placing PCB components on the board. But when did we start using this technology?

Circuit board assembled via SMT

Brief History of SMT

We can trace the origins of SMT back to the 1960s when it was referred to as “planar mounting”. IBM was the first company to showcase it before it was fully adapted by NASA for their Launch Vehicle Digital Computer.

Surprisingly, it took SMT over a decade before more manufacturers outside of the US government began to adopt it. The change began in 1976. By the mid-80s, at least 10% of the PCBs circulating in the market used SMT.

SMT continued to gain notice for its ability to facilitate greater functionality while reducing footprints. Electronics such as digital cameras, camcorders, and video recorders were made possible through SMT circuits.

The mid-90s brought forth new PCB technology. 3D, high-density, and micro-assemblies that only now exist because of SMT.

As we approach the 2030s, we continue to see a decline in electronics with through-hole technology—dropping 11% per year! Meanwhile, those using surface-mount technology are growing by 8%.

You are likely to encounter terms like “Surface Mount Technology” and “Surface Mount Devices” used almost interchangeably. While these terms are closely related, they’re not exactly the same.

A surface mount technology machine

Key Terms Related to SMT

Some key phrases you may come across when dealing with SMT include:

Surface Mount Equipment (SME): Refers to the assembling machinery that attaches components to the circuit boards.
Surface Mount Assembly (SMA): Refers to the entirety of the surface mount assembly process. This includes gathering the necessary pieces, mounting them, and verifying the quality of the PCB. People also refer to it as SMT assembly.
Surface Mount Device (SMD): Components that use surface mount packages. Just like through-hole components, they can be electromechanical, active, or passive. You may also refer to them as Surface Mount Components or Surface Mount Device Components (SMD).
Surface Mount Packages (SMP): Refers to the packaging around that component that allows surface mounting to occur. People mainly talk about packaging concerning integrated circuit systems. However, it can refer to any SMC.

SMT vs. Through-Hole Technology

As we discussed in the introduction, the most obvious difference between the two technologies is how they facilitate mounting. SMT allows manufacturers to attach components directly onto the surface of the PCB without drilling any holes.

SMCs can either have short leads or be completely lead-less. This feature reduces parasitic capacitance and inductance, which makes SMT ideal for high-speed PCBs.

PTH/THT components have leads that the manufacturer must insert into the PCB through a set of drilled holes.

After successful insertion, the manufacturer solders the component into the PCB from the other side. Because of this, we can only mount PTH/THT components on one side of the PCB.

On the other hand, SMT components can be mounted on both sides. Moreover, SMCs tend to be smaller than THT components.

This makes SMCs ideal for PCBs that require a higher number of components.

Manufacturers seldom use THT for large productions. This is because we can’t reliably automate THT. Thus, manufacturers typically relegate it to prototyping and smaller productions where manual assembly is feasible.

We can run large automatic SMT assemblies. SMT has made it easier for us to design and create multi-layer PCBs.

However, THT components are more stable and exhibit stronger mechanical bonds than SMT.

Thus, we still use THT-mounted PCBs in heavy-stress situations like aerospace and the military.

What Are the Types of Surface Mount Devices?

PCB with active/passive components

SMD components can either be active or passive. We can identify each type by:

Nature of energy source
Functionality
Power gain
Flow of current
The requirement for an external source

Active Components

These are SMDs that:

Deliver power or energy to the circuit.
Produce energy form of voltage current.
Capable of providing power gain.
Can control the flow of current.
Require an external source for the operation.

Some common examples of active SMD components include transistors, diodes, and integrated circuits.

Transistors

As with THT transistors, SMD transistors have several purposes, including switching, signal modulation, oscillation, voltage regulation, and amplification.

However, the most glaring difference between SMD and THT transistors is how they sit on the PCB. Whereas most THT transistors sit upright, SMD transistors typically lie flat.

Comparing THT to SMD transistors

Small-outline transistors (SOT/SC) are the most common types of SMD transistors – with the SOT-23 variations being the most ubiquitous. Other package types include:

Dual Flat No-Lead (DFN)
Quad Flat No-Lead (QFN)
Transistor Outline (TO)

Diodes

Diodes are dual-lead components that force a circuit’s current to flow unidirectionally (rectification). This ultimately enables the circuit to convert alternating current (AC) into direct current (DC).

Most SMD diodes come in rectangular packages with their leads extending from the widths. The three main SMD diode package types are:

Small OutlineDiode (SOD)
- SOD123
- SOD323
Surface Mount Assembly (SMA)
- SMA
- SMB
- SMC

SMD diodes are also available in flat no-lead and power packages (TO-220, TO-252, and TO-263).

Integrated Circuits (ICs)

Integrated circuits (ICs) are essentially miniature embedded packaged circuits. They’re available in the following SMD package types:

Small Outline Integrated Circuit (SOIC)
Small Outline Package (SOP)
- Thin Small Outline Package
- Think Shrink Small Outline Package
Quad Flat Package (QFP)
Ball Grid Array (BGA)
Plastic Leaded Chip Carrier (PLCC)

You can also find SOT, DFN, and QFN IC packages.

3d Rendering Sot-236 Ic

Passive Components

Passive SMD components are devices that:

Utilize power or energy from the circuit
Store energy in the form of power or circuit
Are incapable of providing power gain
Cannot alter or control the flow of the current
Do not require any external source to operate

Some examples include capacitors, resistors, and inductors.

Resistors

SMD resistors fall under two main categories: chip (R) and network (RA/RN). Chip resistors are the most common among the two types.

Each chip resistor package contains a printed code. The first two (sometimes three) digits indicate the resistance value. The last digit is a multiplier with a power of ten. For instance, “105” is equivalent to “1 M Ω,” while “672” would be equivalent to “6.7K Ω”.

Network resistor chips feature high-grade ceramics with metal electrodes on either end of the chip. The chips consist of a group of resistors that have similar properties. They use similar identification codes to chip resistors.

Microscopic Resistors

Capacitors

There are two main SMD capacitor types: ceramic and tantalum. Ceramic capacitors consist of interlocked ceramic dielectric blocks featuring metal electrodes. A layer of plated tin covers the inner electrodes and is linked to the end terminations (NiSn).

Tantalum SMD capacitors use tantalum as their anode material. The main advantage of these capacitors is that they offer high capacitance values in a compact package.

Box Capacitor Smd Form Factor

Inductors

There are three types of SMD inductors:

Multilayer chip inductors
Wire-wound inductors
Ferrite Beads

They come in a wide assortment of shapes and sizes. As with most components, you can find their values in their encasements. These values are usually represented in the form of standard two-digit exponents.

For example, 100 µH would be 101 – 10 x 10^1, and 100 would be 10 x 10^0.

Fully Shielded Power Inductor

Other Common SMD Components

Other notable SMD components include:

Light Emitting Diodes (LEDs): Contain a semiconductor crystal that generates light when current travels through it. The individual product manufacturing guideline determines the polarity of the SMD LED. SMD LEDs are available in different sizes. Like most SMD components, you can identify them by their size and package text.
Transformers: Consists of a wire-wrapped toroidal core. SMD transformer designs include surface-mount headers for PCB connectors. In addition to their structures, SMD transformers differ from THT transformers in their voltage and current outputs, power ratings, and bandwidths.
Crystal Oscillators: Facilitates timekeeping (mainly) for digital circuits. You can identify them by their package labels. SMD crystal oscillators are marked with “XTAL” followed by their frequency.
Fuses: Protects circuits from overcurrents. SMD fuse packages feature codes denoting the voltage. For instance, “F02G1R00A” signifies:
- F- fuse
- 02 – Style
- G – voltage rating
- 1R00 – current rating
- A – time delay rating

Electronic Components

What’s the SMT Assembly Process?

SMT assembly is a complex multi-stage process. It requires ample preparation, equipment, and expertise to execute successfully.

Preparation Stage: Creating The SMT Stencil

The mechanical strength and bond between the component and the PCB largely hinge on how well the manufacturer applies the solder.

The manufacturer uses a stencil to ensure the solder application process goes smoothly. (Framed) SMT stencils are metal frames consisting of nickel, stainless steel, copper, or nickel.

Methods of Creating an SMT Stencil

There are three main ways to create an SMT stencil:

Laser Cutting: Forms the stencil using a computer numerical control (CNC) machine that directs a high-precision laser to cut the necessary shapes into a metal foil. Because of this method’s accuracy and speed, it’s ideal for fine-pitch components that will be used in high-density and complex designs.
Chemical Etching: Similar to how manufacturers use etching to form the traces on a PCB, we can use the same method to create SMT stencils. An operator applies a photoresist with a stencil aperture design to a metal foil. Next, the operator exposes the metal foil to UV light which leaves a developed photomask on the metal foil. Finally, the operator uses chemicals to etch away the exposed areas of the photomask, leaving the SMT stencil behind. Chemical etching is low-cost and ideal for small to medium-sized productions. However, it doesn’t offer a lot of accuracy. Hence, it may be better suited to larger apertures.
Electroforming: First, the manufacturer applies an aperture design to a mandrel. Using the mandrel as a model, the manufacturer forms the stencil atom by atom through the process of electrophoretic deposition. The manufacturer then removes the mandrel to expose the stencil. While complex, this method produces highly precise stencils with smooth walls and uniform thickness. Much like laser cutting, it’s best suited for fine-pitch components and productions that require high accuracy.

In instances that do not require high-accuracy apertures for fine-pitch components, manufacturers may use CNC-guided drills or punches in the place of lasers.

Automatic Cnc Laser Cutting Machine

Ultimately, the method you choose will depend on your budget and the accuracy requirements and size of the production.

Solder Application

Next, a solder paste printer applies solder onto the PCB pads with the aid of the stencil. This is where the components will go.

Next, the manufacturer performs a solder paste inspection (SPI) using a machine equipped with cameras and sensors.

The inspector must ensure that the applied solder paste has the correct shape, density, and alignment. This step is important as it will help ensure that the solder joints are strong when the final soldering is complete.

Component Placement

Placement machines pick and place PCB components into their required paces.

These machines efficiently pick the entire surface mount components and put them in their required place on the board.

Although these machines tend to be expensive, they ultimately deliver the perfect results.

Alternatively, human operators can manually place the components on the board.

This is best suited to smaller productions as it’s more time-consuming and has a higher risk of errors than automated placement.

Most SMD pick-and-place machines use a conveyor system. Feeders supply the components to a machine, which then uses nozzles to pick them up.

Before placing the components on the boards on the conveyor belt, the machine must align them carefully.

It achieves this using a set of cameras and optical sensors. Once alignment is complete, the pick-and-place machine sets the component onto the PCB and then unloads the PCB.

Automatic Pick Place Machine

Soldering

Once the placement process is complete, the manufacturer can then solder the SMD components to the board. There are two main ways to do this:

Reflow Soldering: The manufacturer places and heats the PCB in an oven hot enough to melt the solder paste. This process forms solder joints between the components and the PCB.
Wave Soldering: This involves passing a circuit board over a pan of molten solder. The main advantage of this technique is it enables manufacturers to process, and solder boards subsequently. It’s time-saving and ideal for large productions.

Inspection and Testing

Once the soldering process is complete, the manufacturer must inspect it to ensure the joints are strong. They use an Automated Optical Inspection (AOI) to do this.

The AOI machines use sensors, cameras, and image-processing software to find any defects in the PCB assembly.

Once the AOI machine confirms there are no defects, an operator moves the PCB to the cleaning stage, where the manufacturer removes any residual flux and impurities from the PCB.

They utilize a set of cleaning systems that use a combination of water, solvents, and other chemicals. Cleaning prevents corrosion and ensures the reliability of the PCB assembly.

Next, the manufacturer runs functional tests on the PCB assembly using a set of test fixtures and software. This ensures that the PCB assembly meets all functional specifications and operates correctly.

Automated Technology Industrial Robotic

What Are the Advantages and Disadvantages of SMT?

SMT has several advantages that you may already be likely aware of.

Advantages

If you are considering using the technology in your PCB assembly, here are a few good reasons to do so:

Reduced Manufacturing Costs

SMT reduces manufacturing costs by facilitating the design and creation of smaller PCBs. Because they have smaller footprints and lower profiles, Designers can place SMT components close to each other on the board.

Improved Work Efficiency

As we mentioned before, most of the SMT process is automatic. Hence, it eliminates mistakes and improves overall work efficiency.

SMT allows manufacturers to complete tasks that may have taken a week in a single workday. This increase in productivity shortens the time to market for electronics.

This improved work efficiency reduces waste and can potentially result in increased profits for manufacturers and electronic makers.

Compact and Lightweight Designs

In contrast to PTH-assembled panels, the overall structure of the boards you manufacture and design for SMT tends to be more straightforward. Surface-mount technology assembled boards don’t demand too many technicalities, such as drilling.

Disadvantages

These disadvantages include:

Mechanical Stress

Mechanical stress, particularly for prolonged periods, can weaken solder between SMT components and PCBs.This makes them less durable.

Because SMT components are small and delicate, they’re highly susceptible to damage, especially during the handling and assembly phase.

As such, manufacturers must perform ample functional tests after they complete assembly.

This, along with effective inspection, can be a challenge in itself because of the size of the components.

In addition to the above, solder issues such as solder bridges, “tombstoning”, and subpar solder joints are more likely to occur with SMT.

SMT soldering requires a great deal of care and precision.

Environmental Concerns

Soldering SMT assemblies requires tremendous amounts of energy for heat, regardless of which method you use (reflow or wave).

This can be detrimental to both the assembler and the environment.

Temperature Stress

SMT solder joints tend to be more prone to damage caused by temperature/thermal cycling. PCBs are subjected to varying degrees of heat and cold during their lifespans.

These temperature cycles can weaken the solder between the SMT packages and the PCB.

Incompatibility with Some Components

Designing for SMT PCB assemblies can be complex as it requires you to use components that thermally and mechanically complement each other.

For instance, some components generate more heat and electric noise than others. Because of the proximity between SMT components, these components may cause interference.

As such, you’ll need to pair said components with components that are resistant to the generated heat and noise.

This limits the types of components you can use for your design.

Future Trends in SMT

Additive manufacturing (3D printing) is an emerging technology in PCB manufacturing. So far, the industry has applied it to prototyping PCBs.

However, it has the potential to help us create SMT stencils more sustainably and efficiently.

One of the biggest disadvantages of SMT is its negative impact on the environment, largely due to the extreme amounts of heat the soldering process generates.

Modern reflow ovens allow us to regulate and control temperature more precisely. This also decreases the risk of sensitive components incurring thermal damage.

The development of new solder alloys and fluxes has made lead-free soldering more accessible to manufacturers.

These materials yield better performances and are more environmentally friendly.

The increased proliferation of AI has provided designers with the ability to craft more efficient designs.

It also allowed manufacturers to optimize the SMT assembly process using Industrial IoT devices.

Supervisors can survey multiple assembly processes remotely. This improves productivity, efficiency, and accuracy.

Process Creating Printed Circuit Board Computer

Conclusion

Despite all these aforementioned technological advancements, SMT has not made THT/PHT redundant. This is because the industry has yet to eradicate some of SMT’s weaknesses.

However, because SMT has so many advantages and overwhelming support, the industry will continue to develop new approaches and techniques to improve it.

For instance, SMT is more cost-effective (in the long run), requires less human supervision, and has a variety of different component types and packages.

It’s a versatile technology worth exploring and learning about. Using SMT for your large-scale projects is a good idea.

But make sure you stay up to date with the latest developments and news to stay ahead of the curve.