Feb 12, 2020|General

In recent years, SMT PCBs have become one of the most popular and effective ways of producing electronic circuits. In this guide, you will learn more about what they are, why companies use them, and what you should be careful of if you plan to use them.

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What are SMT PCBs

Surface-mounted technology for printed circuit boards, or SMT PCBs, is a process where components are placed directly on top of the boards instead of threading the parts through the board. This helps to reduce manufacturing costs and increase the speed that printed circuit board manufacturers can make boards at.

Common Abbreviations

The circuit board manufacturing industry uses a lot of acronyms for components. Here are the most common abbreviations in the surface mount technology industry and what you should know about them.

SMA

SMA stands for surface-mount assembly. This refers to the overall process of creating a circuit board with components mounted on top of it, including everything from getting all of the parts to performing quality control checks after assembling them.

SMC

SMC stands for surface-mount components. These are the parts and pieces placed on a printed circuit board during the assembly process, as well as solder and other materials that help bind components to the board. A PCB manufacturer may have specialized equipment to deal with SMCs of various types and sizes.

SMD

SMD is the shorthand for surface-mount devices. These include various active, passive, and electromechanical components. The primary difference between SMD and SMC is that SMD only consists of the actual devices, not the extra solder or other materials used. People usually use this term when they want to be specific about the components they’re referring to.

SME

SME is the abbreviation for surface-mount equipment. These are assembling machines that place equipment on prepared circuit boards, making them an essential part of the process. SMT is most cost-effective when automated, so companies usually design surface-mount equipment to handle as much of the assembly process as possible.

SMP

SMP stands for surface-mount packages. These are case forms that help to hold chips and other components. Most case forms have a four-digit identification number, with the first two numbers detailing the length and the second two numbers specifying the width, both measured in 1/100 of an inch (or 2.54 mm).

SMT

SMT is the industry shorthand for surface-mount technology. This includes all assembling and mounting techniques used for surface-mounting things on printed circuit boards. This is related to SME, but SMT represents the processes and techniques while SME represents the machines that a PCB manufacturing company uses.

Similarly, what is the difference between SMT and SMD? The devices placed on boards are separate from both the processes and the assembly machines.

SMT devices

There are several types of SMT devices currently available on the market. Here are the most common types of devices and what you should know about them.

Passive SMDs

Passive SMDs mainly include resistors and capacitors. Rarely, they may also include coils, crystals, or other components that have individual requirements. Passive SMDs come in various predetermined package sizes that help machines understand where to place them.

Transistors and Diodes

Transistors and diodes are circuit board components that help direct and control electrical flow. Since they usually only work one way, transistors and diodes come with three connections, instead of two, which help ensure they only fit onto boards in one way.

Integrated Circuits

Integrated circuits are collections of electronic chips placed together in a single chip, usually (but not always) made of silicon. These are the heart of most printed circuit boards because they are fundamentally smaller, faster, and more affordable than circuits created in other ways.

Most manufacturers add the integrated circuits during the final step of their manufacturing process.

SMT Advantages

The main advantages of using SMT PCB assembly when manufacturing printed circuit boards include:

Low Size: Size is one of the most significant issues for many products, and surface-mounting has the ability to minimize the depth required for manufacturing most components. This is particularly important for many modern devices like smartphones, tablets, and smartwatches, which require the smallest possible SMT PCB board.

Low Weight: SMT systems can weigh as little as a tenth of what through-hole components required. This places less stress on the circuit board itself and allows companies to cut costs by using less material, which is a significant part of why they use the PCB SMT process at all

Added Flexibility: By placing some components directly on the surface, companies can have more space for parts that can’t be put on the surface. This provides added flexibility in manufacturing and allows for creating electronic products that would otherwise be impossible to manufacture at the desired specifications.

SMT Disadvantages

While surface-mounted technology is useful, there are a few drawbacks you should know about.

First, small lead spaces make repairs harder. Surface-mounted parts are often placed extremely close to each other, possibly even closer than many repair tools can work with. This makes repairing damaged boards significantly harder, and that’s a serious concern if your parts have a high failure rate.

Second, it is harder to ensure that solder connections will withstand thermal cycling. Choosing a assembler that has experience with smaller parts is important to avoid failures. Using smaller, thinner components fundamentally means the parts are more likely to fail, so SMT is not a good process if you need to minimize failure rates and dont have the technology to mitigate risks.

Finally, SMT is not usually suitable for components that generate a lot of heat or have a high electrical load. This is mainly because the solder melts under too much heat. Design processes are fundamentally bad if they cannot actually do what you need them to.

Candor uses an innovative press fit copper coin technology to help with heat dissipation on high powered SMD components. Most manufacturers use a combination of through-hole and surface-mounting processes to create circuit boards. Determining what you can and cannot use SMT for is the job of circuit designers and other professionals.

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Assembly Techniques

Assembly techniques vary, but for safety and stability, most companies manufacture printed circuit boards that are at least 18″ by 24″, then cut along predetermined lines to separate the finished board into useful components. This is much more effective for SMT assembly than trying to print each board separately.

The surface-mounting process itself starts with using solder pads (small dots of various metals) that the assembly machine adds solder paste to. After adding the paste, the machines pick up and place components onto the boards.

Once everything is in place, the boards go through a special heating process that’s just hot enough to melt all the solder particles in the solder paste without damaging the other components. With the correct design, the melting paste also pulls each component into position. After this, the board is cleaned and cooled to remove all extra particles and paste, then examined for defects before being shipped out.

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