SMT Manufacturing: Everything You Need to Know - Wevolver

Introduction

Surface mount technology (SMT) is an aspect of electronic assembly where electronic components, also called surface mount devices (SMD), are directly mounted onto the surface of a printed circuit board (PCB). Because of its cost and quality efficiency, SMT has become highly sought after in the industry.

What is Surface Mount Technology (SMT)?

Surface mount technology (SMT) is an assembly and production method that applies electronic components directly onto the surface of a printed circuit board (PCB). This process allows for automated production to complete more of the required assembly to create a working board. Any electrical component mounted in this manner is referred to as a surface-mount device (SMD). Unlike conventional assembly, SMT does not require components to be inserted through holes, rather components are soldered onto the board directly through reflow soldering.

Initially called Planar Mounting, SMT was first developed and applied by IBM to build small-scale computers in the 1960s, thereby becoming a replacement for its predecessor, Through-Hole Technology. However, it did not take off until 1986 when surface-mounted components managed to reach 10% market popularity. By 1990, surface-mounted devices or SMDs could be found in the majority of all high-tech printed circuit assemblies (PCAs).[1]

SMT components were designed to have small tabs where solder could be applied to attach SMDs to the surface of the PCB. In the era of Through-Hole Technology, components were mounted through lead holes that were drilled into PCBs. The holes were sized to fit each component to hold each piece tightly, then the grip was soldered. With SMT, the hole drilling step is bypassed as SMDs are quickly sorted and attached to the top of the PCB with little to no hole leads; thereby significantly cutting the process of device assembly.

The SMT assembly process can be tedious and time-consuming if done manually, due to the precision it requires to create high-quality surface mount assembly (SMA). So for better efficiency, most SMT manufacturing is done through automated assembly machines, especially when production is large scale.[2]

SMT components are significantly smaller than through-hole components, which makes for the production of sleek and attractive electronic devices suitable for modern times. Therefore these days, SMT is used in almost every electronic device ranging from toys to kitchen appliances, to laptops and smartphones.[3]

SMT Manufacturing Process

The SMT manufacturing process is broadly grouped into 3 stages, namely: solder paste printing, components placement, and reflow soldering. However, due to the demands of the SMT production process, these stages are further analyzed in the following outline:

1. SMC and PCB Preparation

This is the preliminary stage where the SMCs are selected and PCB designed. The board usually contains flat, commonly silver, tin-lead, or gold plated copper pads devoid of holes, called solder pads. Solder pads support the pins of the components like transistors and chips.

Another vital tool is the stencil, which is used to provide a fixed position for the next phase of the process (solder paste printing), according to the predetermined positions of solder pads on the PCB. These materials, along with others that are to be used in the manufacturing process must be properly examined for flaws. 

2. Solder Paste Printing

This is a critical phase in the SMT process. During this phase, a printer applies solder paste using the prepared stencil and squeegee (a tool for cleaning in printing) at an angle ranging from 45° to 60°. Solder paste is a putty-like mixture of powdered metal solder and sticky flux. The flux serves as a temporary glue to hold the surface mount components in place as well as cleansing the soldering surfaces of impurities and oxidation.

The solder paste, on the other hand, is used to connect the SMC and solder pads on the PCB. It is pertinent that each pad is coated in the correct quantity of paste. Otherwise, there will be no connection established when the solder is melted in the reflow oven. In the electronics manufacturing industry, a reflow oven is an electronic heating device used in surface mount technology (SMT) to place electronic components on printed circuit boards (PCBs).

3. Components Placement

Next, pick-and-place machines are used to mount components on the PCB. Each component is removed from its packaging using a vacuum or a gripper nozzle, and the placement machine places it in its designed location. The PCB is carried on a conveyor belt while the electronic components are placed on it by the quick and accurate machines, some of which can place 80,000 individual components per hour.

Accuracy is required in this process because any erroneous placement soldered into a position can be costly and time-consuming to rework.

4. Reflow Soldering

After SMCs are placed, the PCB is then conveyed into the reflow soldering oven, where it passes through the following zones to undergo the soldering process:

Preheat zone: this is the first zone in the oven, where the temperature of the board and all the attached components is raised simultaneously and gradually. Temperature is cranked up at the rate of 1.0℃-2.0℃ per second until it enters 140℃-160℃.

Soak zone: here, the board will be kept at a temperature between 140℃ and 160℃ for 60-90 seconds.

Reflow zone: the boards then enter a zone where the temperature is ramped up at 1.0℃-2.0℃ per second to the maximum of 210℃-230℃ to melt the tin in the solder paste, welding the component leads to the pads on the PCB. While this is going on, the components are kept in place by the surface tension of the molten solder.

Cooling zone: this is the final section that ensures solder freezes upon exiting the heating zone to avoid joint defects.

If the printed circuit board is double-sided then these processes may be repeated using either solder paste or glue to hold the SMCs in place.

6. Cleaning and Inspection

After soldering, the board is cleaned and checked for flaws. If any is found, the defects are repaired and then the product is stored. Common ways used for SMT inspection include the use of magnifying lenses, AOI (Automated Optical Inspection), flying probe tester, X-ray inspection, etc. Instead of the naked eye, machines are used for quick and accurate results.

                                                   Soldering of PCB with electronic components

SMT: Pros and Cons

SMT has proven beneficial for PCB assembly (PCBA), PCB manufacturing, and electronics production in many ways including these:

• Allows for smaller components

• SMT process encourages increased automation

• Maximum flexibility in building PCBs

• Improved reliability and performance

• Reduced manual intervention for component placement

• Smaller, lighter boards

• Ease of PCB assembly, using both sides of the board without the hole limitations that exist in the conventional method

• Can co-exist with through-hole components, even on the same board

• Increased density i.e  more SMD components in the same space, or the same number of components in a much smaller frame

• Low Cost of materials

• Simplifies the production process and reduces the production cost. [4]

Conversely, disadvantages of SMT to electronic manufacturing include:

• Small volume

• Easily broken due to fragility

• High requirements for soldering technology

• Components can be easily dropped or damaged when installed.

• It is not easy to use visual inspection, which is difficult to test.

• Miniaturization and numerous solder joint types complicate the process and inspection.

• Large investment in equipment such as the SMT machine 

• Technical complexity requires high training and learning costs.

• Rapid development requires continuous follow-up. [5]

SMT vs SMD

SMT and SMD are frequently misunderstood and used simultaneously. Indeed, any technology and its actual components can be deeply entwined, creating confusion. Such is the case of SMT and SMD. This is why knowing the difference between an SMT assembly and individual SMD components is important.

In a simpler vein, SMT is the process in the technology, while SMD is the device involved in the technology. SMT is the technology that uses the method of directly placing and soldering electronic components on a PCB. These components are also sometimes called surface mount devices or SMDs. They are designed to be mounted on a printed circuit board (PCB).

SMDs make for devices produced faster, with more flexibility and less cost, without sacrificing functionality. They promise more functionality because smaller components allow for more circuits on small board space. This miniaturization is the major feature of SMD.[6]

Both SMT and SMD work together to provide users with faster, more energy-efficient, and more dependable PCBs.

Closing Thoughts

Smaller size, quicker production, and reduced weight are the major allures of SMT, leading to much easier electronic circuitry design and production, especially crucial in complex circuits. This higher level of automation has saved time and resources throughout the electronics manufacturing industry. As such, even though there is always a chance of developing new technology, SMT has undoubtedly secured its relevance.

References

1. SCRIBD. Surface-Mount Technology: History. 2019.[cited 2022 Jun 8]. Available from: https://www.scribd.com/document/106437192/SMD

2. Das S. SMT Machine and SMT Machine Manufacturers. 2021 [cited 2022 Jun 8]. Available from: http://www.electronicsandyou.com/smt-machine-and-smt-machine-manufacturers.html

3. Geospace Technology. SMT ASSEMBLY TECHNOLOGY: ALL YOU NEED TO KNOW. 2021.[cited 2022 Jun 9]. Available from: https://geospacemfg.com/blog/smt-assembly-technology/

4. Blog Industry News. Top Benefits of Surface-mount Technology (SMT) PCB Assembly. 2020. [cited 2022 Jun 9]. Available from: https://www.pcbnet.com/blog/benefits-surface-mount-technology-smt-pcb-assembly/

5. PCBWay. Advantages and Disadvantages of Surface Mounting Technology. 2019.[cited 2022 Jun 9]. Available from: https://www.pcbway.com/blog/PCB_Assembly/Advantages_and_Disadvantages_of_Surface_Mounting_Technology.html

6. History Computer (HC). Surface-Mount Technology. 2022. [cited 2022 Jun 9]. Available from: https://history-computer.com/smt-surface-mount-technology/

Contents

Surface mount technology is a part of the electronic assembly that deals with the mounting of electronic components to the surface of a PCB. Electronic components mounted this way are called surface-mounted devices (SMD).

SMT was developed to minimize manufacturing costs while making efficient use of board space. The introduction of surface mount technology has enabled manufacturers to fabricate smaller size complex circuit boards. There are various advantages and disadvantages of surface mount technology which we will discuss over the course of this article.

The advent of surface mount technology

Surface-mount technology was developed in the 1960s and was broadly used in the 1980s. By the 1990s, they were used in most of the high-end PCB assemblies. Conventional electronic components were redesigned to include metal tabs or end caps that could be attached directly to the board surface. This replaced typical wire leads which needed to pass through drilled holes. SMT led to much smaller components and enabled component placement on both sides of the board. Surface mounting enables a higher degree of automation minimizing labor costs and expanding production rates that results in the development advanced of boards.

Salient features of SMT and through-hole technology

SMT allows electrical components to be mounted on the board surface without any drilling. Most electronic applications prefer to use surface mount components since they are compact and may be installed on either side of a printed circuit. They are suitable for applications with higher routing densities. These components have smaller leads or no leads at all and are smaller than through-hole components.

The process involved in SMT assembly is:

• Apply solder paste to the fabricated circuit board using stencils. Solder paste is made up of flux and tin particles.
• Attach the surface mount components.
• Use a reflow method for soldering.

In through-hole technology, the component leads are inserted into the drilled holes on the board. These leads are then soldered to pads on the opposite side using wave soldering or re-flow soldering tools. Since through-hole mounting offers strong mechanical bonds, it is highly reliable. However, drilling PCBs during production tends to increase manufacturing costs. Also, through-hole technology limits the routing area for signal traces below the top layer of multi-layer PCBs.

Major differences between through-hole technology and surface mount technology

• SMT frees up the limitation on board space posed by the through-hole mounting manufacturing process.
• Through-hole components involve higher manufacturing costs than SMT components.
• You require advanced design and production skills for using SMT when compared to through-hole technology.
• SMT components can have a higher pin count as compared to through-hole components.
• Unlike through-hole technology, SMT enables assembly automation which is suitable for high production volumes at lower costs when compared to through-hole production.

• SMT components are more compact leading to higher component density as compared to through-hole mounting.
• While surface mount leads to lower production costs, capital investment for machinery is higher than needed for through-hole technology.
• Through-hole mounting is better suited to the production of large and bulky components that are subjected to periodic mechanical stresses or even high-voltage and high-power parts.
• SMT makes it easier to achieve higher circuit speeds because of its reduced size and fewer holes.

Factors to consider before choosing SMT or through-hole technology

• Stability of the component when exposed to external stress
• Ease of thermal management/ heat dissipation
• Availability of the part and its alternative
• Cost-effectiveness of assembly
• High performance and life-span of the package
• Facilitate rework in case of board failure

Advantages of surface mount technology

SMT has many advantages over conventional through-hole technology:

• Surface mount technology supports microelectronics by allowing more components to be placed closer together on the board. This leads to designs that are more lightweight and compact.
• The process for SMT production setup is faster when compared to through-hole technology. This is because components are mounted using solder paste instead of drilled holes. It saves time and labor-intensive work.

• Components can be placed on both sides of the circuit board along with a higher component density with more connections possible per component.
• Due to the compact size of the package, higher-density traces can be accommodated on the same layer.
• The surface tension of molten solder pulls components into alignment with solder pads, which automatically corrects minor placement problems.
• Compared to through holes, these do not expand in size during the operation. Hence you can reduce the inter-packaging space.
• Electromagnetic compatibility is easily achievable in SMT boards because of their compact package and lower lead inductance.
• SMT enables lower resistance and inductance at the connection. It mitigates the undesired effects of RF signals and provides better high-frequency performance.
• More parts can fit on the board easily due to their compactness, resulting in shorter signal paths. This enhances signal integrity.
• The heat dissipated is also lesser than through-hole components.
• SMT reduces board and material handling costs.
• Enables you to have a controlled manufacturing process. This especially opted for high-volume PCB production.

Ensure the manufacturability of your circuit design using the Better DFM tool.

Disadvantages of surface mount technology

Even though SMT has several advantages, the technology also comes with it certain disadvantages:

• When you subject components to mechanical stress, it is not reliable to use surface mounting as the sole method of attachment to the PCB. This is because you need to use component connectors to interface with external devices that are periodically removed and re-attached.
• Solder connections for SMDs might be damaged through thermal cycles during operations.
• You would need highly skilled or expert-level operators and expensive tools for component-level repair and manual prototype assembly. This is because of the smaller sizes and lead spaces.
• Most SMT component packages can’t be installed in sockets that enable easy installation and replacement of failed components.
• You use less solder for solder joints in SMT, therefore the reliability of solder joints becomes a concern. Void formation might lead to solder joint failures here.
• SMDs are typically smaller than through-hole components leaving lesser surface area for marking part IDs and component values. This makes identifying components a challenge during prototyping and repairing the PCB.
• The solder can melt when exposed to intense heat. Therefore, SMT cannot be implemented in electrical load circuits with high heat dissipation.
• PCBs that use this technology requires more installation costs. This is because most of the SMT equipment such as the hot air rework station, pick and place machine, solder paste screen printer, and reflow oven are expensive.

• Miniaturization and a variety of solder joints can make the procedure and inspection more difficult.
• Due to compact size, there is an increased chance of solder overflow that can result in short circuits and solder bridge.

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When to use surface mount technology?

The majority of products manufactured at this time utilize surface mount technology. But SMT is not suitable in all cases. Consider SMT considered if:

• You need to accommodate a high density of components.
• The need is for a compact or small product.
• Your final product needs to be sleek and light despite component density.
• The requirement specifies the high-speed/frequency functioning of the device.
• You need to produce large quantities with automated technology.
• Your product should produce very little noise (if any at all).

Guidelines for SMT component placement

Here are some recommendations for SMD placement to maintain good signal and power integrity for your board.

• Keep the components as near as possible to minimize the routing distance.
• Adhere to the signal path as per the schematic while placing the components.
• Never place the components in the return path of sensitive signals. This leads to signal integrity issues.
• For high-speed devices, place the bypass capacitors closer to their power pins. This will reduce parasitic inductance.
• Arrange the SMD together for power supply circuits. This will help you to provide shorter routing and reduce the inductance in the connections.
• Try to keep SMT components on one side of the board to reduce costs associated with stencils and assembly.
• Maintain the minimal spacing between the test points and SMT components as specified by your manufacturer. This spacing may vary depending on the component’s height.

To facilitate the assembly process ensure that all component names, polarities, orientations, and placements are marked properly in the assembly drawing. The footprints present in the drawings should match with the actual parts. Consult your manufacturer for their kitting guideline if you are considering consigned assembly. Prepare your BOM accordingly.

Soldering techniques employed in SMT

Solder reflow and wave soldering are widely used to mount components onto the board. Depending on the nature of the components, the designer can choose one of these methods for surface mounting technology.

Wave soldering: Since the solder will flow through the holes to form a connection, wave soldering is mostly used for through-hole components. You can use wave soldering for most of the surface-mount components also.

Solder reflow: This process is generally preferred in SMT. Here, the solder on one pin melts and reflows faster than the other. The only disadvantage is that it causes a tombstoning effect, where the component peels away from the non-melted pad. This effect is common for surface mount components like resistors, capacitors, and inductors.

Surface mount device packages

SMD packages come in a broad range of shapes and sizes as given below:

Common passive discrete components: These components are mostly resistors and capacitors and are a part of most electronic devices available today. Given below are SMD package details for capacitors and resistors.

Transistors: The common type of packages for transistors are as follows:

• SOT-23 (Small Outline Transistor) with dimensions 3 x 1.75 x 1.3mm
• SOT-223 (Small Outline Transistor) with dimensions 6.7 x 3.7 x 1.8Mmm

Integrated Circuit (IC) packages

Integrated Circuit packages come in a wide range as given below:

• Small Outline Integrated Circuit (SOIC)

Small Outline Package (SOP)

TSOP (Thin Small Outline Package)  is thinner than SOIC

• Quad Flat Pack (QFP)

Quad flat packs are generic square, flat IC packages.

• Ball Grid Array (BGA)

BGA packages include an arrangement of solder balls on the chip underside in the place of pins. The ball spacing typically is 1.27, 0.8, 0.5, 0.4, and 0.35mm

• Plastic Leaded Chip Carrier

The chip is enclosed in a plastic mold. It can either be square or rectangular in shape.

Measurement of SMD size

Surface mount component standards are specified by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association (JEDEC.org). JEDEC is an independent semiconductor engineering trade organization and standardization body that has its headquarters in Arlington, Virginia, United States.

You can measure SMD size in inches in the Imperial system and millimeters in the Metric system. For the 0201 imperial components, the dimensions are 0.02 x 0.01 inches. For the 0201 metric components 0.2 x 0.1 mm.

Understanding the advantages and disadvantages of surface mount technology is essential to comprehend its role in the electronics industry. This will always help in optimizing your design and assembly skills. Follow the guidelines listed in this article to completely benefit from the SMT process for component placement. Please comment below if you have any queries on employing surface mount technology for your design. We will be happy to help you.

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