What are the drawbacks of SMT?

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Introduction

Printed circuit board assembly, in particular, has significantly benefited from surface mount technology (SMT). SMT eliminates the requirement for drilled holes and manual insertion by mounting electronic components directly onto a PCB's surface. This technique offers several advantages, such as smaller and thinner PCBs, flexibility in design, reduced material costs, simplified automation, and high signal transmission capabilities.

Advantages of SMT in PCB Assembly

Ultra-Thin and Miniature PCBs

The size and weight of surface-mount components are significantly lower than their through-hole counterparts. Boards are smaller and thinner because their compactness makes better use of space on printed circuit boards. As a result, products can be designed to be more compact without compromising functionality or performance.

Flexibility

SMT provides great flexibility in terms of PCB materials and designs. It enables the use of flexible PCBs (flex PCBs) and rigid-flex PCBs that can bend or conform to specific shapes or spaces. This flexibility opens up opportunities for innovative designs while maintaining excellent electrical performance.

Lesser Expense on Materials and Cardboard

Unlike through-hole components that require manual insertion, SMD components can be automatically placed on the board using pick-and-place machines or automated assembly lines. This automation streamlines the production process, reduces labour costs, and improves efficiency.

Simplified Manufacturing Process

When compared to through-hole components, surface-mount devices (SMDs) can be automatically positioned on the board through the use of pick-and-place machines or automated assembly lines. Because of this automation, production is more streamlined, costs less to operate, and more efficient overall.

Frequency and High Signal Transmission

Fast signal transmission with less radio frequency interference is made possible using SMT components, which are compatible with high-density double-sided and multi-layer printed circuit board assembly. Additionally, SMT assemblies' lack of leads improves their resilience to vibration and guarantees dependable signal integrity.

Drawbacks of SMT for PCB Build-Up

High Initial Investment

Implementing SMT assembly requires investment in specialized equipment such as reflow ovens, solder paste screen printers, pick-and-place machines, and hot air rework stations. These machines can be costly but are essential for efficient and accurate assembly.

Difficult to Inspect

Due to the small sizes and numerous solder joints of most SMD components, inspection becomes challenging. Identifying defects or faults can be time-consuming, especially when dealing with complex packages like ball grid arrays (BGAs), where solder balls are located under the component.

Fragility

Comparing surface-mount components to their through-hole counterparts, SMD parts are more delicate and fragile. They can be easily damaged if mishandled or dropped during the manufacturing process or while in use. Special care must be taken to handle and package these components using anti-static measures in clean room environments.

Expensive Small Batch Production

Technical complexity necessitates skilled operators and specialized equipment, making prototyping or small-batch manufacture of SMT PCBs an expensive ordeal. Due to the high initial setup expenses, SMT technology may not be cost-effective for small volumes.

Limited Power Capabilities

Surface mount technology isn't suitable for all electrical components, active or passive, because of power constraints. Components generating high heat may not be suitable for surface mounting as it can cause solder melting and compromise the integrity of the connection.

Despite these challenges, SMT remains a highly recommended practice in the manufacturing industry due to its numerous benefits. It enables manufacturing efficiency, design flexibility, better performance in harsh conditions or high-frequency applications, and more compact devices without compromising functionality or performance attributes. With ongoing advancements and innovations in SMT technology aimed at addressing existing limitations and improving production processes further, we can expect to see continued growth and impact on the industry.

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 s and was broadly used in the s. By the s, 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:

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• 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.

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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 imperial components, the dimensions are 0.02 x 0.01 inches. For the 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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