Surface Mount Technology (SMT) adds to the benefits of manufacturing boards more efficiently and with reduced weight for multiple reasons:

• Components are placed directly on the board surface rather than connecting through holes in the board (through-hole technology).
• Many components for SMT, often referred to as surface mount devices or SMDs, are smaller and lighter than their counterparts, partly due to the reduced or elimination of leads required in through-hole manufacturing.
• Through standardization of components and design tools, fabrication can be largely automated.

SMT was originated in the 1960’s when IBM used this approach in designing small-scale computers, and was subsequently adopted for use in guidance systems in the space program for Saturn rockets. Since those early days, the concept has been constantly refined, and improved.

SMT Advantages

SMT allows engineers to design boards with components on both sides of the board material. By eliminating or reducing the need for drilling and component holes, there is no restriction on placement of circuit elements on opposing sides.

Design flexibility is another advantage of SMT construction. There is no conflict in combining SMT and through-hole manufacturing methods on the same board. This gives PCB designers a free reign to create specialized circuits without the need for multiple boards, when one can provide the functionality needed.

Other distinct advantages of SMT printed circuit boards:

Size – reduced component size equates to more components being contained on a single board, and subsequently fewer boards being required for a product. With today’s demand for miniaturized products and lighter weight, such attributes are critical.

Dual-sided mounting – without the need to design for drilling and connecting through board materials, components can be place on both surfaces of the PCB.

Quality – Manufacturing processes that place components and perform soldering functions can actually improve placement reliability. Tolerance for placement is improved by the ability of surface tension of the soldering function to actually correct slight alignment issues.

Reliability – SMT connections are less prone to failure from the impact of vibration or shaking.

Speed of manufacturing – Especially when combined with adherence to design for manufacturing (DFM) when circuits are designed, SMT increases production efficiency through elimination or reduction of drilling operations and lower setup times.

Cost reductions – Many of the smaller-sized surface mount devices (SMDs) or components actually have a lower cost than their larger through-hole versions.

SMT – the Down Side – Disadvantages

SMT technology is not without its disadvantages:

Connecting – SMT is not practical in conditions where mechanical stress factors come into play, such as where physical connections will be attached and detached on a frequent basis.

Joint Size – Due to the very nature of SMT, soldered connections are smaller, facilitating the smaller components. This results in a smaller amount of solder utilized in each contact, which could compromise the reliability of some solder joints.

High-power applications – Where large components are required in a circuit such as fuses, large capacitors, or large connectors, SMT is not the best solution for PCB fabrication. In such instances, the practical approach is often to combine the use of SMT with through-hole design for larger components such as transformers or semiconductors that require heat sink considerations. In practice, such larger devices are sometimes combined on the same board, but utilizing through-hole fabrication.

Prototyping – This is unlikely to be an effective application for SMT. If components need to be added or replaced, specialized tools and skills may be required, especially when the board consists of a high density of component placement.

SMT in Use

SMT construction has taken over as the primary design and fabrication standard in electronic devices today. Taking a look inside nearly any consumer product, automobile, or computer will reveal considerable use of SMT boards. With their attributes of reliability, light weight, reduced size, and manufacturing cost benefits, SMT boards are utilized in any situation that calls for reliable, high-volume PCB fabrication.

Design for Manufacturing (DFM) software tools provide circuit designers with guidelines for SMT component placement that increases manufacturing productivity and reduces the potential for designs that cannot practically be fabricated. This saves time and money by reducing the need for rework and redesign of complex circuits.

Industry standards for most SMDs have further simplified design and manufacturing considerations to work in harmony, producing PCBs of consistent quality that can be produced efficiently with automated manufacturing processes and equipment.

Just what is surface mount technology? Prior technology of printed circuit board design and fabrication consisted mostly of components situated on the board and connected to conductors through holes and typically soldered in place. This through-hole methodology requires obvious manufacturing steps of drilling holes in the board material, inserting leads through those holes properly and consistently, and connecting them in place securely through soldering processes.

While many of these fabrication functions are today highly automated to provide quality and efficiency, they are nonetheless steps in the manufacturing process that require attention to detail and introduce potential for flaws and quality issues when not executed with exacting precision.

Surface mount technology (SMT) began to be widely used in the 1980s with the introduction of improved manufacturing processes and surface mount devices (SMDs). Nearly every electronic device that contains printed circuit boards and is mass produced today includes some level of SMT-manufactured boards. SMT boards are typically smaller through the very nature of smaller SMD components that can be positioned in a higher density on the board.

SMT vs. Through-Hole PCB Fabrication

SMT manufacturing of PCBs provides a number of advantages over the previous through-hole technology:

• Smaller components – SMDs are not only smaller themselves, but they also greatly reduce the space and processes needed for connecting to the board without the need for leads, placement and drilling of holes, and soldering. SMDs are connected directly to the board surface. SMD components are typically one quarter to one tenth the size and weight of through-hole devices – an obvious advantage for PCB designers and the devices they will be used in.
• Higher density of components – this results in smaller boards or even fewer boards.
• Construction facilitates the mounting of components on both sides of the board.
• Manufacturing efficiency – reduced cost through simplified setup and reduction of hole drilling operations.
• Reduced cost – many SMD components are less expensive than their leaded counterparts.
• Reliability – SMT fabrication is generally less susceptible to impact from vibration or shaking.
• SMD use can reduce resistance and inductance.
• Smaller board size and shorter paths can improve performance.

There are, of course trade-offs or disadvantages in SMT board construction:

• Construction of prototypes or manual fabrication is more difficult.
• Board repair of components is also more challenging, not easily done by manual means.
• Use of breadboard materials for construction is not feasible with SMDs.
• SMD construction is not suitable when there are requirements for high power or large high-voltage parts such as in power circuits.
• Thermal cycling potting compounds can damage SMD solder connections.
• Through-hole fabrication is less vulnerable to damage from exposure to harsh environments such as repeated shock or vibration. Since leads actually pass through holes and are soldered, the connections are less likely to fail than surface-mounted devices.
• Capital outlay for SMT equipment is considerable.
• SMT design requires more advanced skills than traditional through-hole methods.
• Through-hole still retains a strong foothold in prototyping and testing.
• Not all components are available as SMDs. In such cases through-hole design will remain the only alternative.

SMT Application in Practice

SMT technology is utilized nearly exclusively in the manufacturing of today’s electronic devices. SMT provides the ability to generate mass-produced, smaller, lighter boards with fewer fabrication steps and less setup time, reducing cycle times and fabrication complexity. This makes the resulting PCBs less expensive to produce and more cost-effective for use in electronics or other products.

Modern computer aided manufacturing capabilities increasingly automate placement of components that previously required manual or assisted operations. SMD manufacturers also continue to develop components that simplify assembly through reduced size and ease of placement and connection to board surfaces. Some applications require a hybrid use of through-hole and SMT boards to take advantage of the particular benefits of each technology. The two technologies can co-exist side-by-side without issues.

Automated processes, reduced size and weight, and simplified fabrication have combined to make SMT board manufacturing the primary method utilized in today’s electronic devices. Sophisticated equipment required to manufacture SMT PCBs can be a significant investment, resulting in the use of outsourced manufacturing for many companies needing SMT boards.