PCB Surface Mount Technology
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 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.