July 16, 2020 by Emmanuel Ikimi
Electronic components' rapid miniaturization has given rise to ever-smaller device geometries, even as performance and computing power increases.
Surface mount technology (aka SMT) allows semiconductor engineers to fit more components onto the same printed circuit board (PCB). This article will outline how these components work as well as their benefits, applications, and limitations.
SMT is an electronic assembly technique where components are mounted directly onto a PCB, as opposed to inserting them into sockets within the board (aka "through-hole technology" or THT). IBM was the first to use surface-mounted components in the 1960s for its groundbreaking work on launch vehicle digital computers (LVDCs) for NASA space shuttles. Today, SMT is used in virtually every type of electronic application.
Surface mount devices (SMDs) are electronic components produced using SMT processes. Examples include passive elements such as resistors, chip inductors, transient voltage suppression (TVS) diodes, and ceramic capacitors. Unlike through-hole devices, SMDs have minimal lead terminals, allowing manufacturers to shrink them further.
After selecting the required components, the process starts with design engineers developing a schematic or layout showing where each one fits on the board. PCB design is a software-based process using computer-aided design (CAD) tools. Following several iterations, the engineer sends the final schematic to a PCB manufacturer for fabrication and sources the required components.
SMDs ship in a wide range of packages. The most common types are standard tape-and-reel or cut tape packaging. For the former, the manufacturer loads units of SMDs into small pockets on continuous embossed tape wound around a base for easy transport.
EIA Standard 481 contains specifications for this packaging type. On the other hand, cut tape packaging delivers the SMDs in short strips of tape. Alternative packaging includes tubes and trays.
Surface mount components usually contain small lead contacts (pins) of various styles (e.g., "J" shaped and "gull-wing" types) that extend from the sides or on the body of the package. In some variants (e.g., ball grid array or BGA SMDs), the lead contacts are underneath the device as a grid pattern. The mounting technique depends on the pin configuration and pin count of SMDs that vary from package to package.
A PCB logic board showing passive surface mount components.
Two standard techniques for soldering SMDs onto PCBs are reflow soldering and wave soldering.
Reflow soldering is one of the most widely adopted techniques for attaching SMDs onto PCBs. The idea is to create high-integrity solder joints through four stages:
Preheating the PCB, solder paste (a mix of solder and flux), and SMD itself to a specific temperature (aka the "soak" or "dwell" temperature)
Passing the preheated materials into a second 60 to 120 seconds thermal soak zone of a specific temperature until the entire assembly reaches thermal equilibrium
The assembly enters into a "reflow zone" where solder around the SMDs is melted at the maximum allowable (aka peak) temperature of the process without causing thermal damage to any of the components. Reflow soldering is done using a reflow oven with a unique thermal profile setting to distribute the heat precisely and in a predictable manner.
Cooling the entire assembly at a specific rate to allow the soldered joints to solidify.
Wave Soldering
Wave soldering is ideal for large-scale PCB fabrication and is suitable for both surface mount and through-hole components. It typically involves the following stages:
Spraying flux onto the PCB to eliminate oxides from the surface of the PCB, prevent secondary oxidation during the thermal process, and lower the surface tension of solder paste.
Preheating the PCB
Passing the PCB over a wave soldering machine to apply the solder
Controlled cooling the PCB to room temperature
Engineers working on a manufacturing floor.
After soldering SMDs on a PCB, it is essential to test the integrity of the soldered joints. Automated optical inspection (AOI) machines are suitable for this purpose. AOI machines have side-facing and downward-facing cameras for viewing the PCB to identify common soldering issues such as insufficient solder or gaps, solder bridging, solder balling, lifted pins, and tombstoning. Engineers may also carry out visual inspections to identify soldering problems.
Compared to THT, SMT provides a host of benefits, including the following:
Reduced manufacturing costs
A high degree of repeatability with fewer errors
Optimization of board space
Both sides of an SMT component are solderable
Achieves greater PCB complexity due to higher component density on the board
Surface mount components are cheaper than through-hole types
Despite providing a host of benefits within the electronic industry, surface mount technology has several limitations. Firstly, SMT is not suitable for components that carry high electrical energy or generate significant heat, as the solder integrity may be compromised. Moreover, it is not suited for large-volume or pilot-scale fabrication. Also, due to their miniature sizes, SMT assemblies are difficult to repair, often requiring specific expertise, costly downtime, and sophisticated equipment.
Surface mount technology is a breakthrough method that will continue to shape electronics design for many years to come. Its main benefit is that it allows large numbers of components to be integrated into the same PCB while maintaining a high degree of accuracy and repeatability. Used alongside through-hole components, SMDs help to increase design complexity while conserving valuable board space.
