When talking about PCB assembly, the term surface mount technology or SMT will eventually be mentioned. This term refers to soldering (mounting) SMDs or surface mount devices on a printed circuit boards surface.
This process is much more complex than the traditional through-hole. This is also a fairly new technology for assembling circuit boards. The finished PCBs offer better electrical conductivity and are more reliable.
How is SMT carried out?
Solder paste is applied instead of the traditional solder bar or wire used in through-hole assembly. In traditional plated through hole technology, the leads of the circuit components are threaded through a hole on the board surface. The leads are then soldered on the underside of the board. In SMT, there are no leads pulled through holes. Most often, this process of PCB assembly is carried out by machines designed to attach and secure components in place. In some instances, components can be mounted by hand, too.
Manufacturing units that utilize SMT include clean room and ESD-safe. Clean room environment helps in preventing any damage to sensitive components such as semiconductors. One source of damage is static electricity.
This new technology is rapidly gaining preference in the word of electronics. Most of the steps are carried by special machines but yields a lot of benefits in terms of manufacturing and design.
In terms of design, SMT allows for more savings in weight. Smaller amounts of solder are used. Components are flushed closer to the board, removing excess lengths and leads. There is reduced mass from the components used, which improves resistance to vibrations, reduces electrical noise and improves shock resistance. Also, the circuits can be manufactured in a more compact manner.
In terms of manufacturing, PCB assembly through SMT can reduce the cost of production. There is an overall reduction in the cost of the boards and materials handling. The entire manufacturing process is also more controlled, producing more uniform and reliable products.
Surface Mount Device (SMD)
As has been previously mentioned, SMT mounts SMDs on the surface of printed circuit boards. SMD or surface mount device is an electrical component similar to popular circuit board components (e.g., resistors, diodes, capacitors, etc.) but without the leads. SMDs have tiny metallic legs that are very close to the body of the components. The metallic legs are attached or soldered onto the PCB surface, on the copper traces, through the use of solder paste. This material is a ball of gel or flux material and inside it is a small amount of solder.
As with PCB components, SMDs are classified as passive and active, depending on their function during the operation. The following are some of the most commonly used SMD components for PCB assembly:
• Surface mount resistor networks
• Tubular passive components for SMT
• Ceramic capacitors for SMT
• Surface mount discrete resistors
• Surface mount tantalum capacitors
• SOT or small outline transistors
• LCCC or leadless ceramic chip carriers
• SOJ or small outline J packages
• PLCC or plastic leaded chip carriers
• BGA or ball grid array
• SOIC and SOP or small outline integrated circuit
• QFP, SQFP or fine pitch SMD packages
Equipment used for MST is very different from plated through-hole technology. There are so many, depending on the end use of the PCB assembly. The most commonly used ones include the following:
SMT Solder Past Screen Printer – This screens the solder paste applied on the surface of the board. This makes sure that the solder paste is applied only on designated areas where SMDs are later attached and eventually soldered.
SMT Pick-and-Place machine – This machine is programmed to pick SMDs from reels and place them in the proper places on the board.
SMT Reflow Soldering – This is where reflow soldering happens, where the solder paste is melted.
SMT Baking Oven or Curing Oven – This machine is responsible for curing the adhesives on the board and baking the solder paste.