Surface Mount Assembly Technology (SMT) printed circuit boards involves the installation of components on the surface of the board by soldering SMD (surface mounted device) components to the pad.
This type of installation allows you to place components not only on one side of the PCB (single-sided boards) but also on both (two-way boards). The development of surface mount technology dates back to the 1960s, when the development of the installation of hybrid microcircuits began, for which it was difficult to obtain holes in the ceramic substrate. However, the emergence of smd editing on laminated boards began relatively recently. Advantages of surface mounting are the use of smaller components and a greater density of their placement. Large holes were replaced by smaller ones to conduct a signal between the sides of the board and the inner layers. A smaller trace and a decrease in component height also contributed to the miniaturization of boards and increased functionality. An example of a surface-mounted PCB is shown in Figure.
The main trend of PCB technology used in surface mounting technology is the use of smaller passive components - capacitors, resistors, inductors and chokes. In addition, built-in passive components are used - resistors and capacitors, which are located inside the layers of the printed circuit board. The use of built-in passive components releases an additional area for large, active components.
In the use of active components used in surface mounting (SMD), there are two opposite trends. On the one hand, the size of memory components (RAM, SDRAM, etc.) is getting smaller, since transistors are now increasingly manufactured on a silicon chip. On the other hand, microprocessors and specialized integrated circuits (ASICs) are becoming more and more due to the increased functionality of large crystals. Cases for both types of devices were transferred from the peripheral location of the leads to the matrix pins. Enclosures with matrix terminals include BGA-cases and smaller components - CSP and DCA / FC. Figure 2 shows an example of a BGA chip used for surface mounting on a printed circuit board. The advantages of matrix technology smd installation include the reduction of the area occupied by the component, by eliminating the leads emerging from the housing. Besides,
From the very beginning of development, the size and pitch of the outputs of the matrix housings in the technology of surface mounting of printed circuit boards were larger than those used at that time with peripheral pinouts with small steps of 0.4 and 0.5 mm, respectively. However, as the number of leads began to increase along with the increase in the functionality of the components in the matrix housings, the size of the solder balls and the pitch were significantly reduced, especially considering the DCA technology.
Expanding the functionality and further miniaturization of SMD devices led to an increase in the density of the components on the board, so now, when surface mounting of PCBs have adhered to strict rules.
A particular advantage of surface mount technology is the reduction in production costs as a result of automation of assembly processes. Solder paste, which is a mixture of a metal powder of solder, flux and thixotropic agents, is applied in strictly controlled quantities (by thickness and area) with the help of screen printing, and also with the use of dispensers. Assembly machines are able to accurately set even the smallest smd-components on points solder paste (or "bricks"). The increased stickiness of the flux in the solder paste keeps the components in place. The assembled (surface mounted) printed circuit board is then moved through a convection / radiating solder reflow furnace or a soldering chamber in the vapour phase (or condensation phase) for melting the solder. Automatic machines that perform operations at all stages of installation - paste screen printing, component installation and solder reflow soldering, - are connected by conveyor belts to create process flow lines. In fact, the last stage - washing the boards - can also be part of the sequence of the installation process.
Of course, depending on the volume of production and capital costs, different levels of automation of SMD PCB mounting can be used. Nevertheless, with the constant miniaturization of surface-mounted products, as well as stringent requirements for reproducibility with high accuracy of solder paste volumes and the location of components, it is necessary to pre-design surface mounting on the basis of full automation.
Mixed technologies include a combination of surface mount technology (SMT) and mounting in holes on a single printed circuit board. The absence of components in surface-mounted enclosures is almost always the reason for the use of their analogs mounted in holes. In general, surface-mounted products are soldered first to the top side of the printed circuit board using a convection or radiant reflow oven or in the vapor phase (surface mounting is performed primarily because the components mounted in the holes will interfere with the application of the jaws: and the installation of components by PIP- technologies). Then the components are soldered into the holes on the board. In fact, the soldering process is carried out at the bottom of the board. With a large number of components mounted in the holes, soldering with a wave of solder is used. If there are surface-mounted components on the underside of the board, they can also be soldered with a wave of solder, but they are first installed and fixed in place with glue. If the components mounted in the holes a little or the underside of the board cannot be soldered with a wave of solder, it is preferable to use manual soldering.