TWI Knowledge Summary

Surface mount technology

by Simon Mason

Surface mount technology is used in the electronics industry in the design and assembly of circuit boards. It is the use of flat pads on the surface of a board combined with components designed such that their connecting leads can be soldered onto the pads. It has replaced, to a great extent, conventional through-hole technology, in which the components had leads which passed through the board and which were soldered on the opposite side to the component body. Surface mount became mainstream in the 1980s and its uptake has been accelerated by the drive towards smaller, faster, cheaper electronic products. It's main advantages over through-hole technology are:

1. Both sides of a circuit board can be used (which helps to reduce the size of an assembly).
2. It becomes easier to design and make multilayer circuit boards, therefore leading to a decrease in size.
3. Surface mount lends itself to automated assembly and therefore, when used for volume production, it can offer lower costs.
4. Smaller components can be used, allowing increased circuit density.

However, initial costs in switching to surface mount are high because of equipment purchase. Because of this, in countries where labour is cheap, or in small companies which cannot afford the capital expense, through-hole soldering still has a major role. Surface mount design is used particularly for the high volume, leading edge electronic products such as mobile telephones and computer boards, which are manufactured by, or for, the large multinational OEMs (Original Equipment Manufacturers).

The standard surface mount process consists of three basic stages. Firstly, solder paste is printed on the surface of the circuit board through a stencil which matches the component patterns. The components are then placed onto the printed paste. The paste is tacky enough to keep the components in position as the board passes down the production line. Once all the components have been placed, the assembly then passes through a reflow oven, which melts the solder to form the joints. Once out of the oven, that part of the assembly is then complete. The board may then be packaged or possibly the second side assembled. An alternative process involves dispensing adhesive on the board, onto which components are placed. The adhesive is cured to hold the components in place. The board is flipped and passed over a solder wave which provides the solder for the joints.

The main development issues at the moment are the assembly of finer pitch components, such as flip chip and micro lead-frame devices and the continued implementation of lead-free solder. The finer pitch assembly is being driven by the need to make circuitry small or to increase functionality in an existing space. The lead-free solder issue is driven by environmental and marketing considerations, and impacts particularly the reflow cycle since these alloys have higher melting points than typical tin/lead solder.

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.