Surface mount technology (SMT)

Surface mount technology is a collection of scientific and engineering methods needed to design, build, and test products made with electronic components that mount to the surface of the printed circuit board without holes for leads. This definition notes the breadth of topics necessary to understand SMT and also clearly says that the successful implementation of SMT will require the use of concurrent engineering. Concurrent engineering, means that a team of design, manufacturing, test, and marketing people will concern themselves with board layout, parts and part placement issues, soldering, cleaning, test, rework, and packaging—before any product is made.

Considerations in the Implementation of SMT

The main reasons to consider implementation of SMT include
• reduction in circuit board size
• reduction in circuit board weight
• reduction in number of layers in the circuit board
• reduction in trace lengths on the circuit board, with correspondingly shorter signal transit times
and potentially higher-speed operation
• reduction in board assembly cost through automation

However, not all of these reductions may occur in any given product redesign from through-hole technology (THT) to SMT. Obviously, many current products, such as digital watches, laptop computers, and camcorders, would not be possible without the size and cost advantages of SMT. Important in all electronic products are both quality and reliability.

• Quality = the ability of the product to function to its specifications at the conclusion of the assembly process.
• Reliability = the ability of the product to function to its specifications during its designed lifetime.

Most companies that have not converted to SMT are considering doing so. All, of course, is not golden in SMT Land. During the assembly of a through-hole board, either the component leads go through the holes or they don’t, and the component placement machines typically can detect the difference in force involved and yell for help. During SMT board assembly, the placement machine does not have such direct feedback, and accuracy of final soldered placement becomes a stochastic (probability-based) process, dependent on such items as component pad design, accuracy of the PCB artwork and fabrication (which affects the accuracy of trace location), accuracy of solder paste deposition location and deposition volume, accuracy of adhesive deposition location and volume if adhesive is used, accuracy of placement machine vision system(s), variations in component sizes from the assumed sizes, and thermal issues in the solder reflow process. In THT test, there is a through-hole at every potential test point, making it easy to align a bed-of-nails tester. In SMT designs, there are not holes corresponding to every device lead. The design team must consider form, fit and function, time-to-market, existing capabilities, testing, rework capabilities, and the cost and time to characterize a new process when deciding on a change of technologies.