Surface Mount Technology
- An Introduction

    Surface Mount Technology(SMT) is a new technology to mount components on Printed Circuit Boards. Earlier in Through Hole Technology (THT), components were/are mounted on printed circuit boards by inserting component leads through holes in the board and then soldering.

    In SMT, components are mounted on printed circuit boards by soldering the component leads or terminals to the top surface of the board. This deceptively simple difference changes virtually every aspect of electronics: Design, Materials, Processes, Assembly of Component packages and substrates.

    THT had reached its limit as far as cost, weight, volume and reliability are concerned. With the advent of microprocessor and growth of computer technology, circuit complexity has increased to the point that requires making well over 100 connections to the board. Through-hole packages with leads on 0.1" centers became extremely inefficient for such large lead counts. SMT overcomes the limitations of existing techniques. It has a predominant effect on

Size/Weight issues

    The size of the component is limited by the packaging requirement. For THT, leads must rugged enough to survive the insertion process without damage. Lead diameters, drilling and imaging tolerances puts a limit on how closely leads are spaced, which in turn limits the size of component packages. In contrast, SM components have either closely spaced leads or no leads at all. As a result component packages can be made two to five times smaller than their through-hole equivalent. The smaller packages increases the interconnectivity, which is the number of solder joints that can be realized per unit area on the PWB. In SMT, as each side of the board is independent, components can be mounted on both sides.

Performance issues

Frequency response

    Surface mount packages has reduced component related parasitic reactances because of shorter component leads. Parasitic reactances affect the performance of high frequency circuits. Hence using leadless devices, operation to atleast 3GHz is possible.

Package propagation delay

    The time required for an electrical signal to propagate from the input to the output of the integrated circuit is the package propagation delay. It is the measure of the speed of the digital circuit. The propagation delay in surface mount packages is less as parasitic reactances,(primary cause of propagation delay) are smaller and more uniform.

Electromagnetic interference

    Through hole component leads serve as tiny antennas that radiate and receive undesirable signals. On the other hand, surface mount leads do not penetrate the board and thus reduce this problem. Hence sensitive circuits can often be combined onto a single board, simplifying shield design.

Cost

    Cost improvement is inherent in each of the elements discussed above. Though assembly costs for an automated SMT line are not appreciably different from that for an automatic through hole line, significant cost reduction occurs at the overall system level. Actual savings are highly dependent on design of the individual product and cannot easily be generalized.

Quality

    The higher level of automation in SMT results in improved quality and more consistent output.

Limitations

Mechanical Strength

    The solder joints of surface mount components become both the electrical and mechanical contacts to the board . The amount of solder available is generally much less than at a through hole joint; so its mechanical strength is less. Since leads do not penetrate the board, they cannot reinforce the joint.

CTE Mismatch

    The coefficient of thermal expansion for ceramic component body is less than that of the organic printed wiring board. The difference in expansion over temperature must be entirely absorbed within the solder joints. For leadless components, over the full product temperature range, large stresses can be built in the joints, causing eventual failure.

Process Flow

        Solder Paste is screen printed on the Substrate.     Surface Mount Components are then placed on the substrate The Solder is then reflowed, during which the component leads form an electrical and mechanical contact with the substrate. It is then finally cleaned to remove the fluxes.