Introduction
What is Surface Mounting? In conventional board assembly technology the component leads are inserted into holes through the PC board and connected to the solder pads by wave soldering on the reverse side (through-hole assembly). In hybrid circuits (thick and thin film circuits) “chips”, i.e. Ieadless components, are reflow soldered (see paragraph 7.2) onto the ceramic or glass substrate in addition to the components already integrated on the substrate. Surface mounting evolved from these two techniques .

In through-hole technology the components are placed on one PCB side (component
side) and soldered on the other (solder side) (Figure 1, top), whereas in surface mount
technology the components can be assembled on both sides of the board (Figure 1,
bottom). The components are attached to the PCB by solder paste or non-conductive
glue and then soldered.

In the near future mixed assemblies, i.e. a combination of leaded and surface mounted
components, will prevail, since not yet all component types are available as surface
mount version.

Automatic assembly machines are a must for an expedient production; there are systems
for simultaneous and for sequential assembly.

The following explanations point out what is new in surface mounting:
• Up to now connecting materials with large differences in the thermal coefficient of expansion, such as plastic boards and ceramic components, by rigid soldering has been regarded as a serious problem. Practice has shown, however, that this is feasible owing to the elasticity of board and solder; of course, component size and thermal stress are
subject to certain restrictions (see section “Restriction and special features of
SMD”).
• Components for surface mounting have to withstand high thermal stress during the soldering procedure. Not all component types meet these requirements; therefore new components suitable for surface mounting are constantly developed (see section “Restriction and special features of SMD”).
• In some cases the components are nonconductively glued to the PCB before soldering.
• As compared to through-hole technology there is a closer interrelation between the
individual steps in design and production.
• Automatic assembly gains prior importance.

What are SMDs ?
The abbreviation SMD* for Surface Mounted Device is the most common designation for
those components. SMDs are designed with soldering leads or short leads and are much
smaller than comparable leaded components. In contrast to conventional components,
the leads of which must be inserted into holes, SMDs are directly attached to the
surface of the PCB and then soldered.

In Figure 2 and the section below the various SMD types are summarized. Surface
mountable components include “chips”** with cubic dimensions, cylindrical SMDs,
plastic packages with solder pins (SOT, SO, VSO package), chip carrier packages,
miniature IC packages (Quad Flat Pack, Flat Pack), TAB components and special SMDs
such as inductors, trimmers, quartz crystals, switches, plugs, relays, etc.
* Other terms used: SMC (Surface Mounted Component), SMT (Surface Mount Technology), SMA (Surface Mount Assembly).
** “Chip” should only be used when confusion with semiconductor chip as used in semiconductor technology can be excluded.

SMD Types:
• Cubic components (“chips”) - Preference types 0805,1206,1210,1812, 2220, etc.
• Cylindrical components - MELF1 , MINIMELF, MIKROMELF, TUBULAR
(e.g. tubular capacitors), SOD 80 (MELFsimilar diodes)
• SOT 23,143, 89,192
• SO24 to 28 pins (SOIC)
• VSO3 40 pins
• CHIP CARRIER - Plastic case (PLCC4), Ceramic case (LCCC5)
• ICs with gullwing leads - Flat Pack, Quad Flat Pack
• Ball Grid Array, Chip size package
• Special packages for: Inductors, SAWs6 , trimmers, quartz crystals, switches, plugs, relays, etc.

Resistors, ceramic capacitors and discrete semiconductors represent at 80% the largest
part of the SMD spectrum. In the range of SMDs the cubic shape prevails over cylindrical
versions, as the latter can only have two pins thus being exclusively suitable for resistors,
capacitors and diodes.

If development of a special SMD package is not advisable for electric or economic reasons,
the DIP package can be converted into a surface mountable version by bending
the leads. An important factor for automatic assembly is the components’ adequate and uniform geometry. Some packages are already standardized (IEC) or are proposed for standardization (JEDEC Recommendation).

The spectrum of active and passive components available covers ICs, transistors, diodes,
ceramic multilayer capacitors, NTC thermistors, as well as SIFERRIT miniature ferrites, and the product menu is growing larger almost daily.

For more than ten years OSRAM Opto Semiconductors has offered its customers
optoelectronic SMDs and thus has gained considerable experience in the field of SMD
production through continual modernization and development.

Advantages of Surface Mounting The three major benefits of surface mounting
• rationalization
• miniaturization
• reliability
are discussed in the following. A consistent concept as regards components,
board layout, assembly machines, processing and testing is essential for an
efficient application of surface mount technology; in other words, the aim should be an
optimized overall concept. The component price, for example, should not be seen isolated,
but with regard to the total cost including placement, soldering and testing which may already be considerably lower than with conventional board assembly technology. In the following the advantages of surface mounting are analyzed as to component, PC board, automatic assembly, reliability and rework.

Components
• SMDs are much smaller than leaded components, thus enabling smaller board size, higher packing density, reduced storage space and finally smaller equipment to be obtained.
• Light weight makes them ideal for mobile appliances.
• No leads means high resistance to shock and vibration.
• Cutting and bending of leads are eliminated.
• Parasitic inductance and capacitance due to leads are substantially lowered making SMDs particularly suitable for RF applications.
• Automatic assembly machines ensure accurate placement.
• MlKROPACKs, PLCCs and similar packages permit a considerably higher number of pins.
• Closer capacitance tolerances can easily be obtained for capacitors with low capacitance
values.
• The growing demand for SMDs results in lower production costs, so that further cost reductions can be anticipated.

Printed Circuit Board
• Surface mount technology makes PC boards smaller. When using SMDs on both sides of the board, size can be reduced by more than 50 per cent. On the other hand, maintaining the PCB size implies reduced packing density and thus higheryields and higher reliability.
• In many cases the printed circuits can be shrinked and reduced in number. Owing to the compact “leadless” con-struction the electrical characteristics can easily be reproduced,

• thus cutting the cost for adjusting RF circuits.
• Surface mount technology does not require a special PCB material; standard materials such as FR4, FR2, CEM1 are quite suitable, but of course, special materials, e.g. for RF circuits, can be used, too. For normal packing density the printed circuit precision should meet current requirements.
• The elimination of through-holes entails a further cost reduction. This is quite an important factor, as the cost for the drilling of holes can amount up to 10% of the total PCB cost.
• Mixed assembly with leaded components is possible. The reason for using this assembly variation was explained in the beginning.

Assembly
The average cost per component for automatic assembly can be considerably cut by
surface mounting, because the smaller number of assembly machines entails less
capital investment, maintenance, servicing and factory space.
• A major advantage of surface mounting are the high component placement rates
attained by automatic placers. Fast machines can place several ten thousand components on the PCBs per hour.
• Automatic placement systems for SMDs feature high placement reliability. Failure rates of less than or equal to 20 ppm (parts per million) can be obtained by machines capable of identity checking and defective recognition. This means that out of a million placed components only max. 20 are not at all or incorrectly assembled.
• In mixed assembly any ratio of SMDs and leaded components is possible, thus
facilitating transition to SMD technology.
• Some automatic placement systems can handle a wide range of different components.