At present three assembly machines are usually required for leaded components:
• Insertion machine for radial-leaded components
• Insertion machine for axial-leaded components
• Insertion machine for DIPs.

Reliability
The demands on quality and reliability of PCB assemblies increase steadily. In this
respect SMDs have at least to meet the standard set by conventional through-hole
technology. Following general statements can be made:
• The failure rate of SMDs does not exceed that of leaded components. Omission of leads means one point of contact less. Owing to their small size and light weight SMD assemblies feature a higher resistance to mechanical stress (vibration, shock) than the corresponding assemblies with leaded components.
• A quality approval for SMDs used in hybrid circuits can be usually applied to surface mounting, as well.
• High requirements are placed on the solderability of SMDs. The specifications for wetting, leaching and storage have to be observed (see section “Soldering Techniques”).
• In many cases the soldering methods are the same as with other mounting methods. The known advantages and disadvantages apply to surface mount technology as well. One should bear in mind, however, that the criteria for judging solder joints are different for wave
soldering and reflow soldering (see section “Soldering Techniques”). For example, the filling of through-holes with solder is only possible with the wave soldering method, with reflow soldering the amount of solder is too small.

• If components have to be replaced because of incorrect assembly, reliability of the board—although correctly assembled then—is diminished. Hence, automatic placement systems with their high degree of placement reliability enhance board reliability.

Rework
Elimination of component preparation, high placement reliability provided by automated
systems, and careful planning of each step of the design and production process considerably reduce expensive rework of PCB assemblies with SMDs. Restrictions and Special Features of Surface Mounting Maximum packing density—one of the primary
goals in surface mount technology— requires the use of miniature components, i.e. certain IC packages (e.g. VSO or MIKROPACK). This involves problems, not necessarily
resulting from surface mount technology as such, but from miniaturization in general.
• The use of high-pin-count ICs may require new PCB design (fine etching and
super-fine etching) and an increased number of layers (multilayer) because the space between the IC pins is too narrow for printed circuits.
• Due regard must be paid to heat dissipation. The high packing density may cause thermal problems. Special PCBs with good thermal conductivity can aid heat removal, if necessary.
• Using ceramic components is restricted. Due to the different thermal expansion coefficient of ceramic and PCB material, ceramic SMDs with edges longer than 6 mm should not be used on phenolic resin laminated paper and epoxy glass fiber boards.

• Not all SMDs are suitable for dip or wave soldering. This has to be considered when designing a PC board.
• Some components are not yet available as SMD version. Not all SMDs available are standardized.
• High voltages naturally require certain minimum spacings.
• Visual inspection of solder joints becomes difficult if the leads are partially beneath the component body. Therefore, soldering methods should be optimized so that visual inspection will become unnecessary. • Test methods have to be adjusted to SMD assemblies. Development of new adapters may be required.
• Repair of SMD assemblies may be more costly as compared with conventional
PCB assemblies.

Fixing SMDs by Glue
New in surface mounting is the gluing procedure required for fixing the components
when the PC board is to be turned upside down for soldering. The glue has to meet
numerous requirements. It must provide reliable fixing of the components (also of
heavy ones) on all kinds of PC boards. Furthermore, it should feature uniform viscosity
to ensure easy handling; a pot life of at least several days is advisable. The glue should
feature short curing time at low temperature.

After curing the glue must not show chemical reactions in order not to impair board or
components. On the one hand the adhesive is required to withstand high thermal stress,
and on the other hand it must permit removal of SMDs from the assembled board in
case of repair. For repairs the component body is heated, so that the adhesive becomes
soft and allows the component to be removed without damaging the printed circuit
below it. The glue has to be non-toxic, as odorless as possible, and free of solvents. Besides, it should feature good heat conductivity. Development of new adhesives
is under way.

The component outline should be such that the adhesive can easily be applied, i.e. the
distance between component body and board must be closely tolerated (Figure 3).
There are three methods of dispensing the glue:
• by applicator
• by pin transfer
• by screen printing.
Not all adhesives are equally suitable for all methods. Component and glue dot have to be shaped such that the component is reliably wetted while the contact area remains free of glue.

Soldering Techniques
An appropriate soldering method is particularly important for obtaining good electrical
contact and inhibiting short circuits. The choice of the soldering procedure depends
on the PCB design (single or double-sided, multilayer, etc.), the components supplied,
and the production facilities. While many SMDs are suitable for all soldering methods,the soldering technique for ICs, for example, has to be chosen very carefully. Besides manual soldering, which should only be used for repair purposes, there are several automated soldering methods such as bath soldering (wave and dip soldering) and reflow soldering.

With bath soldering the solder is applied during the soldering process itself, whereas
with reflow soldering the solder is applied before. For this reason the preconditions for
bath soldering, e.g. component orientation and configuration are quite different from
those for reflow soldering. The reflow method is particularly advisable for soldering certain ICs.

Wave Soldering
With wave soldering, a heated solder batz is used. The solder bath temperature lies between 240 and 260C and the dwell time is 1 to 3 seconds. Before soldering the flux is
applied. High packing density on the PCB side to be wave soldered involves the problem of solder bridges and shadows (not completely wetted leads and pads). Therefore, PCB
layout, i.e. component configuration, should match the soldering method used.
Dual-wave soldering best meets requirements of surface mounting. The first turbulent
wave sends up a jet of solder to ensure good wetting of all metallization areas, while
the second more laminar wave removes the excess solder (solder accumulations and
bridges).

Reflow Soldering
In reflow soldering a specific amount of solder, in the form of solder paste, is applied to
the PC board. Rerflow soldering is the prefered process for soldering SMD components.
After attaching the SMDs the reflow process is performed by one of the following methods:
• infrared soldering
• hot gas soldering
• heat collet soldering
• vapor phase soldering
The latest reflow technique is vapor phase soldering, where the entire PC board is uniformly
heated until a defined temperature is reached; there is no possibility of overheating.
The defined temperature (e.g. 210C) in a saturated vapor zone is obtained by heating
an inert (neutral) fluid to the boiling point. A vapor lock above this primary vapor zone
prevents the expensive primary medium from escaping (Figure 4).

Other methods are hot gas and infrared soldering in continuous- type furnace. Hot gas and infrared soldering have become the most common reflow soldering methods. For heat collet or pulse soldering a collet or a soldering iron is used to transfer the heat to the component leads. It is important to force the leads into reliable contact with the solder pads before and during the soldering process. This method is preferably used for Flat Pack packages.

Iron Soldering
Manual soldering with temperaturecontrolled miniature iron should only be used in exceptional cases (repair, etc.), because this method is not only uneconomic, but can
also damage components or PC board. Fluxes, Cleaning Agents Wave soldering requires no other fluxes than those used for conventional techniques (e.g. collophony F-SW32 per DIN 8511).

Most of the solder pastes required for reflow soldering, however, contain aggressive
fluxes the residues of which must be removed by a cleaning process. Conductive Adhesion
Conductive adhesion is not a soldering process, but shall be described here for the sake of completeness. It is not very often used since most conventional PC boards with a surface of tin or solder tin are not suitable for gluing. If components or PC board permit gluing, silver filled mixed epoxy resin adhesives can be recommended. These can be spread by an applicator, screen printing, or by pin transfer.

The times required for curing are between 1 min and 12 h depending on the temperature.
The thermal stress imposed on the components is less than with soldering, but the
adhesion process must be performed separately after soldering the other components.
Assembly Variations

Figure 5 shows the PCB assembly variations possible with SMDs: Assemblies exclusively
with SMDs in the top row (Figure 5a and 5b), mixed assemblies, i.e. SMDs combined with
leaded components in the middle (Figure 5c and 5d), and mixed assembly consisting of
dip solderable components (on solder side) and non-dip-solderable components (on component side) in the last row (Figure 5e). The versions illustrated in Figures 5b, d, e
require double-sided PC boards.

In mixed assemblies with SMDs and leaded components (Figure 5c and 6) the leaded
components are usually placed first, then the board is turned over and the glue applied.
Subsequently the SMDs are placed, the glue is cured and after a renewed turn over the
board is wave soldered.

The second variant shown in Figure 7 differs from the first in so far as the glue is applied by screen printing at first; the following production steps are executed as illustrated in Figure 7. This procedure has the advantage that the glue can be applied by screen printing, however, it has to be taken into account that because of the already mounted SMDs vacant board space is required for the mounting tools of the insertion machines, which are needed for cutting and bending the leads of conventional components. The procedure for double-sided SMD mounting is as follows:
• Screen printing of solder paste
• SMD placement
• Reflow soldering
• Insertion of leaded components
• PCB turn over
• Application of glue
• Placement of SMDs on the reverse side
• Curing of the glue
• PCB turn over
• Mounting of components requiring special handling
• Fluxing, wave soldering

Here both reflow and wave soldering are used. Assemblies including leaded components
always require wave soldering. The aim is a uniform mounting procedure with the exclusive use of SMDs. Figure 8 shows examples for totally surface mounted assemblies with reflow soldering (top) and wave soldering (bottom). Figure 9 is a flow chart for the various assembly and exclusive use of SMDs.