LeadedSurfaceMountTechnology (SMT)
7
7.1
7.2
Introduction
Traditional through-hole Dual In-Line Package assemblies reached their limits in terms of
improvements in cost, weight, volume, and reliability at approximately 68L. SMT allows
production of more reliable assemblies with higher I/O, increased board density, and reduced
weight, volume, and cost. The weight of printed board assemblies (PBAs) ...using SMT is reduced
because surface mount components (SMCs) can weigh up to 10 times less than their conventional
counterparts and occupy about one-half to one-third the space on the printed board (PB) surface.
SMT also provides improved shock and vibration resistance due to the lower mass of components.
The smaller lead lengths of surface mount components reduce parasitic losses and provide more
effective decoupling
The smaller size of SMCs and the option of mounting them on either or both sides of the PB can
reduce board real estate by four times. A cost savings of 30% or better can also be realized through
a reduction in material and labor costs associated with automated assembly.
Types Of Surface Mount Technology
SMT replaces DIPs with surface mount components. The assembly is soldered by reflow and/or
wave soldering processes depending on the mix of surface mount and through-hole mount
components. When attached to PBs, both active and passive SMCs form three major types of SMT
assemblies, commonly referred to as Type 1, Type II, and Type III (see Figure 7-1).
Type I is a full SMT board with parts on one or both sides of the board.
Type II is probably the most common type of SMT board. It has a combination of through-hole
components and SMT components. Often, surface mount chip components are located on the
secondary side of the Printed Board (PB). Active SMCs and DIPs are then found on the primary
side. Multiple soldering processes are required.
Type III assemblies are similar to Type II. They also use passive chip SMCs on the secondary side,
but on the primary side only DIPs are used.
2000PackagingDatabook
7-1
LeadedSurfaceMountTechnology(SMT)
Figure 7-1. Surface Mount Technology Board Types
PLCC
Passive
Components
Solder Paste
Type I
SO
Solder Paste
Type II
Type III
SO
PLCC
Passive
Components
Passive
Component
SO
PLCC
DIP
Passive
Components Only*
DIPs
Passive
Components Only*
NOTE: Intel does not recommend active devices be immersed in solder wave.
A5664-02
The process sequence for Type III SMT is shown in Figure 7-2. Leaded components are inserted,
usually by automatic equipment. The assembly is turned over, and adhesive is applied. Next,
passive SMCs are placed by a "pick-and-place" robot, the adhesive is cured, the assembly is turned
over, and the wave-soldering process is used to solder both leaded and passive SMCs in a single
operation. Finally, the assembly is cleaned (if needed), inspected, repaired if necessary, and tested.
For this type of board, the surface mount components used are chip components and small pin
count gull wing components.
7-2
2000PackagingDatabook
LeadedSurfaceMountTechnology(SMT)
The process sequence for Type I SMT is shown in Figure 7-3. For a single sided type I, solder
paste is
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