Surface - mount technology (SMT) is a method for producing
electronic circuits in which the components are mounted or
placed directly onto the surface of printed circuit boards (PCBs).
An electronic device so made is called a surface
mount device (SMD). In the industry it has largely
replaced the through - hole technology construction
method of fitting components with wire leads into
holes in the circuit board.
An SMT component is usually smaller than its through
Hole counterpart because it has either smaller leads or
no leads at all . It may have short pins or leads of various
styles , flat contacts, a matrix of solder balls ( BGAs ),
or terminations on the body of the component.
• Surface Mounting was originally called “Planar Mounting".
• Surface-Mount Technology was developed in the 1960s ,became widely
used in the late 1980s. Much of the pioneering work in this
Technology was by IBM which Launched
Vehicle Digital Computer( autopilot ) used in the
Instrument Unit that guided all Saturn IB
and Saturn V vehicles.
• Components were
mechanically redesigned to have small metal
tabs or end caps that could be directly soldered to
the surface of the PCB.
• Components became much
smaller and component placement on both sides of a
board became far more common with surface mounting
than through-hole mounting, allowing much higher circuit densities.
SMD Surface-mount devices (active, passive and electromechanical
SMT Surface-mount technology (assembling and mounting technology)
SMA Surface-mount assembly (module assembled with SMT)
SMC Surface-mount components (components for SMT)
SMP Surface-mount packages (SMD case forms)
SME Surface-mount equipment (SMT assembling machines)
SURFACE MOUNT DESIGN
It depends on a number of factors
Package moisture sensitivity
Thermal and solder joints
• As the packaging density
increases, thermal problems are
compounded, with a potential
adverse impact on overall
1. Design for manufacturability:
Includes the considerations of land pattern, placement, soldering, cleaning,
repair, and test are important issues.
2.Land Pattern Design:
•The surface mount land patterns, also called footprints or pads, define the
sites where components are to be soldered to the PC board.
• The design of land patterns is very critical, because it determines solder
joint strength and thus the reliability of solder joints, and also impacts
solder defects, clean ability, test ability, and repair or rework.
3.Design for Testability:
In SMT boards, designing for testability requires that test nodes be accessible
to automated test equipment (ATE).
SOLDER PASTE APPLICATION
Solder paste, a sticky mixture of flux and
tiny solder particles, is first applied to all
the solder pads with a stainless steel or
nickel stencil using a screen printing
Components to be placed on the boards are usually delivered to the production line
in either paper/plastic tapes wound on reels or plastic tubes. Numeric Control pickand-place machines remove the parts from the tapes, tubes or trays and place them
on the PCB.
There are different types of auto-placement machines :
In-line: It employs a series of fixed-position placement stations.
Sequential: Components are individually placed on the PC board in succession.
SMD Pick and Place Machine
Placement of Components on PCB
Soldering is a process in which two or
more metal items are joined together by
melting and flowing a filler metal into the
joint, the filler metal having a relatively
low melting point.
We have further 2 types of soldering in
Surface Mount Devices :
• Wave soldering is a large scale soldering
process, used to solder the electronic
components on PCB.
• Then name is derived from the use of
waves in molten solder.
• The process uses a tank to hold a quantity
of molten solder; the components are
inserted into or placed on the PCB and the
loaded PCB is passed across a pumped
wave or waterfall of solder.
• Used for both through-hole printed circuit
assemblies, and surface mount.
• If a wave soldering process is used in SMT,
then the parts must be glued to the board
prior to molten solder wave to prevent
them from floating off when the solder
paste holding them in place is melted.
Steps Included In Reflow Soldering:
Pre Heating→→ Soldering→→ Cooling
•They first enter a pre-heat zone, where the
temperature of the board and all the components is
gradually, uniformly raised.
•The boards are then conveyed into the reflow
•The boards then enter a zone where the temperature is
high enough to melt the solder particles in the solder
paste, bonding the component leads to the pads on the
•If the circuit board is double-sided then this printing,
placement, reflow process may be repeated using either
solder paste or glue to hold the components in place.
During infrared soldering, the energy for heating up the solder joint will be
transmitted by long or short wave electromagnetic radiation
•No compressed air
•No componentspecific nozzles (low
•Fast reaction of
•Central areas will be heated
more than peripheral areas
•Temperature can hardly be
controlled, peaks cannot be
•Covering of the neighboured
components is necessary to
prevent damage, which
requires additional time for
•Surface temperature depends
on the component's reflection
characteristics: dark surfaces
will be heated more than
CONVENTIONAL HOT GAS SOLDERING
During hot gas soldering, the energy for heating up the solder joint will be transmitted
by a gaseous medium. This can be air or inert gas (nitrogen)
hot gas and nitrogen
nozzles allow high
reduced process time
•Thermal capacity of the heat
generator results in slow
reaction whereby thermal
profiles can be distorted
•A rework process usually
undergoes some type of error,
either human or machinegenerated, and includes the
1. Melt solder and component
2. Residual solder removal
3. Printing of solder paste on
PCB, direct component
printing or dispensing
4. Placement and reflow of
REFLOW v/s WAVE SOLDERING
• Reflow soldering is the most common method of attaching surface mount
components to a circuit board, although it can also be used for throughhole components.
• Because wave soldering can be simpler and cheaper, reflow is not
generally used on pure through-hole boards.
• When used on boards containing a mix of SMT and THT components,
through-hole reflow allows the wave soldering step to be eliminated.
• As through-hole components have been largely replaced by surface
mount components, wave soldering has been supplanted by reflow
soldering methods in many large-scale electronics applications. However,
there is still significant wave soldering where SMT is not suitable (e.g.,
large power devices and high pin count connectors).
• Many different types of solder that can be used in wave soldering, tin/lead- based
solders are the most common.
• But these are replaced by Lead free solder pastes, Because of highly toxic in nature.
• The next best metals to use are nickel, brass, aluminium, tungsten, and lastly steel.
Lead free solder
Pure tin solder
• Most common composition of solder is 63% tin, 37% lead. And another one is 11%
tin, 37% lead, 42% bismuth, and10% cadmium.
CLEANING SMT ASSEMBLIES
• After soldering, the boards may be washed to remove flux residues
and any stray solder balls that could short out closely spaced
• require cleaning regardless of the solder flux type used to ensure a
thoroughly clean board.
• Rosin flux is removed with fluorocarbon solvents, which require
extra rinsing or drying cycles.
• . Water soluble fluxes are removed with deionized water and
detergent, followed by an air blast to quickly remove residual water.
• Proper cleaning removes all traces of solder flux, as well as dirt and
other contaminants that may be invisible to the naked eye.
REPAIR / REWORK
•Finally, the boards are visually inspected for missing or misaligned components
and solder bridging.
•If needed, they are sent to a rework station where a human operator corrects
• They are then sent to the testing stations to verify that they operate correctly.
Fix problems and add parts that can’t survive
the high temperature of the reflow oven
Reworking usually corrects some type of
error, either human- or machine-generated,
and includes the following steps:
•Melt solder and remove component (s)
•Remove residual solder
•Print solder paste on PCB, directly or by
•Place new component and reflow.
Thoroughly clean site and solder
new device to PBC
•A specially formulated alloy in wire form is designed to melt at the low
temperature of around 136 degrees F, 58 degrees C. It eliminates the
potential for damage to the circuit, adjacent components, and the
•Liquid flux and a soldering iron are used to melt this low temperature
alloy that is specially formulated to stay molten long enough to react
with existing solder. The SMT device can then be easily removed with a
Apply Low Residue Flux
to all the leads on the
SMD you're removing
With a soldering iron,
melt the low
Easily lift device off
the board with a
Surface-mount components are usually smaller than their counterparts with leads, and
are designed to be handled by machines rather than by humans. The electronics industry
has standardized package shapes and sizes (the leading standardisation body is JEDEC).
SURFACE MOUNT ADVANTAGES
Surface mount technology has been used on hybrid circuits for many years. Its greatest
recent advance has been its use on printed circuit board (PCB) assemblies.
The benefits of this technology as applied to PCB’s are substantial:
• Ease in automation results in higher production
throughput and better yields.
• Closer spacing of components coupled with the
ability to use both sides of the PCB for component
mounting results in significant savings in board real
• Reduced board cost by elimination of plated
through-holes, a major portion of PCB costs.
• Simpler and faster automated assembly. Some
placement machines are capable of placing more
than 136,000 components per hour.
• Better mechanical performance under shake and
• Many SMT parts cost less than equivalent throughhole parts.
SURFACE MOUNT DISADVANTAGES
Manual prototype assembly or component-level
repair is more difficult and requires skilled operators
and more expensive tools, due to the small sizes and
lead spacings of many SMDs.
SMDs cannot be used directly with plugin breadboards requiring either a custom PCB for
SMT is unsuitable for large, high-power, or highvoltage parts, e.g. transformers so It is common to
combine SMT and through-hole on one side.
Unsuitable for connectors that are used to interface
with external devices that are frequently attached
Solder joint dimensions in SMT quickly become much
smaller as advances are made toward ultra-fine pitch
technology. The reliability of solder joints become
more of a concern, as less and less solder is allowed
for each joint prone to vibrations and temperatures.
At last we conclude that this technology will be very
useful & effective in the fabrication of electronics
circuits on PCB.
• Through hole component technique has been
replaced by SMT, with higher I/O capacity, and high
• Weight, size and volume reduced.
• Surface mounting lends itself well to a high degree of
automation, reducing labour cost and greatly
increasing production rates.
• SMDs can be one-quarter to one-tenth the size and
weight, and one-half to one-quarter the cost of
equivalent through-hole parts.
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