What is Surface Mount Technology (SMT).pdf

A guide to surface Mount Technology (SMT)
Electronics manufacturing process

1. A Guide to Surface Mount Technology
(SMT)
Certification Course Content available on :
https://www.udemy.com/course/introduction-to-electronics-manufacturing-process
1: What is Surface Mount Technology?
Surface Mount Technology (SMT) is the primary method used today for building electronic
circuits. In simple terms, it’s the process of mounting electronic components directly onto the
surface of a Printed Circuit Board (PCB).
This is a major shift from the older method, known as Through-Hole Technology (THT), where
components had long wire leads that were inserted into holes drilled through the PCB and
soldered on the opposite side.
In SMT, the components, known as Surface Mount Devices (SMDs), have small metal tabs or
contacts. These contacts are soldered directly to pads (or "land patterns") on the surface of the
board. This single change revolutionized electronics, allowing for the creation of the small,
powerful, and complex devices we use every day, from smartphones to medical implants.

Chapter 2: SMT vs. Through-Hole Technology (THT)
The move from THT to SMT was driven by the need for miniaturization and automation. Here’s
a direct comparison:
Feature Surface Mount Technology
(SMT)
Through-Hole Technology
(THT)
Mounting Components (SMDs) solder
directly to the board's
surface.
Component leads are
inserted through holes and
soldered underneath.

2. Size & Density Extremely high.
Components are tiny,
allowing for densely packed
boards. Both sides of the
PCB can be used easily.
Low. Components are bulky,
and the holes take up
valuable board space, limiting
density.
Automation Highly automated.
Pick-and-place machines can
place thousands of
components per hour,
lowering costs for mass
production.
Difficult to automate.
Requires manual insertion or
complex robotic systems,
making it slower and more
expensive.
Mechanical Strength Lower. Solder joints on the
surface are not as robust
against mechanical stress or
vibration.
Very high. The component
leads going through the
board create a strong
mechanical bond.
Performance Excellent. Shorter
connections mean lower
resistance and inductance,
which is ideal for
high-frequency and
high-speed circuits.
Good, but limited. Long
leads can act as antennas,
causing noise and signal
integrity issues at high
frequencies.
Prototyping & Rework Difficult. Tiny components
require specialized tools (like
hot-air stations) and skill to
solder or repair by hand.
Easy. Large components and
strong joints make it ideal for
hand-soldering, prototyping,
and repairs.
Best For Mass-produced, compact
electronics: smartphones,
laptops, medical devices, IoT
sensors.
High-reliability, high-power, or
mechanically-stressed
applications: power supplies,

3. industrial connectors,
aerospace.

Chapter 3: The SMT Assembly Process
The SMT assembly process is a highly automated and precise, three-stage workflow.
Step 1: Solder Paste Printing
The process begins with a bare PCB. A machine called a solder paste printer uses a
stainless-steel stencil, which is like a silk-screen for electronics, to apply a precise amount of
solder paste to the component pads. This paste is a sticky, grey mixture of tiny solder spheres
and flux. The flux cleans the surfaces, and the paste temporarily holds the components in place.
Step 2: Component Placement
Next, the board moves to a pick-and-place machine. This high-speed robot is the heart of the
SMT line. It uses vacuum nozzles to pick individual SMDs from reels and trays and places them
onto the solder-pasted pads with incredible speed and precision—often placing tens of
thousands of components per hour. The sticky solder paste holds the components in position as
the board moves.
Step 3: Reflow Soldering
Once all components are placed, the board travels through a long reflow oven. This oven heats
the board in a carefully controlled temperature profile:
1.​Preheat: Gradually warms the board to activate the flux and prevent thermal shock.
2.​Soak: Allows the entire board to reach a uniform temperature.
3.​Reflow: Briefly heats the board to the solder's melting point (e.g., above 217°C for
lead-free solder). The solder spheres in the paste melt, forming liquid solder.
4.​Cooling: Cools the board down, solidifying the solder into a clean, strong, and
conductive joint between the component and the PCB pad.

Chapter 4: Common Surface Mount Devices (SMDs)
SMDs are the building blocks of SMT. They are designed to be small, lightweight, and easily
handled by automated machinery. They are broadly categorized into two types:
Passive SMDs

4. These are the most common components on any PCB.
●​Resistors & Capacitors: These often look like tiny, rectangular "chips," black for
resistors (with a code indicating their value) and brown for capacitors. Their package
sizes are standardized by codes like "0805" (0.08 x 0.05 inches) or "0402" (0.04 x 0.02
inches). ●​Inductors & Diodes: These also come in small chip packages. Diodes will have a small
line or dot to indicate their polarity (direction).
Active SMDs (Integrated Circuits)
These are the "brains" of the circuit.
●​Small Outline Transistor (SOT): A small, black plastic package with three or more
leads, commonly used for transistors and voltage regulators.
●​Quad Flat Package (QFP): A square package with "gull-wing" leads extending from all
four sides. This is a common package for microcontrollers and other complex chips.
●​Ball Grid Array (BGA): An advanced package for high-pin-count components like
processors and FPGAs. Instead of leads, it has a grid of tiny solder balls on its
underside. This allows for hundreds or even thousands of connections in a very small
space, but it makes inspection and rework impossible without special equipment.

Chapter 5: Quality Control: Inspection & Rework
Since SMT components are so small and often densely packed, quality control is critical and
highly automated.
Inspection
After the reflow process, boards are automatically inspected to find any defects.
●​Automated Optical Inspection (AOI): An AOI machine uses high-resolution cameras
and bright, multi-angle lighting to visually scan the board. It compares the finished board
to a "golden" reference board and flags potential defects like missing components,
incorrect polarity, shifted parts, or solder bridges (unwanted solder connecting two
pins).
●​Automated X-ray Inspection (AXI): For components like BGAs where the solder joints
are hidden under the chip, AOI is useless. An AXI machine uses X-rays to see through
the component and inspect the quality of the solder balls underneath. It is essential for
catching defects like voids (air bubbles in the solder) or hidden bridges.
Rework
If a defect is found, the board may be sent to a rework station. Repairing SMT components is
not a job for a standard soldering iron.

5. ●​General Rework: A technician uses a hot-air rework station, which blows a precise,
temperature-controlled stream of air to melt the solder of a single component. The faulty
component is removed with tweezers, the pads are cleaned, and a new component is
carefully soldered in its place. ●​BGA Rework: This is a much more complex process. A BGA rework station uses a
combination of a top-side heater for the chip and a large bottom-side pre-heater for the
entire board. This prevents the board from warping due to thermal stress. Once
removed, the BGA chip may need to be "reballed" (a new set of solder balls applied)
before it can be re-attached.

Chapter 6: Design for Manufacturability (DFM)
Engineers can't just design a circuit; they must design it so it can be manufactured efficiently.
This is called Design for Manufacturability (DFM).
●​Land Patterns: The copper pads on the PCB must be designed perfectly to match the
component's leads. If the pads are too large or too small, it can cause soldering defects.
●​Component Placement:
○​Spacing: Components must have enough clearance between them for the
pick-and-place nozzles and to prevent solder bridges.
○​Orientation: Polarized components (like diodes and some capacitors) must be
oriented correctly. Keeping all similar components in the same orientation (e.g.,
all "0805" resistors horizontal) speeds up the assembly machine.
○​Grouping: Keeping related components (like a microcontroller and its bypass
capacitors) close together improves electrical performance.
●​Thermal Management: High-power components get hot. Designers use techniques like
large copper planes (heat spreaders), thermal vias (holes that transfer heat to other
layers), and mounting points for heat sinks to dissipate this heat and prevent the
component from overheating. ●​Via-in-Pad (VIP): In extremely dense designs (like for BGAs), designers may place vias
directly inside the component pad. This saves space but requires a more complex PCB
manufacturing process where the via is filled and plated flat to create a reliable surface
for soldering.

Chapter 7: Modern Applications of SMT
Surface Mount Technology is the enabling technology behind virtually all modern electronics.
●​Consumer Electronics: This is the most obvious example. Smartphones, tablets,
laptops, smartwatches, and digital cameras all rely on SMT to pack immense processing
power into tiny, thin devices.

6. ●​Automotive: Modern cars are filled with electronic modules. SMT is used for Engine
Control Units (ECUs), Battery Management Systems (BMS) in electric vehicles,
infotainment systems, and advanced driver-assistance (ADAS) sensors.
●​Medical Devices: SMT's high reliability and miniaturization are critical for life-saving
devices. This includes implantable devices like pacemakers and cochlear implants, as
well as diagnostic tools, patient monitors, and medical imaging systems.
●​Telecommunications & IoT: The entire infrastructure of our connected world, from 5G
base stations and Wi-Fi routers to the billions of tiny, low-power Internet of Things (IoT)
sensors, is built using SMT.
●​Industrial & Aerospace: While THT is still used for high-power components, SMT is
used for the complex control and processing circuits in factory automation, robotics, and
avionics systems.