This article introduces what SMT technology is, the historical origin and development process of SMT, introduces common parts and special parts respectively, and finally talks about assembly technology.
Part 1: Modern Surface-mount Technology (SMT)
Now the majority of PCB assemblies are only available in SMT form.
This is because SMT has a high degree of automation, and the components are quickly transported directly to the assembly machine through the surface mount component reel, which greatly improves production efficiency.
Therefore, under the new assembly technology, the original electronic parts can be fixed on the PCB board better and faster, this will be conducive to large-scale PCB assembly.
How much do you know about SMT components? This blog will take you to learn more about SMT components and packaging methods.
Part 2: Common SMT Components (resistors, capacitors, ICs)
Except for connectors and ports which need mechanical strength and high-current components, almost all electronic components are now available in SMT packages.
We will look at some of the common SMT component types and packages we usually come across in PCBs.
1. Resistors
The most common type of component in a PCB is the resistor.
The surface mount device(SMD) package of a resistor comes as a rectangle with leads on both sides or an array of rectangles with multiple resistors.
- 1. Three-figure coding scheme
- 2. Four-figure coding scheme
- 3. EIA96 coding scheme
Three-figure coding scheme
This scheme is normally used for standard tolerance resistors.
The first two figures are multiplied by the base ten power of the third figure to get the resistance value.
For resistor values under 10, the ‘R’ letter is used to indicate the decimal point.
Four-figure coding scheme
This is very similar to the three-figure scheme but has 3 figures and one multiplier figure.
This scheme is used for higher tolerance resistors. For resistor values under 100, the "R" letter is used to indicate the decimal point.
EIA96 coding scheme
This scheme also has three figures but aims to cover high-tolerance resistors.
It consists of two numbers and one letter which is the multiplier.
But the significant value and multiplier are not straightforward and to read values you need to refer to the standard EIA96 table.
|
Code
|
Multiplier
|
|---|---|
|
Z
|
0.001
|
|
Y or R
|
0.01
|
|
X or S
|
0.1
|
|
A
|
1
|
|
B or H
|
10
|
|
C
|
100
|
|
D
|
1000
|
|
E
|
10000
|
|
F
|
10000
|
|
100000
|
10000
|
Capacitors
There are two main types of capacitors. Which are polar and non-polar.
Polar surface mount capacitors have polarity and capacity markings on the top side and have a common cylindrical capacitor shape.
Most SMD capacitors you would encounter would not have any markings on them.
For example 10uf 0805 ceramic capacitors (middle one)
3. Integrated Circuits (ICs)
The bulk of the surface mount devices are ICs, and they have the largest variety of packages.
The following chart demonstrates most of the surface IC varieties with examples.
| Material |
Type
|
Image
|
Symbol
|
Lead count
|
|---|---|---|---|---|
|
Plastic
|
SOP (Small Outline Package) |
|
MAzxcsdfsdfdsf
|
8, 16 |
|
Plastic
|
SSOP (Shrink Small Outline Package) |
|
MB
|
20, 30, 32, 60, 64, 70 |
|
Plastic
|
TSOP (Thin Small Outline Package) |
|
TA
|
20,24,26,28, 32,40,44,48, 50,54,66,64, 70,86 |
|
Plastic
|
QFP (Quad Flat Pack) |
|
GA
|
44, 56, 64, 80, 100, 128, 160,208, 240,272,304 |
|
Plastic
|
TQFP (Thin Quad Flat Pack) |
|
TB
|
44, 48, 64, 80, 100, 120 |
|
Plastic
|
LQFP (Low-profile Quad Flat Package) |
|
TC
|
144, 176, 208
|
|
Plastic
|
SOJ (Small Outline J-lead) |
|
JA
|
20, 24, 26, 28, 32, 36, 40, 42, 50 |
|
Plastic
|
QFJ (Quad Flat J-leaded) / PLCC (Plastic Leaded Chip Carrier) |
|
JB
|
18, 29, 22, 28, 32, 44, 68, 84 |
|
Plastic
|
BGA (Ball Grid Array) /FBGA (Fine Pitch Ball Grid Array) |
|
LA
|
48, 84, 104, 144, 176, 224, 256, 352, 420, 560 |
|
Plastic
|
FLAG (Fine Pitch Land Grid Array) |
|
LB
|
49, 56, 84, Custom designs
|
|
Plastic |
W - CSP (Wafer Level Chip Scale Package) |
|
FB
|
208, 256 |
|
Plastic
|
HQFP (Heat sunk Quad Flat Pack) |
|
FB
|
208, 256 |
Part3: Special SMT Components
Other than the above discussed there are surface mount devices and components that are custom-designed and uncommon to find.
Some of them are listed below.
|
Type
|
Image
|
Symbol
|
Lead count
|
|---|---|---|---|
|
COB (Chip On Board)
|
|
KA
|
Custom
|
|
TCP (Tape Ca)
|
|
VA
|
Custom
|
|
COT (Chip On Tape)
|
|
KC
|
IC Card
|
COB (Chip on Board)
In this custom design, the IC is mounted directly on the printed board and sealed by a plastic cover.
Usually, there are no specifications written on the cover.
Commonly used in custom low-cost products like toys, the chip is not removable.
TCP (Tape Carrier Package)
These are produced by Tape Automated Bonding (TAB).
These are suitable for high-density multi-thin pin applications.
COT (Chip on Tape)
A microchip is directly mounted and sealed on a substrate.
A common use is SIM cards, ID cards, and bank cards.
Part 4: SMT Component Packages
Almost all SMT package types that exist today aim to ease assembly automation.
Modern PCB manufacturing chooses ratios between SMT to through-hole (TH) to achieve space-saving of 50 to 90 percent.
This paved the path for electronic appliances to get smaller and pack more features.
Currently, there are dozens of SMT package types.
One of the main reasons for the diversity of SMT packages is the number of functions or I/Os per device (integrated circuit) has increased, besides, accommodating these ICs will increase the I/O count and reduce contact pitch.
For Example Fine pitch (0.5mm) and Ultra Fine Pitch (0.4mm - 0.3mm).
Over 50% of all SMT components in the market have an I/O count in the 16–64 I/O range, and only 5-10% contain an I/O count over 200.
Many of the high I/O count devices have area array packaging formats such as ball grid arrays (BGAs).
The heart of the surface mount technology is the machine that precisely places SMT components onto the printed board before soldering.
Surface mount placement machines are more advanced than through-hall (TH) insertion machines and usually capable of handling different SMT component types simultaneously.
The following two tables show the relationship of SMT component package I/O counts to the pitch considering the main two variations.
The first number in the matrix cell reflects the design complexity and the second number reflects the difficulty of the manufacturing process.
To have a general understanding of SMT components and packaging we can refer to IPC National Technology Roadmap for Electronic Interconnections, which published the following overall classification.
|
Class A |
Through-hole (TH) components only
|
|---|---|
|
Class B |
Surface mount devices (SMD) only
|
|
Class C |
A mixture of Through-hole(TH) and simplistic SMD
|
|
Class X |
The complex intermix of Through -hall(TH), simplistic SMD, fine pitch (FP), and ball grid array (BGA)
|
|
Class Y |
The complex intermix of Through -hall(TH), simplistic SMD, Ultrafine pitch (UFP), and chip-scale package(CSP)
|
|
Class Z |
The complex intermix of Through -hall(TH), simplistic SMD, Ultrafine pitch (UFP), Chip On Board (COB), Flip Chip (FC), and Tape carrier package(TCP)
|
Part 5: How to Find a Relieved PCB Components Assembler?
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Conclusion
Components assembly requires extreme Accuracy and stability. If you have read the whole article you should now be aware of rough SMT components.
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