DVD-64C v.2a Transcript
DVD-64C v.2a
Component Identification
Below is a copy of the narration for DVD-64C. The contents for
this script were developed by a review group of industry experts and
were based on the best available knowledge at the time of
development. The narration may be helpful for translation and
technical reference.
Copyright  IPC – Association Connecting Electronics Industries. All Rights Reserved.
Introduction
Many things can cause an electronic component on a circuit board to fail. Sometimes the
component is defective. Occasionally the soldered connections are made incorrectly. But a lot of
the time component failures are due to improper component identification – meaning an incorrect
component is mistakenly attached to the circuit board, or the correct component is attached with
an improper orientation.
Knowing how to accurately identify components is critical for people working in the electronics
assembly industry.
This video training program will examine the various types of through-hole and surface mount
components you’ll be encountering. Through-hole components have leads that are designed to be
inserted through mounting holes in the circuit board. Leads are rigid metal wires that extend out
from the component. Through-hole components may have axial leads, also referred to as arms;
or radial leads, also called legs. Surface mount components are designed to be placed directly
onto lands that serve as mounting points on the surface of the board. They come in a variety of
leaded and leadless packages.
Let’s begin by taking a look at some basic information about components. Components are
devices that control the flow of electricity so that an electronic product does what it’s designed to
do. The most important components are semiconductors, and are called active components.
Semiconductors have properties of both conductors and insulators. A conductor is a metal or
material that will allow electricity to flow through it. An insulator is any material that has a high
resistance to the flow of electrical current.
Some components are active – meaning they can amplify or interpret a signal. Active components
include diodes, transistors and integrated circuits, also known as ICs. Other components are
passive – meaning they cannot change an electrical signal – except to reduce it in size or delay it.
An example of a passive component is a capacitor.
When a component is packaged with only one or two functional elements it is called a discrete
component. An example of a discrete component is a resistor that performs the simple function
of limiting the electrical current that flows through it. On the other hand, an IC is a group of
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interconnected elements assembled into a single package that performs multiple functions. A
well- known example of a complex IC is the microprocessor found in computers.
At this point, let’s examine some of the important factors you’ll need to be aware of to begin
identifying and understanding components. We’ll start with component marking. Every
manufacturer of electronic components uses a slightly different system for marking components.
This marking will either be on the component – or on the component packaging. Typically, this
information includes the manufacturer’s name or logo; the component part number or ordering
code; the lot code; the date code; and the country where it was manufactured.
For example, let’s look at the marking on this Ball Grid Array from Freescale Semiconductors.
As you can see, the logo is clearly marked on the component. The part number is MPC8270. If
you were to search that number on the internet, you’d learn that this component is an Integrated
Communications Processor. The letter C refers to the temperature range and ZU refers to the
package type. The letters UPE refer to the processor speed. The last letter on the first line is the
revision level of the die, or chip. The second line specifies the three frequency rates of the
processor. The third line is very important to understand because it contains the lot code and the
date code. These codes allow us to track the manufacturing information for the product in the
event a particular component has a problem and needs to be recalled. In this case, QQ tells us
where these chips were fabricated and assembled. Q is the code for the Freescale manufacturing
facility in Kuala Lampur, Malaysia. The letters KZ specify the lot number. The last four digits
tells us the year and work week that the component was manufactured. The last important
marking is underneath the lot and date code and tells us that the device was manufactured in
Malaysia. You don’t really need to know all the details related to component marking, but this
information is available in case the source of a problem needs to be identified.
Now, let’s take a look at documentation. Every circuit board assembly assembly comes with
documentation that includes a bill of materials – and an assembly drawing. The bill of materials
defines all of the components used in the assembly – by manufacturer and part number, or using
an internal company part number. The bill of materials may also reference the type of component
– its size, value, voltage, wattage and composition.
In addition, there is a component reference designator, or CRD, assigned to each component on
the circuit board. For example, resistors utilize the letter R as a reference designator. If a circuit
board contains 30 resistors, they are usually identified as R1 through R30 – even if they all have
the same part number or value
You can see the importance of reference designators when we look at the actual assembly
drawing. This drawing shows the components and their location on the circuit board. The C
designations on the drawing usually refer to capacitors – and the U designations usually refer to
ICs. We’ll discuss reference designators in more detail when we address the individual
components. The important thing to remember is that the assembly drawing shows the location
of the component by CRD. The bill of materials relates the CRD to a specific component part
number. In addition, the component side of the circuit board is usually silk-screened with the
identical markings from the assembly drawing. This makes the component identification and
insertion process more clear.
Another method of identifying some components is by their value. A value is a numerical
quantity given to the component. This value is usually assigned a plus and minus tolerance
which is the variation allowed from that value. For example, resistor values are units of resistance
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measured in ohms. If a 250-ohm resistor has a 5 percent tolerance, its acceptable operating range
would be 237.5 to 262.5 ohms. We’ll talk more about specific component values and tolerances
later in this video.
Now that you see how the bill of materials and assembly drawing are used to identify the proper
components and their locations on the circuit board, let’s examine another important topic in
component identification – the direction, or orientation in which the component needs to be
installed.
Some components have a positive and negative connection to the circuit board -- and the leads
must be inserted into the positive and negative holes identified on the circuit board and assembly
drawing. Components with this positive and negative connection have what is called polarity.
This works the same way as the positive and negative terminals on a battery. For example, a
flashlight battery has to be inserted with the terminals in the proper position for the flashlight to
work.
For components that have polarity, the positive lead is called the anode and the negative lead is
called the cathode. The symbol for the positive anode is the plus sign and the symbol for the
negative cathode is the minus sign. A plus or minus sign is usually marked on the component, as
well on the assembly drawing and on the component side of the circuit board.
The direction in which a component is installed is also important for components with multiple
leads – even if they do not have polarity. Proper orientation means that pin 1 of the component is
in the same position as the pin 1 hole on the circuit board. The board usually has a square pad, or
a symbol or screened outline of the component shape to show correct orientation.
IC manufacturers use a variety of methods to identify pin 1 on the component package. Markings
may include a notch, a wedge, a dimple, a different edge or corner shape, a stripe or the actual
number. When viewed from above, if the mark is at the top of the component, pin 1 is always to
the left. The numbering of the other pins always proceeds in a counter-clockwise direction
around the component.
When a component is not oriented correctly, the assembly won’t function as intended and will fail
electrical tests. When a component is inserted with polarities reversed, the component can
actually be destroyed. There may also be damage to the circuit board. The result can be costly
repair work, or even a scrapped assembly.
Identifying Through-Hole ICs
Now let’s take a look at the different types of through-hole ICs. Remember, an IC is a group of
interconnected elements assembled into a single package. Common IC packages include singlein-line packages, or SIPs; dual-in-line packages, or DIPs; IC cans; and pin grid arrays, or PGAs.
SIPs consist of a single row of leads and are often resistor networks or diode arrays. The
reference designator for a particular SIP is based on the function. For example, resistor networks
will usually use the letters RN. In many cases a dot or number will indicate pin 1 for orientation.
DIPs are the most common through-hole IC. The DIP consists of two rows of leads and the body
is made of plastic or ceramic. DIPs usually come from the manufacturer in tubes. The tubes
protect the components from ESD and lead damage, and are designed to load the components into
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the feeders of automatic insertion machines for component placement. The most common
reference designator for an IC such as a DIP is the letter U. The orientation marking for DIPs is
either right over pin 1, or on the end where pin 1 is found. As we mentioned earlier, the most
common orientation markings are a notch, wedge, dimple, stripe or number – and the leads are
counted counter-clockwise from pin 1.
IC cans are often transistors or voltage regulators. The cans have a number of long radial leads.
TO-5 and TO-92 are among the most common sizes of IC cans. The reference designator will
depend on the function of the IC can. Typical reference designators may include a U for a
general IC, Q for a transistor, AR for an amplifier or VR for a voltage regulator. IC cans contain
a tab located over the highest numbered pin, or between the highest numbered pin and pin 1. If
you face the tab and look down at the IC can, pin 1 will be located to the right of the tab.
Our last through-hole IC is the most complex. Pin grid arrays have several rows of leads
extending from the bottom of the IC. The rows make up a grid of connection points. PGAs come
in plastic or ceramic packages. For orientation, there is usually a notch or mark directly over pin
1. PGAs are often installed into sockets that have been soldered onto the circuit board. The pin 1
location on the socket will also be marked in the same way as the PGA for proper alignment.
Common Axial and Radial Leaded Components
In this section we’ll be discussing resistors, capacitors, diodes and transistors. Resistors are a
common passive component. A typical circuit board assembly will contain many resistors. The
letter R is the reference designator. Resistors limit the flow of electrical current in a circuit. This
resistance to the flow of current is measured in ohms. The value and tolerance of the resistor is
either printed on the component, or is calculated by decoding color bands into numbers. The IPC
Component Identification Training and Reference Guide, IPC-DRM-18, contains a color code
chart for resistors – and an explanation of how to read the color bands. It may be important that
you understand how to decode the color bands so that you can determine the value and tolerance
of a particular resistor. This way -- you can be sure the correct resistor has been installed on the
circuit board.
Another type of resistor is called a variable resistor, or potentiometer. They may also be called
trimmers or trim pots. Variable means that the value, or resistance of these types of components
can be changed or adjusted, either by turning a shaft or sliding a contact. The maximum value is
usually molded into the component body. Along with the letter R, the reference designator for
variable resistors may be VAR or ADJ. Variable resistors typically have a non-symmetrical lead
pattern that only allows them to be installed in one way.
Now let’s examine capacitors. Capacitors are the most widely used component in electronics.
They also have the greatest variety of shapes and sizes. The basic function of a capacitor is to
store and discharge electricity. A unit of electrical capacitance is called a farad. The three most
common units of measure using farads are microfarads, nanofarads and picofarads. The picofarad
has the smallest capacitance value.
The value and tolerance of the capacitor is either printed on the component, or is calculated by
decoding color bands or coded numbers into numerical values. As with resistors, the Component
Identification Training and Reference Guide contains a color code chart for capacitors and an
explanation of how to read the color bands or number code. Again, it may be important that you
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understand how to decode the color bands and numbers so that you can determine the value and
tolerance of a particular capacitor.
Capacitors are either non-polarized or polarized and are designated by the letter C. Nonpolarized capacitors can be installed in either direction. They are usually small and have low
electrical values. They come with either axial or radial leads.
Polarized capacitors also come with either axial or radial leads. The positive lead, or anode will
usually be indicated by a plus sign. This same plus sign is usually on the circuit board. There
may be other types of markings to identify polarity on the capacitor.
There are also variable capacitors. These types of capacitors can be tuned to a particular value –
depending on the function required. The part number and value range is usually marked on the
component. Along with the letter C, the reference designator for variable capacitors may be
CVAR or CADJ. Variable capacitors often have a non-symmetrical lead pattern that only allows
them to be installed in one way. Sometimes the component shape is outlined on the assembly
drawing to show orientation.
Now let’s turn our attention to diodes. Diodes are semiconductors that allow electrical current to
flow in only one direction. The reference designator for diodes is CR or D. All diodes are
polarized components – meaning it is important to install them correctly. Polarity is usually
indicated by a colored ring, or up to three rings, on the cathode or negative end. The diode
symbol may also point to the negative end.
Another type of diode is called a light emitting diode, or LED. LEDs emit light – and are
typically used as display indicators on the front panels of electronic equipment. The reference
designator for an LED is usually DS, although they may be identified by LED, or by the diode
designators CR or D. LEDs usually have radial leads. Polarity is typically indicated by the
location of the cup, or flag – inside the lens. The cup is associated with the cathode, or negative
lead. In some LEDs, the negative lead will be shorter, or there may be a plus symbol on the
positive side.
The last component we’ll be examining in this section is the transistor. Transistors are discrete
components that amplify electrical signals. The reference designator for a transistor is usually the
letter Q. Orientation may be accomplished by matching marked pin numbers on the transistor to
pin numbers on the drawing or circuit board; by locating pin 1 to the right of the tab on a
transistor can; or by matching the component shape with the outline on the circuit board or
assembly drawing. Sometimes the pattern of holes on the circuit board indicates there is only one
way to install the transistor.
Other Through-Hole Components and Hardware
This section will examine the remaining through-hole components and connecting hardware.
They include inductors, transformers and other coiled components, filters, fuses, switches,
crystals, voltage regulators, thermistors, connectors, sockets, headers and jumpers.
We’ll start with inductors. Inductors consist of a coil of wire that creates a magnetic field when
current flows through the coil. The reference designator for an inductor is the letter L. The
magnetic field produced by an inductor is measured in henrys – either microhenries or
milihenries. The inductor value may be either printed on the body of the component, or
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calculated by decoding the color bars on the component’s body into numbers. Again, the
Component Identification Training and Reference Guide contains a color code chart for inductors
and an explanation of how to read the color bands. There is no required polarity for inductors,
and they usually can only be installed one way.
Closely related to inductors are transformers and other coiled components. These components
also create magnetic fields. Transformers and torroids use the letter T as a reference designator.
Chokes share the letter L with inductors. There’s usually only one way to install transformers,
and the part number and value may be marked on the component. Watch out for those coiled
components that are designed without that information. Variations in wire gauge, number of
turns of wire, wire spacing, internal diameters, numbers of wires, and sometimes the use of ferrite
or other types of cores can change the part numbers and values. Check with your supervisor
when you have trouble identifying a component.
Now let’s take a look at filters. The function of a filter is to allow some signals to pass – and to
block, or filter out other signals. The reference designator for a filter is FL and there is usually
only one way to insert the leads into the holes of the circuit board.
Fuses and circuit breakers protect a circuit from electrical overload by intentionally failing before
the overload gets to the circuit. A circuit breaker is a fuse that can be reset after failing. The
reference designator for a fuse is the letter F. CB designates a circuit breaker. These components
are measured in amps and volts. The value is usually marked on the component along with the
part number.
Next we’ll talk about switches and relays. Switches and relays open and close circuits. Relays are
electrically operated switches. The reference designator for a switch is the letter S or SW. A
relay is usually designated by the letter K. These components are measured in amps and volts,
and utilize a dot or notch for orientation.
Our next component is a crystal. Crystals produce a consistent electrical pulse – similar to the
ticking of a clock. They are available in through-hole or surface mount packages. Crystals are
usually designated by the letter Y. They are measured in frequency – such as kilohertz or
megahertz. A chamfered corner or a dot indicates proper orientation.
Now let’s examine voltage regulators. Voltage regulators keep voltage constant in a circuit by
minimizing peaks and valleys. The reference designator is VR and there is usually an angle on
the body or an indented dot for orientation.
The last electronic component we’ll discuss is the thermistor. A thermistor is a resistor that
changes value based on the temperature of the component. The reference designator for a
thermistor is RT. It can be inserted into the circuit board in any orientation.
Let’s complete this section by discussing connecting hardware. This hardware is available in
both through-hole and surface mount package styles.
Connectors are attached to a circuit board so that wires, cables and other outside connections can
be made to the assembly. The reference designator for a connector is the letter P. Connectors
also use the letter J as a reference designator.
Another type of connecting hardware is a socket. Sockets are soldered onto a circuit board so that
an IC or PGA can be plugged into the socket without being soldered. This makes it easier to
remove and replace a component that might be upgraded or changed in the future. The reference
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designator for a socket is some variation of the letter X. Sockets usually have a notch or dot to
indicate orientation. The component being plugged into the socket will have an identical
orientation marking.
The next type of connecting hardware we’ll discuss is the header. Like connectors, they are used
for outside connections. Headers usually don’t have a housing around their pins. They also use
the letter J or P as a reference designator. Headers usually have alignment or locking tabs for the
connector which mates with it.
The last type of connecting hardware we’ll examine is the jumper. Jumpers connect two pins on
an assembly together -- providing an electrical path between those points. The reference
designator for a jumper is W or E when the jumper is a wire and the letter P when the jumper is a
plug.
Surface Mount Overview
Surface mount components are designed so they can be placed directly onto lands on the circuit
board -- as opposed to being inserted through mounting holes in the board. They have the same
types of electronic functions as the through-hole components. Many through-hole components
have surface mount equivalents -- that are much smaller in size. What this means is that many
more electronic functions can be present in a surface mount component that is the same size as a
through-hole component.
Surface mount devices use the same system of part numbers, reference designators and
component values and tolerances as through-hole components. And the indicators for polarity
and pin 1 orientation are also similar. What’s different is that surface mount assembly is more
compatible with automated processes than through-hole assembly. Because of their size and
shape, many types of through-hole components require manual insertion. Almost all surface
mount components can be placed by machine.
The components come packaged from the manufacturer in one of three forms – on tape and reel,
in tubes or in waffle trays. Let’s take a look at how each of these methods of packaging is loaded
onto a placement machine. Notice how the operator checks the part number on the reel against
the bill of materials. That’s because these components are too small to have part numbers marked
on them. In addition, polarity or pin 1 orientation should be verified before loading tubes into
feeders. It’s also important to check pin 1 orientation when loading trays.
Another difference is that surface mount components don’t have long leads. That’s because they
don’t attach to holes through the circuit board. Instead, they attach to lands on the surface of the
board with either special leads or leadless terminations.
We’ll talk about leaded components first. Surface mount components have four styles of leads –
gull wings, J-leads, L-leads, and I-leads. Gull wings and J-leads are the most common.
The gull wing lead is a metal lead that bends down and away – similar to a seagull’s wing. Some
gull wing components are said to have fine pitch. Pitch is the distance between the center of one
lead to the center of the next. Fine pitch means that the leads are spaced very close together.
J-leads have metal leads that bend down and underneath a component in the shape of the letter J.
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L-leads are found on molded tantalum capacitors and are inward formed, almost touching the
body of the component. I-leads are actually through-hole leads that have been cut short for
surface mounting. The end of the modified lead is placed directly on the surface mount land. As
you can imagine, this connection won’t be as strong or stable as the other types of surface mount
leads and IPC standards do not permit their use on high reliability products.
Next, let’s take a look at the leadless types of surface mount connections. Leadless means there
are no metal leads sticking out of the component body. Instead, the components are attached to
the circuit board by soldering to some type of metallized termination on the component.
The most common type of metallized termination component is the chip component -- with three
or five sided terminations at opposite ends of the component’s body.
Castellations are another form of leadless connection. These half round metallized recesses in the
side of the component are filled with solder when connected to the circuit board land.
In addition, there are “no lead” components. These packages come in two styles – Dual Flat No
Lead, or DFN; and Quad Flat No Lead, or QFN. Both DFNs and QFNs have terminal pads
underneath the component that are soldered directly onto circuit board lands.
The last type of leadless connection is the Array type component. Ball Grid Arrays, or BGAs,
consist of parallel rows of tiny balls of solder on the bottom of the component. These solder balls
are connected to matching rows of lands on the circuit board. There are also Ceramic Column
Grid Arrays, or CCGAs, that connect to matching lands on the circuit board. Another array type
component is the Land Grid Array, or LGA that has a series of flat land terminations that are
soldered directly to matching circuit board lands.
Identifying Surface Mount ICs
In this section we’ll be examining the surface mount ICs. We’ll start with the large scale ICs.
The larger scale ICs usually come from the manufacturer in waffle trays. These complex
components may be packaged in several package styles. The two most popular of these styles are
quad flat packs, or QFPs and plastic leaded chip carriers, or PLCCs.
We’ll talk about QFPs first. The body of a QFP is usually plastic. Sometimes the bodies are
ceramic or metal. QFPs have gull wing leads on all four sides. There are anywhere from a few to
hundreds of these leads. The body of the component is usually marked with a part number, date
code, place of manufacture and the logo of the manufacturer. The reference designator for QFPs,
like all other surface mount and through-hole ICs, is also indicated by the letter U. Orientation
for QFPs is indicated by a dot or a beveled edge over the number 1 lead. Sometimes there may
be a chamfered corner or a stripe for pin 1 orientation.
Now, let’s look at PLCCs. PLCCs are also a popular large scale IC package style. They have up
to 100 J-leads on each of the four sides. Notice that J-leads are not spaced as closely together as
the gull wing leads on fine pitch QFPs. Orientation markings for PLCCs are not the same as for
QFPs. A center dot indicates pin 1.
Next, let’s discuss some of the large scale leadless ICs. These include Leadless Chip Carriers, or
LCCs; QFNs; and the Array style components – BGAs, CCGAs and LGAs.
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LCCs are most commonly used in aerospace applications because they are extremely rugged.
Smaller, hybrid varieties are currently being designed and used for military and
telecommunications applications. As we mentioned earlier, LCCs utilize half round metallized
recesses in the side of the component called castellations. There can be up to 124 castellations on
the larger LCCs. Like the leads on ICs, castellations are counted counter-clockwise from the pin
1 orientation mark.
The QFN is a cousin to the QFP. Instead of having gull wing leads that extend out from the
component, the QFN contains perimeter lands on the bottom that connect to corresponding lands
on the circuit board. The bottom of the package also contains an exposed thermal pad that
improves heat transfer from the IC to the circuit board. QFNs can have a single row of lands – or
multiple rows of lands. These lands are counted counterclockwise from the number one pad –
starting with the row on the component edge.
The other large scale leadless ICs are the array style components. We’ll start with the BGA.
Instead of leads, the BGA contains from 25 to 625 or more tiny solder balls on the bottom of the
component. These solder balls connect to a matching set of lands on the circuit board. The body
of the BGA may be made of plastic, ceramic or metal. BGAs utilize the same orientation
markings as the other large scale ICs.
A cousin to the BGA is the CCGA. As stated earlier, CCGAs have solderable columns instead of
the tiny solder balls to connect to the matching lands on the circuit board. CCGA packages are
becoming increasingly popular as an alternative to ceramic BGA packages for applications
requiring very high-density interconnections with higher board level reliability.
The last type of array style component we’ll mention is the Land Grid Array, or LGA. For
identification purposes, an LGA is simply a ceramic BGA or CCGA without the balls or columns.
There are simply a series of flat pads, or lands, that are soldered to corresponding lands on the
circuit board.
At this point, let’s take a look at the smaller scale ICs. These smaller packages with leads on two
sides are called Small Outline Integrated Circuits, or SOICs. SOICs are the surface mount
equivalents of through-hole DIPs. These components come from the manufacturer in tubes or on
tape and reel.
SOIC bodies are most commonly plastic -- and have gull wing leads. When they are provided
with J-leads, they are referred to as SOJs. Like the larger scale surface mount ICs, orientation is
indicated by a dot or beveled edge over the number 1 lead, or by an end notch or stripe on the
body of the component.
Other styles of SOICs are all gull wing leaded components that usually begin with the letters SO
for small outline. The remaining letters refer to the size of the component package, or to the
thickness of the body, where T refers to thin packages.
A leadless cousin to the SOIC is the DFN. DFNs are smaller versions of QFNs since they have
two rows of terminal pads, rather than four. The bottom of the package also contains an exposed
thermal pad that improves heat transfer from the IC to the circuit board. The lands are counted
counterclockwise from the number 1 terminal pad.
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Identifying Chips, MELFs, SOTs and DPAKS
In this last section we’ll be discussing chip components, MELFs, transistors and diodes. We’ll
start with chip components. Chip components usually have ceramic bodies with metallized
terminations on either end. The most common types are chip resistors and chip capacitors.
Inductors have recently been introduced in this style package. These components are usually
very small and come from the manufacturer on tape and reel.
Chip components are typically placed on circuit boards using high-speed chip shooters. Because
the components are so small, there’s often no identification marking on the body. This makes
reading the value of the component impossible. That’s why it’s critical that the part number on
the reel is verified against the bill of materials, and that the feeder containing the reel of
components is loaded into the correct feeder location on the chip shooter.
Like their through-hole counterparts, chip resistors use the letter R as a reference designator and
chip capacitors use the letter C. Chip resistors and capacitors may have numerical value and
tolerance codes, rather than color codes. These codes are sometimes printed on the component
body if the component is large enough. More often, the value and tolerance is printed on the label
of the tape reel, along with the part number. The Component ID Reference Manual contains an
explanation of how to read the number codes for resistors and capacitors.
Another type of chip capacitor is the molded tantalum capacitor. This type of capacitor has
inward formed L-leads that look like metallized terminations. Molded tantalum capacitors are
polarized. Polarity is indicated on the positive, or anode end by a plus sign, a line or the letter A.
The top of the component may also be beveled on the anode end. The physical size of the molded
tantalum capacitor is described by one of four letters: A, B, C or D. A is the smallest and D is the
largest.
Next, let’s examine MELFs. MELF stands for Metal Electrode Leadless Face, and is a type of
package. MELFs have metallized terminations on each end of a cylindrical body. MELF
components include resistors, capacitors, diodes and inductors. Like chip components, MELFs
usually come from the manufacturer on tape and reel. The label on the reel will usually display
the part number.
Reference designators and value coding are pretty much the same for MELFs as for their surface
mount chip and through-hole counterparts. For example, the color band system for specifying
value and tolerance on MELF resistors, capacitors and inductors is the same system that we used
for through-hole components. MELF polarized capacitors usually have a beveled anode to
indicate polarity. Polarity is indicated on MELF diodes by a band on the negative, or cathode
end.
The last surface mount package styles we’ll discuss are small and large transistors and diodes.
Small Outline Transistors, or SOTs, are small rectangular transistor or diode packages. SOTs
usually have three or four gull wing leads on two sides of the package. SOTs come in different
sizes. The most popular SOT is the SOT-23. As with any transistor or diode, the reference
designator is Q for transistors and D or CR for diodes. Orientation of SOTs is determined by the
lead and land patterns. There is only one way to place the component.
Diode Packages, or DPAKs, accommodate more complex groups of large transistors or diodes.
DPAKs have three gull wing leads. Like SOTs, they can only be placed on the board in one
orientation. Both SOTs and DPAKs have equivalent through-hole transistors and diodes.
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Summary
This program has presented many of the most important details of component identification. First,
you were introduced to through-hole components and surface mount components – and took a
look at the relationship between the component, the circuit board, the assembly drawing and bill
of materials, and component reference designators. We also discussed component values and
tolerances, polarity and orientation.
Then we continued with the identification of through-hole components. We covered ICs,
common axial and radial components, and the remaining types of through-hole components and
connecting hardware.
Next, we discussed the differences between through-hole and surface mount components. Then
we identified the surface mount components. First we looked at the larger scale leaded and
leadless ICs. Next, we examined the small outline ICs. And we concluded with a discussion of
chip components, MELFs, SOTs and DPAKs.
Identifying components correctly is one of the most critical jobs in electronics assembly.
Improper component ID can result in damage to the component and circuit board. It’s important
to understand part numbers and reference designators, as well as component values and
tolerances. And you need to be able to verify polarity and pin 1 orientation. Careful attention to
detail will help to make sure that your company’s products work properly and reliably. The more
knowledge you have, the better you can do your job.
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