Which is positive and negative in SMD LED?

In the PCBA processing industry, due to the small size of SMD LEDs, low power consumption, long service life, high brightness, environmental protection, sturdiness and durability, suitable for mass production, fast response, shockproof, energy saving, high resolution, shock resistance, Advantages such as design will be applied to light-emitting diodes in many fields. It is often necessary to weld the SMD LEDs during use. At this time, it is necessary to judge the positive and negative poles of the SMD LEDs to complete the welding. So how can we quickly determine the positive and negative poles of the SMD LEDs?

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1. Look at the diode pin

In the conventional case, the long leg is positive and the short leg is negative. If the two legs are the same length, the small dot in the LED is the positive pole and the larger one is the negative pole.

2. Use testing tools

Use a multimeter to test the light-emitting diode, connect the red test lead to positive and the black test lead to negative. Note: When testing light-emitting diodes, if the low range cannot be measured, you can use RX10K to test. When the two test leads are in contact with the positive and negative poles of the diode, when the resistance is small, the black test lead is connected to the anode of the diode, which has a higher resistance. When it is large, the black test lead is the cathode of the diode.

What state is displayed when the multimeter tests the positive and negative poles of the light-emitting diodes?

The red and black test leads of the multimeter Rx1k file are alternately connected to the two leads of the flashing light-emitting diode. When one of the measurements is found, the needle first swings to the right for a certain distance, and then the needle starts to jitter (vibrate) slightly at this position, with a swing amplitude of one Around the small grid. This phenomenon shows that the integrated circuit inside the self-flashing light-emitting diode starts to oscillate under the action of the 1.5V battery voltage inside the multimeter, and the output pulse current makes the pointer jitter, but the voltage is too low to make the light-emitting diode light. However, this phenomenon shows that the connection of the red and black test leads of the multimeter is correct, that is, the black test leads of the multimeter are connected to the positive electrode of the self-flashing light-emitting diode.

3. Look at the missing corner of the diode

In the LED energy-saving lamp lighting industry, 5050 SMD LEDs are often used. This SMD LED is square. One of the four right angles has a small corner, and the end with the small corner is the negative electrode. One end is the positive electrode.

4. Look at the diode product identification

SMD LEDs packaged in 0805 and 0603 will have a "T" or inverted triangle symbol on the bottom. One side of the "T" is the positive pole and the other side is the negative pole; the triangle symbol's "side" is close to the positive, "corner" "The one that is close is the negative electrode.

                   

5. Look at the shape of the LED

From the side of the light-emitting diode, there will be two lead wires in the shape of the tube, the smaller one is the positive electrode, and the larger one is the negative electrode.

Many surface-mounted devices (SMDs) are polarized components, such as light-emitting diodes (LEDs), diodes, tantalum capacitors, ICs, etc. Polarity identification of these electrolytic SMD components can avoid SMD polarity reversal during PCB assembly. This article will guide you to identify the polarity of SMD LEDs, capacitors, diodes, inductors, and ICs(including BGAs) quickly and easily.

Part 1. What is SMD Polarity?

Polarity means the direction of SMD components when soldering on printed circuit boards. SMD polarity has two meanings as below:

1. For LEDs, diodes, tantalum capacitors, and so on, polarity means the active/negative electrode of the component is in the same direction as the PCB's active/negative electrode mark.

2. For ICs (including BGAs), and multi-pin inductors, polarity means the component's first pin/ball is in the same direction as the PCB's first-pin mark.

If the SMD polarity is reversed on the circuit board, the whole PCBA will fail. That's why both engineers and PCBA manufacturers should be aware of the SMD polarity identification.

Part 2. What SMD components are Polarized?

Only polarized SMDs require polarity identification. Below are the polarized and non-polarized SMD components.

SMD components

Non-polarized

Polarized

Resistors

All resistors

/

Capacitors

Ceramic capacitors

Electrolytic capacitors, tantalum capacitors

Inductors

Chip inductors

Multi-pin inductors

Diodes

/

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All diodes including LEDs

ICs

/

All ICs including BGAs

As you can see, all resistors are non-polarized components, and all diodes, LEDs, and ICs are polarized.

Part 3. SMD Capacitor Polarity Identification

There are 3 types of SMD capacitors.

Ceramic capacitors are non-polarized components.

Tantalum capacitors and aluminum electrolytic capacitors have polarity. Their positive electrode should be placed in the same direction as the PCB pad's positive electrode.

SMD tantalum capacitor polarity identification

SMD tantalum capacitors are polarized components. For tantalum capacitors, the polarity is marked by:

• 1. The positive electrodes of the PCB and tantalum capacitor are both marked by a color strip.
• 2. The positive electrodes of the PCB and tantalum capacitor are both marked by a "+" symbol.
• 3. The positive electrode of the PCB pad is marked by a bevel.

Aluminum electrolytic capacitor polarity identification

For aluminum electrolytic capacitors, the polarity is marked by:

• 1. The negative electrode of the aluminum electrolytic capacitor is marked by a color strip or block. Therefore, the opposite side is the positive electrode.
• 2. The positive electrode of the PCB pad is marked by a color strip.
• 3. The positive electrode of the PCB pad is marked by a "+" symbol.

Part 4. SMD LED Polarity Identification

The positive/negative electrode of LEDs must be in the same direction as the positive/negative electrode of the PCB. SMD LED polarity is marked by:

• 1. The negative electrode of the LED is marked by the green color blocks.
• 2. The negative electrode of the PCB pad is marked by a color strip.
• 3. The negative electrode of the PCB pad is marked by a silk screen sharp corner.
• 4. The negative electrode of the PCB pad is marked by a silk screen 匚 frame.

Part 5. SMD Diode Polarity Identification

SMD diode polarity is marked by:

• 1. The negative electrode of the diode is marked by a color strip.
• 2. The negative electrode of the diode is marked by a groove.
• 3. The negative electrode of the diode is marked by a color strip on the glass.
• 4. The negative electrode of the PCB pad is marked by a vertical bar.
• 5. The negative electrode of the PCB pad is marked by a silk screen sharp corner.
• 6. The negative electrode of the PCB pad is marked by a silk screen 匚 frame.

Part 6. SMD Inductor Polarity Identification

There are 2 types of SMD inductors.

Chip inductors are non-polarized components.

Multi-pin inductors are polarized components. The first pin of the multi-pin inductor should be in the same direction as the first pin of the PCB pad. Multi-pin inductor polarity is marked by:

• 1. The first pin of the multi-pin inductor is marked by a round point.
• 2. The first pin of the multi-pin inductor is marked by the number 1.
• 3. The first pin of the PCB pad is marked by a round point.
• 4. The first pin of the PCB pad is marked by a round circuit.
• 5. The first pin of the PCB pad is marked by a * symbol.

Part 7. SMD IC Polarity Identification

All ICs (including BGAs) are polarized. Below we list the polarity identification methods for IC packages BGA, SOIC, SOP/QFP, and QFN.

BGA polarity identification

BGA is an IC packaging that uses solder balls to replace the pins. The polarity of BGA means that the first ball of the BGA is in the same direction as the PCB pad of the first ball. The BGA polarity is marked by:

• 1. The first ball of the BGA is marked by a round point.
• 2. The first ball of the BGA is marked by a round circle.
• 3. The first ball of the BGA is marked by the number 1.
• 4. The first ball of the PCB is marked by a round point.
• 5. The first ball of the PCB is marked by a round circle.
• 6. The first ball of the PCB is marked by the letter A.
• 7. The first ball of the PCB is marked by the number 1.
• 8. The first ball of the PCB is marked by a bevel.

SOIC polarity identification

The polarity of SOIC is marked by:

• 1. The first pin of the SOIC is marked by a color strip/round point/round/circle/groove/bevel/slope.
• 2. The first pin of the PCB is marked by a "*" symbol/round circle.

SOP/QFP polarity identification

The polarity of SOP/QFP is marked by:

• 1. The first pin of the SOP/QFP is marked by a pit.
• 2. The first pin of the SOP/QFP is marked by a point that is different from other points in size/shape.
• 3. The first pin of the PCB is marked by a round point/round circle/color strip.

QFN polarity identification

The polarity of QFN is marked by:

• 1. The first pin of the QFN is marked by a round point/"+" symbol/< symbol/bevel/point that is different from other points.
• 2. The first pin of the PCB is marked by a "*" symbol/round circle.

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Conclusion

This article gives a detailed illustration of SMD polarity and identification methods for the polarity of capacitors, inductors, LEDs, diodes, and ICs. If you still have problems with SMD polarity, welcome to leave your question via our online chatting window on the right of the PCBONLINE webpage. If you need SMD components or PCB assembly, do not miss PCBONLINE, the 24-year-old source factory.

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