Chip Fixed Resistors Guide for Beginners

A fixed resistor is a passive electronic component that is designed to have a specific, unchanging electrical resistance. The primary function of it is to restrict the flow of electric current in a circuit, following Ohm's law, which states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them. Fixed resistors are used in a wide variety of electronic circuits for various purposes, such as setting biases, limiting current, dividing voltage, terminating transmission lines, and many other applications. They come in various sizes, shapes, and resistance values to suit different circuit requirements. Here we will provide a detailed introduction to Chip Fixed Resistors (SMD Resistors).

What Is Chip Fixed Resistor?

Chip Fixed Resistor, a type of metal glaze resistor, combines metal powder and glass enamel powder, and is manufactured by screen printing on a substrate. It is moisture-resistant, heat-resistant, and has a low temperature coefficient. It significantly saves circuit space and cost, enabling more refined designs. Its key features include high moisture resistance, high temperature tolerance, high reliability, uniform appearance dimensions, precision, and low temperature coefficient and resistance tolerance. And its production process involves vacuum deposition of an alloy resistor film on a ceramic substrate, followed by a protective glass layer and three layers of electroplating, making chip fixed resistors widely used in electronic devices.

Main Features

1. Small volume, light weight.

2. Suitable for reflow soldering and wave soldering.

3. Stable electrical performance, high reliability.

4. Low assembly cost, compatible with automatic mounting equipment.

5. High mechanical strength and superior high-frequency characteristics.

What is the Material of the Chip Fixed Resistor?

Chip fixed resistors, also called SMD resistors, are electronic components designed to be soldered directly onto the surface of a printed circuit board (PCB), as opposed to through-hole components which have leads that go through the PCB. The materials used in the construction of SMD resistors typically include:

1. Substrate

The base material of an SMD resistor is usually a ceramic substrate, which provides a stable base for the resistive element.

2. Resistive Element

The resistive material is typically a thin film of metal alloy (such as nickel-chromium, also known as Nichrome) or a metal oxide (such as tin oxide). This material is deposited onto the substrate in a precise pattern to achieve the desired resistance value.

3. Electrodes

The electrodes are the conductive pads at each end of the resistor that allow it to be soldered to the PCB. They are usually made of a silver-based or copper-based material that is plated with tin or another solderable metal to ensure good solderability.

4. Protective Coating

After the resistive element and electrodes are applied, the entire component is often coated with a layer of protective material, such as a glass or epoxy resin, to protect it from the environment, physical damage, and to ensure stability of the resistance value over time and temperature variations.

5. Markings

Some chip fixed resistors have markings to indicate their resistance value or tolerance. These markings are usually printed on top of the protective coating using a laser or ink that can withstand high temperatures.

The specific materials and manufacturing processes can vary depending on the type of SMD resistor (e.g., thick film, thin film, metal film, carbon film) and the precision, power rating, and temperature coefficient required for the application.

What is the Composition of the Chip Fixed Resistor?

Chip fixed resistor is widely used in electronic circuits due to its small size, reliability, and ease of mass production. Here is an explanation of its main composition and related characteristics:

1. Substrate Material

The base material of a chip fixed resistor is typically a ceramic substrate, such as alumina (Al2O3) or a more modern alternative like a ceramic composite. Alumina is preferred for its excellent thermal conductivity and mechanical stability.

2. Resistive Element

The resistive element itself is usually a thin film or a thick film of resistive material deposited onto the ceramic substrate. This resistive material is chosen based on its stability, temperature coefficient, and resistance range. Common materials include nichrome (NiCr), tantalum nitride (TaN), or various metal alloys.

3. Termination Material

The ends of the resistive element are connected to metal terminals, which are typically made of materials like nickel (Ni) or silver (Ag). These terminals serve as the connection points for soldering the chip resistor onto the circuit board.

4. Protective Coating

To protect the resistive element and ensure reliability, a protective coating is often applied over the resistor. This coating can be a thin layer of glass or polymer to guard against environmental factors such as moisture and contaminants.

Main Working Parameters of the Chip Fixed Resistor

1) Size: Chip resistors come in various sizes, designated by codes such as 0402, 0603, 0805, etc., which refer to their dimensions in metric units (e.g., 0402 is 0.04 inches by 0.02 inches).

2) Resistance Range: Chip resistors can have a wide range of resistance values, from fractions of an ohm to several megaohms, depending on the specific type and application.

3) Tolerance: The tolerance of a chip fixed resistor specifies how closely its actual resistance matches its nominal (specified) resistance value. Common tolerances include ±1%, ±5%, and ±10%.

4) Power Rating: This indicates the maximum amount of power the resistor can dissipate without damage. It is determined by factors such as the size of the resistor and its ability to dissipate heat.

5) Temperature Coefficient: This parameter indicates how much the resistor's resistance changes with temperature. It is expressed in parts per million per degree Celsius (ppm/°C).

How to Identify the Resistance of the Chip Fixed Resistor?

Identifying the resistance of a chip fixed resistor, such as a surface-mount device (SMD) resistor, can be done through several methods. Here are the most common ways to determine the resistance value:

1. Color Code (for some larger SMD resistors)

Some larger SMD resistors may have a color code similar to traditional through-hole resistors. The color bands indicate the resistance value according to a standard color code chart.

2. hree-Digit Marking System

For most SMD resistors, there is a numerical marking system. A three-digit code (e.g., 102) is used where the first two digits are the significant figures, and the third digit is the multiplier, which represents the number of zeros that follow the two significant figures. For example, 102 means 10 × 10^2 ohms, or 1 kilohm.

3. Four-Digit Marking System

For higher precision resistors, a four-digit code may be used. The first three digits are the significant figures, and the fourth digit is the multiplier. For example, 4702 means 470 × 10^2 ohms, or 47 kilohms.

4. EIA-96 Marking System

For even higher precision resistors, the EIA-96 marking system is used. This system uses a three-character code where the first two characters represent a value from the EIA-96 series, and the third character is a letter that indicates the multiplier (decade). You'll need an EIA-96 lookup table to decode these values.

5. Using a Multimeter

The most straightforward way to identify the resistance of any resistor, including SMD resistors, is to use a multimeter. Set the multimeter to the resistance measurement mode, and then touch the probes to the two terminals of the resistor. The multimeter will display the resistance value.

6. Manufacturer's Datasheet

If you have the part number of the resistor, you can look it up in the manufacturer's datasheet. The datasheet will provide detailed information about the resistor, including its resistance value.

7. Measuring with a Microscope or Magnifying Glass

For very small SMD resistors where the markings are difficult to read with the naked eye, you may need to use a magnifying glass or a microscope to read the codes.

Remember that the actual resistance may vary slightly from the marked value due to manufacturing tolerances. Always measure the resistance with a multimeter if you need the exact value for critical applications.

How to Test the Resistance of the Chip Fixed Resistor?

Testing the resistance of a chip fixed resistor, also known as a surface-mount resistor, can be done using a digital multimeter (DMM). Here's a step-by-step guide to measure the resistance:

1. Select the Multimeter Mode

Set your digital multimeter to the resistance measurement mode. This is usually indicated by the Ω (omega) symbol. If your multimeter has multiple resistance ranges, start with a range that is higher than the expected resistance value of the resistor.

2. Turn Off the Circuit Power

If the resistor is part of a circuit, make sure to turn off the power to the circuit before attempting to measure the resistance. This is important for safety and to ensure an accurate measurement.

3. Access the Resistor

If the resistor is soldered to a circuit board, you may need to remove it to get an accurate reading, as other components can affect the measurement. If you're testing a resistor that's not installed in a circuit, you can proceed with the measurement directly.

4. Connect the Multimeter Probes

Touch the probes to the two terminals of the resistor. It doesn't matter which probe goes on which side of the resistor for the purpose of measuring resistance.

5. Read the Measurement

The multimeter display will show the resistance value. If the reading is close to zero or shows a very low resistance, it could mean that the resistor is shorted or that you're measuring it while it's still connected to other components in the circuit.

6. Adjust the Range

If the multimeter displays "OL" or "overload," it means that the resistance is higher than the selected range. Switch to a higher range on the multimeter and try again.

7. Compare with the Expected Value

Compare the measured resistance with the expected value of the resistor. The actual value should be within the tolerance range of the resistor. For example, a 100Ω resistor with a 5% tolerance should measure between 95Ω and 105Ω.

8. Repeat the Test

To ensure accuracy, you can repeat the measurement a few times. If you consistently get the same reading, you can be confident in the measurement.

Remember that the resistance of a resistor can change due to factors like temperature, age, and physical stress. If you suspect that a resistor is damaged or not functioning correctly, it's a good idea to replace it with a new one of the same value and tolerance. Always consult the datasheet for the resistor to understand its specifications and how it should behave under different conditions.

Soldering or Replacing of Chip Fixed Resistors

During the production and processing of surface-mount fixed resistors, they are mass-produced and therefore attached through bulk soldering using SMT (Surface Mount Technology) machines. However, for individual needs or when replacement is required, manual soldering is necessary. The commonly used tools for this purpose include:

Tools

1. Soldering Iron

A soldering iron is indispensable for manual soldering of components. Here, I recommend using a soldering iron with a fine, pointed tip, as it allows for precise and convenient soldering of individual or several pins when dealing with surface-mount chips that have closely spaced leads.

2. Solder Wire

Good quality solder wire is also important for surface-mount soldering. If possible, when soldering surface-mount components, try to use fine solder wire, as it is easier to control the amount of solder applied, thereby avoiding waste and the trouble of having to remove excess solder.

3. Tweezers

The primary function of tweezers is to conveniently pick up and place surface-mount components. For example, when soldering a surface-mount resistor, tweezers can be used to hold the resistor and position it on the circuit board for soldering. The tips of the tweezers should be sharp and flat to facilitate the handling of components. Additionally, for certain chips that require protection from static electricity, anti-static tweezers are necessary.

4. Solder Wick (Desoldering Braid)

When soldering surface-mount components, it is easy to apply too much solder. This is particularly true when soldering densely packed, multi-pin surface-mount chips, where it is common for adjacent pins or even multiple pins to become shorted by excess solder. In such cases, traditional solder suckers are ineffective, and a braided solder wick is needed instead.

5. Rosin

Rosin is the most commonly used flux in soldering because it can dissolve oxides from the solder, protect the solder from oxidation, and increase its fluidity. When soldering through-hole components, if the component is rusty, it should be scraped clean, then touched with a soldering iron while on the rosin, before applying solder. When soldering surface-mount components, rosin not only serves as a flux but can also be used in conjunction with copper wire to act as a solder wick.

6. Solder Paste

When soldering items like iron parts that are difficult to tin, solder paste can be used. It can remove oxides from the metal surface and is corrosive in nature. When soldering surface-mount resistors, the adhesive properties of solder paste can sometimes be utilized to make the solder joints shiny and robust.

7. Hot Air Gun

A hot air gun is a tool that uses the hot air blown out by its heating element to solder and desolder components. It requires relatively high craftsmanship to use. From removing or installing small components to large-scale integrated circuits, a hot air gun can be employed. In different situations, there are specific requirements for the temperature and airflow of the hot air gun. If the temperature is too low, it can cause poor soldering of components; if too high, it can damage the components and the circuit board. Excessive airflow can blow away small components.

8. Magnifying Glass

For some surface-mount chips with extremely fine and closely spaced pins, it is difficult to inspect the pins after soldering to ensure they are properly soldered and to check for any short circuits using the naked eye. A magnifying glass can be used in such cases to conveniently and reliably examine the soldering of each pin.

9. Alcohol

When using rosin as a flux, it is easy to leave excess rosin on the circuit board. For aesthetic purposes, a cotton ball soaked in alcohol can be used to clean off the residual rosin from the circuit board.

In addition to the items mentioned above, there are other tools and materials such as sponges, PCB cleaning solvent, stiff brushes, and glue that may be needed.

Soldering Steps

Before soldering, apply a layer of rosin flux to the pad and wait for it to evaporate. Tin the pad using a soldering iron, with the iron in contact with the pad for no more than 1.5 seconds. Use anti-static tweezers to place the surface-mount fixed resistor onto the pad and hold it in place. Add solder and use the iron to melt it at the solder joint, ensuring the solder wets the joint properly. Quickly remove the iron. Remove any solder balls. The evaporation time is 5-10 seconds. The contact time between the iron and the pad is 1 second. The anti-static tweezers can be flat-tipped and anti-slip. Use a blower to clean the solder joints.

Welding Requirements

Insert the resistor accurately into the designated position. Ensure that the markings are facing up and the direction of the characters is consistent. After completing the installation of the same specification, proceed to install another specification, trying to keep the height of the resistors consistent. After soldering, trim off any excess leads that protrude from the surface of the printed circuit board flush with the root.

Precautions

1. The temperature of the soldering iron should not be too low or too high, with 270℃ being the optimal temperature.

2. SMD resistors or capacitors must be placed correctly.

3. Before soldering, it is advisable to practice repeatedly on a defective circuit board.

Conclusion

In summary, a chip fixed resistor is a fundamental component in electronic circuits, composed of a ceramic substrate with a resistive film and metal terminals, offering precise resistance values across a wide range of applications.

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