Previously, INFINITECH has thoroughly analyzed the characteristics, application areas and selection guidelines of SMDS. On this basis, we will further explore the common fault performance and causes of chip resistance, share effective detection methods and solutions, and provide practical suggestions to prevent resistance failure. With a deep understanding of these, you will be able to more skillfully deal with issues related to chip resistance, ensuring the stability and reliability of circuit design.

This article will cover the following points:

> Analysis of common fault phenomena and causes of chip resistance

> Methods and measures to avoid resistance failure

> Tips and precautions for manual welding and machine mounted patch resistors

> Influence of different welding materials on resistance welding

> Recommendations for patch resistance layout in PCB design

Analysis of the common fault phenomena and causes of chip resistance

Patch resistors are extremely common in electronic devices, and their small size and reliability make them the first choice for modern circuit designs. However, in the course of use, the patch resistor may encounter various failures, which may be caused by a variety of factors. The following are some common chip resistance failure phenomena and their causes:

Fault phenomenon and cause analysis

1. Resistance changes

• Long-term overload: When the patch resistor is subjected to a load exceeding its rated power for a long time, its internal material may change due to overheating, resulting in increased resistance.

• Thermal stress: Repeated thermal expansion and contraction may cause damage to the internal structure of the resistor and affect the resistance value.

• Material aging: Over time, resistive materials may naturally age, especially carbon or metal oxide film resistors, causing resistance values to drift.

2. Unweld

• Improper welding: During manual welding, if the welding temperature is too high or the welding time is too long, the electrode may burn, thereby reducing the strength of the solder joint.

• Mechanical stress: During assembly or transportation, the board may be shaken or bent, which puts pressure on the external electrodes and causes derailing.

Step 3 Damage

• Electrical corrosion: Under certain conditions, oxidation, dissolution, or mechanical damage may occur on the surface of the resistive material, especially in high humidity or corrosive gas environments.

• Oxidation of the outer electrode: oxidation of the electrode surface will reduce its conductivity, affecting the welding quality and electrical connection.

• Overload: Transient overvoltage or overcurrent can instantaneously destroy resistance, resulting in an open or short circuit.

• Design Flaws: If an adequate safety margin is not considered in the circuit design, the patch resistor may be damaged due to operating conditions beyond its specification range.

4. Matrix fracture

• Mechanical stress: Strong external force or continuous vibration may cause the resistance base material to break.

• Thermal stress: Uneven heating and cooling can cause stress to develop within the matrix, especially in the case of sharp changes in temperature.

5. Short circuit/open circuit

• Physical damage: Direct physical impact or cutting may result in an interruption (open circuit) or short circuit in the internal connection of the resistor.

• Manufacturing defects: Internal manufacturing defects, such as cracks or foreign objects, can cause failure early in use.

Solutions and preventive measures

• Select the right specifications: ensure that the power and resistance value of the patch resistor meet the actual needs of the circuit, and consider the appropriate derating use.

• Improved heat dissipation: For high power applications, additional heat dissipation measures should be considered, such as the use of heat sinks or improved air circulation.

• Welding quality control: Use the correct welding temperature and time to avoid excessive heating.

• Environmental protection: For environmentally sensitive applications, consider sealing or coating protection.

• Periodic detection: Periodically check the resistance of the patch in the circuit to monitor changes in resistance value and other abnormal signs.

How can I identify and repair damage caused by surges, welding crack problems, vulcanization effects or overloading

1. Damage caused by surge

• Detection method: Use a digital multimeter to measure the resistance value, if the resistance value deviates too much from the nominal value or displays infinity (open circuit) or zero ohms (short circuit), it may have been damaged. Observe whether the resistance surface has obvious physical damage, such as cracks, burn marks, etc.

 

• Solution strategy:

  • Replace damaged resistors and ensure that replacements with adequate surge absorption capacity are selected.

  • Incorporate surge protection devices such as TVS diodes, gas discharge tubes or varistors into the circuit design.

  • Strengthen the layout design of the circuit, such as increasing the decoupling capacitance, optimizing the layout of the ground wire to reduce the impact of surges.

2. Welding crack problem

• Detection method:

  • Visual inspection, looking for solder joints are complete, whether there are signs of cracks or welding.

  • Use X-ray inspection or microscope to look inside the solder joint.

  • Test by shaking or tapping to see if there is a loose sound.

• Solution strategy:

  • Reweld faulty joints to ensure good weld quality.

  • Improve welding process, use appropriate solder and welding temperature.

  • Consider reducing mechanical stress in the design, such as adding supports or using more flexible materials.

3. Vulcanization effect

• Detection method:

  • Use a multimeter to measure the resistance change.

  • Observe the resistance surface for white crystals or discoloration.

• Solution strategy:

  • Clean the affected area to remove the sulfide.

  • Use seals or coatings to protect resistors in environments with high humidity or risk of vulcanization.

  • Consider using a resistance material with better vulcanization resistance.

4. The load is damaged

• Detection method:

  • Check the resistance value of the resistor. If the resistance value is abnormal, it may be caused by overheating.

  • Look for signs of burning, discoloration, or expansion on the surface of the resistor.

• Solution strategy:

  • Recalculate the power requirements of the resistors in the circuit to ensure that the selected resistors can withstand the actual load.

  • Improve the heat dissipation capacity of the circuit, such as using heat sinks or improving air flow.

  • Consider using resistors with a larger power margin to reduce the risk of overload.

For all types of failures, an important preventive measure is to consider possible failure modes at the circuit design stage and take appropriate preventive measures, such as rational layout, use of high-quality components and adequate thermal design.

Methods and measures to avoid resistance failure

Avoiding resistance faults in circuit design is very critical, because resistance, as a basic electronic component, is widely used in various circuits. Here are some key considerations for avoiding resistance failures in circuit design, including layout and wiring considerations and protection measures:

1. Select an appropriate resistance type and specification

• Thermal stability: Choose resistors with good thermal stability, such as metal film resistors or precision metal foil resistors, whose resistance values change less with temperature.

• Power rating: Ensure that the power level of the resistor is higher than the power it actually consumes in the circuit, and it is usually recommended to use derated to extend the service life.

• Resistance value accuracy: According to the circuit needs to select the appropriate precision of the resistance to ensure circuit performance.

2. Layout and wiring precautions

• Thermal management: Ensure that high power resistors have sufficient cooling space to avoid adjacent heating elements, using heat sinks or enhanced air flow if necessary.

• Avoid mechanical stress: The patch resistor should be firmly fixed to the circuit board to avoid mechanical stress due to vibration or thermal expansion.

• Signal integrity: In high-speed digital circuits, the parasitic effects of resistance, such as parasitic inductors and capacitors, should be considered to reduce signal reflection and attenuation.

• Layout sequence: In mixed-signal circuits, analog and digital circuits should be arranged separately, and sensitive resistors should be kept away from noise sources.

3. Protective measures

• Overvoltage protection: A transient voltage suppressor (TVS) or Zener diode is added to the input to prevent excessive transient voltage from damaging the resistance.

• Overcurrent protection: Use a fuse or PTC thermistor to limit excessive current and avoid overheating of the resistor.

• Isolation measures: In multi-layer PCBS, ensure that the layer below the resistor is not a signal layer or a power layer to reduce coupling and interference.

4. Circuit design and test

• Redundant design: In critical circuits, multiple resistors can be considered in parallel or series, so that even if one is damaged, the circuit can still partially work.

• Thermal simulation: Thermal simulation is performed during the design phase to ensure the stability and safety of the circuit under extreme conditions.

• Testing and verification: After the circuit design is completed, adequate testing is carried out, including temperature cycle test, power cycle test, etc., to verify the performance and reliability of the resistance.

5. Regular maintenance and inspection

• Monitoring and recording: During the operation of the equipment, the key resistance parameters in the circuit are regularly monitored, such as resistance values and temperature, and the data is recorded for trend analysis.

• Fault detection: Use the circuit diagnostic tool to check the circuit regularly to find and replace the resistance that is about to fail.

Tips and precautions for manual welding and machine mounted patch resistors

Manual welding and machine mount chip resistors (surface mount technology, SMT) have their own characteristics and requirements. Here are tips and considerations for both methods:

Tips and precautions for manual welding of patch resistors:

1. Tools required:

   • Use a pointed and temperature-controlled soldering iron to avoid excessive temperature damage to components.

   • Fine solder wire, easy to control solder quantity.

   • Electrostatic tweezers for precise placement of components.

   • Suction tape or ball for handling excess solder.

   • Rosin or lead-free flux to improve solder joint quality.

2. Preparation before welding:

   • Clean work areas and components to ensure no dust or oil stains.

   • Apply a small amount of flux to the pad and pre-tin it.

3. Welding process:

   • Carefully place the component on the pad with tweezers to ensure correct position.

   • Use a soldering iron to heat one end of the element while adding solder to ensure that the solder is sufficiently wetted.

   • After confirming that the solder joint is good, quickly remove the soldering iron and repeat the process to complete the welding on the other end.

   • The welding time should not be too long to avoid damage to the components.

4. Inspection and cleaning:

   • Check whether the solder joint is full, no porosity, and whether the resistance is stable.

   • Remove welding slag and excess solder, keep solder joints clean.

Tips and precautions for machine mounting (SMT) :

1. Machine Settings:

   • Adjust the nozzle and mounting pressure of the mounter according to the size and type of resistance.

   • Set the correct amount of paste to ensure that the paste is evenly distributed on each pad.

2. Solder paste printing:

   • Use a steel mesh template to print solder paste to ensure consistent thickness and avoid bridging or short circuits.

   • Control printing speed and pressure to avoid solder paste overflow or insufficiency.

3. Component positioning:

   • The machine vision system is used to ensure accurate positioning of components and avoid deviation.

   • For multi-pin or high-precision components, a higher level of positioning accuracy is used.

4. Reflow welding:

   • Set the appropriate reflow welding temperature curve to ensure temperature control during melting and curing of the solder paste.

   • Avoid overheating or cooling too quickly to reduce thermal stress and welding defects.

5. Quality control:

   • Use AOI (Automatic Optical Inspection) system to check solder joint quality and find bad solder joints in time.

   • For complex components, X-ray inspection may be required to ensure internal weld quality.

Whether it is manual welding or machine mounting, it is necessary to strictly adhere to the code of operation and use the appropriate tools and materials to ensure the quality and reliability of the final product. Although the machine mounting degree of automation is high, the efficiency is fast, but the early equipment investment and maintenance cost is also high, suitable for mass production; Manual welding is more flexible and suitable for small batches or maintenance occasions.

Influence of different welding materials on resistance welding

In surface mount technology (SMT), different welding materials such as solder paste and red glue will have different effects on the welding effect of the resistance and the performance of the final board.

Effects and effects of solder paste:

Solder paste as an indispensable material in electronic assembly, its core function is to establish a solid electrical connection. It usually contains metal alloy components, such as traditional tin-lead alloys, and is now more inclined to use environmentally friendly lead-free alloys, which can melt and solidify between the resistor and the circuit board under heating conditions, forming a reliable metal contact interface to ensure the smooth flow of current. In addition, the solder paste also assumes the role of mechanical fixing, which can tightly adhere the resistance components to the circuit board, effectively resist the influence of external vibration and thermal stress, and ensure the stability of the circuit structure.

The solder joint formed by solder paste not only has good conductivity, but also has a certain heat conduction ability. This is especially important for high power resistors because the solder joints help distribute heat generated by the resistors evenly to the board, preventing local overheating, thereby improving the overall system's heat dissipation efficiency and operational reliability.

However, the quality of the solder paste and the temperature control of the welding process have a direct and significant effect on the shape, size and reliability of the solder joint. Any improper operation can lead to a range of welding defects, such as weak welding (incomplete fusion), solder holes (internal air bubbles) or insufficient solder joint size, which can weaken the electrical and mechanical properties of the solder joint and even cause circuit failure. Therefore, in practical applications, strict control of solder paste selection and welding parameters is the key to ensure the quality of electronic assembly.

Application and function of red glue:

As a non-conductive adhesive, red glue's main responsibility is to accurately position and temporarily fix electrical components such as resistors on the circuit board before the welding process, effectively avoiding the risk of component displacement or falling off during the subsequent high-temperature welding process such as wave soldering. It is worth noting that the red glue itself does not have conductive properties, so it does not interfere with the electrical performance of the resistor, and it cannot directly establish an electrical connection.

On the basis of the red glue assisted positioning, in order to achieve the electrical connection between the components, it is also necessary to use reflow welding or wave soldering technology, the use of solder paste to complete the final welding operation. The excellent temperature stability of red glue ensures that it can maintain good bonding force under high temperature welding environment and will not fail due to thermal stress.

The cured red glue can provide additional mechanical support for components on the board, increasing the stability of the components, although this support is mainly focused on positioning rather than withstand heavy loads or severe shocks. This property of red glue is important for improving the reliability and durability of electronic devices, especially in precision circuit designs where component positioning accuracy is required.

Comparison and selection

• Welding sequence: In the process using red glue, the resistance is first temporarily fixed by red glue, and then the electrical connection is completed by reflow soldering of the solder paste. In the process using only solder paste, the fixing of the resistance and the electrical connection are done in one go.

• Application Scenario: Solder paste is suitable for most SMT processes, especially those with double-sided reflow boards that do not require additional fixing steps. Red glue is often used in single-sided or mixed-technology circuit boards where components need to be secured before wave soldering.

• Cost and efficiency: Solder paste process is relatively simple, low cost, high efficiency, suitable for mass production. The red glue process adds additional fixing steps that may increase cost and processing time, but is necessary in some cases (such as the fixing of large or heavy components).

Recommendations for the layout of patch resistors in PCB design

In PCB design, the layout of the patch resistance can not only improve the circuit performance, but also enhance the reliability and production efficiency of the circuit. Here are some suggestions for the layout of the patch resistor:

1. Consider thermal management

• Distributed layout: High power resistors should be distributed as far as possible to avoid local overheating, while ensuring sufficient space for heat dissipation.

• Near the edge: If possible, place the high-power resistor near the edge of the PCB to facilitate natural heat dissipation or the addition of heat sinks.

• Thermal isolation: In multi-layer PCBS, avoid placing other sensitive elements or signal wiring below the high-power resistor to reduce the thermal impact.

2. Reduce Electromagnetic interference (EMI)

• Away from sensitive elements: Keep the resistor away from highly sensitive analog circuits or high-frequency circuits to reduce potential electromagnetic interference.

• Signal path: For the resistance on the signal path, try to keep the signal line straight and short to reduce signal reflection and delay.

3. Signal integrity

• Matching network: In high-speed digital circuits, ensure that resistors together with capacitors and other components form a matching network to optimize signal integrity.

• Decoupling capacitor: In the power line, the patch resistor should be used in conjunction with the decoupling capacitor to filter out the power supply noise.

4. Power and safety considerations

• Derated use: Select a resistor with a power rating higher than the actual power requirement to ensure long-term stability and safety.

• Protection circuit: In critical circuits, consider using fuses or PTC thermistors to protect the resistance and avoid overload damage.

5. Ease of production and testing

• Accessibility: Design with ease of production and testing in mind, ensuring that all components are easily accessible, especially resistors that require later commissioning or replacement.

• Clear marking: The position and value of each resistor are clearly marked on the PCB for easy assembly and maintenance.

6. Layout consistency

• Alignment: Align resistors of the same type or function as much as possible, which is not only beautiful, but also conducive to automated assembly and inspection.

7. Optimize cable routing

• Reduce crossing: When planning cable routes, minimize crossing routes to avoid unnecessary cable length and complexity.

• Ground plane: Where possible, use a large area of ground plane to provide a good grounding reference and reduce signal interference.

8. Consider future upgrades

• Leave room: Leave some room in the design for possible future upgrades or modifications, such as adding additional resistors or adjusting circuit layout.

By following the above recommendations, you can ensure that the layout of the patch resistors on the PCB meets the needs of circuit performance, with good reliability and manufacturing feasibility.