introduction

Chip resistors are also called chip resistors or surface mount resistors. They are an important part of modern electronic equipment. Like other chip components (SMC and SMD), it is a new generation of leadless or short-leaded miniature electronic components suitable for surface mount technology (SMT). The welding surfaces of the leading ends are on the same plane. The main function of chip resistors in the circuit is to reduce the voltage, share a part of the voltage, that is, voltage division, current limiting protection circuit, shunt, etc., and can also be used as time circuit components and sensors.

Chip resistors are composed of four parts: a substrate, a resistive film, a protective film, and an electrode.

Substrate: The substrate material generally uses 96% Al2O3 (aluminum oxide) ceramics. Basically, it should have good electrical insulation, good thermal conductivity, electrical properties and certain strength mechanical properties at high temperatures.

Resistive film: The resistive film is printed on the ceramic base with a resistive paste with a certain resistivity, and then sintered to form a thick film resistor. Resistance paste generally uses RuO2 (nail dioxide). In recent years, base metal-based resistive pastes have been used, such as oxide-based (TaN-Ta), carbonized-based (WC-W) and Cu-based materials, in order to reduce costs.

Protective film: Cover the resistive film with the protective film. The main function of the protective film is to protect the resistor. On the one hand, it plays a role of mechanical protection, and on the other hand, it makes the surface of the resistor body insulative, preventing the resistor from contacting adjacent conductors and causing malfunctions. The protective film is generally a glass paste with a low melting point, which is printed and sintered.

Electrode: The electrode is to ensure that the resistor has good solderability and reliability. Generally, a three-layer electrode structure is used: inner electrode, middle electrode, and outer electrode. The role of the inner electrode: to connect the inner electrode of the resistor. The middle electrode is a nickel-plated layer, which has a blocking effect, improves the heat dissipation of the resistance, and buffers the thermal shock of welding. The outer electrode is a tin-lead layer, whose main function is to make the electrode solderable.

2 Chip resistor failure mode

The main failure modes and failure mechanisms of chip resistors are:

1) Open circuit: The main failure mechanism is that the resistive film is burned out or falls off in a large area, the matrix is broken under force, and the lead cap and the resistor body fall off.

2) The resistance drift is beyond specification: the resistive film is defective or degraded, the substrate has movable sodium ions, and the protective coating is not good.

3) Port fracture: Process defects occur during the welding of the resistor body, the solder joints are contaminated by impurities, and the ports are damaged by mechanical stress.

4) Short circuit: migration of silver ions and corona discharge occur.

Among them, open circuit and resistance increase are the main failure modes of chip resistors.

3 Reasons for failure

The causes of resistance failure can be attributed to six types: welding, temperature, humidity, organic matter, surge static electricity, and cracking.

1) Welding

The reason for the discontinuity or voids of the surface electrode at the edge of the solder is that during the soldering process, the Ag in the surface electrode close to the terminal electrode is largely lost during the soldering process, and "melted" in the solder, forming a partial area of the edge surface electrode. The Ag layer is hollow. During long-term use, due to Ag migration or corrosion, the expansion of voids causes the silver layer to open.

2) Temperature

In any case, the electrical load will accelerate the aging process of the resistor, and the effect of the electrical load on accelerating the aging of the resistor is more significant than the accelerated aging effect of increasing temperature. The reason is that the temperature rise of the contact part of the resistor body and the lead cap exceeds the resistance. The average temperature rise of the body. Generally, the life span is shortened by half for every 10°C increase in temperature. If the overload causes the temperature rise of the resistor to exceed the rated load by 50°C, the life of the resistor is only 1/32 of the life under normal conditions. It can pass the accelerated life test of less than four months to assess the working stability of the resistor during 10 years.

3) Humidity

Under the action of direct current load, electrolysis will cause the aging of the resistor. Electrolysis occurs in the groove of the grooved resistor. When moisture is present, intense electrolysis will occur. If the resistive film is a carbon film or a metal film, it is mainly electrolytic oxidation; if the resistive film is a metal oxide film, it is mainly electrolytic reduction. For high-resistance thin-film resistors, the effect of electrolysis can increase the resistance, and film damage may occur along the side of the groove spiral. Conducting a DC load test in a hot flash environment can comprehensively assess the resistance to oxidation or reduction of the resistor base material and film, as well as the moisture resistance of the protective layer.

4) Organic matter

During the formation of the organic protective layer, condensation polymerization volatiles or solvent vapors are emitted. The heat treatment process causes part of the volatiles to diffuse into the resistor, causing the resistance to rise. This process can last for 1 to 2 years. For example, silver electrode sulfide corrosion leads to open circuit.

5) Surge static electricity

Surge and static electricity can cause excessive current through the resistor. Because the volume of the surface mount resistor is relatively small, and the resistive film is only coated on one side of the porcelain body, its heat dissipation area is very small. Especially in the central part of the resistance film, because it is far away from the terminal electrode with faster heat transfer, once it is impacted by a large current, a large amount of heat is generated instantaneously and cannot be dissipated in time. pit.

6) Cracking

There are many reasons for the cracking of the porcelain body, and the resistance porcelain body material itself will have some very small cracks. When the resistance is installed, the terminal is locally heated, which will cause micro-cracks due to thermal stress, or further expand the original micro-cracks; for manual welding, local overheating of the terminal electrode is more likely to occur. The semi-finished printed board is subjected to mechanical stress during the splitting process, which leads to cracks in the resistance porcelain body.

In the early stage of failure, because the cracks are very fine or not completely disconnected, after a period of work, because the cracks gradually increase or organic matter enters the inside of the resistor from the cracks, the failure is manifested as an increase in the resistance value of the resistor or an open circuit.

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Figure 6 Resistor open circuit

4 Solution

In view of the failure modes and causes of the above resistors, and to improve the reliability of the use of chip resistors, the following measures can be taken.

1) Three defenses

Use three-proof paint with strong adhesion, wide operating temperature range and high surface resistance, such as Nanjing Duterun CHEMITECH 260. Improving the resistance to moisture can also prevent organic pollution. The key point is to choose the appropriate three anti-paint and coating method, coating thickness guarantee.

2) Anti-surge and static electricity design

Static electricity and surge can cause damage to the electrical performance of the resistor, so protective measures need to be taken in the design. The PCB uses a four-layer board design to form a complete ground plane to enhance the leakage prevention path. At the same time, the ports use TVS or ESD devices to improve surge and static resistance. The power supply pins of key components are added with a small capacitor of 1000P to the ground.

3) PCB imposition design

The PCB design needs to meet the IPC standard for the distance edge requirements of the resistor devices. For the PCB boards of device-intensive products, consider hollowing out during the imposition to reduce the mechanical stress on the resistor devices on the PCB during board splitting.

4) Welding

Strictly control the soldering time and soldering temperature of the resistor in the SMT process according to the standard, and minimize the manual soldering of the chip resistor. Ensure that no invisible damage is caused to the resistance during the welding process.

5) Potting

The function of potting is to prevent dust, salt spray, mildew, and vibration. The main material of potting is two-component silica gel or epoxy resin. Among them, the sulfide produced by the two-component silica gel is easy to cause resistance vulcanization. Need to consider the use of anti-sulfurization resistors in the design, or isolate the potting layer from the printed board device surface.

5 concluding remarks

This paper analyzes the structure, failure modes, and failure causes of chip resistors, and proposes improvement measures to improve the reliability of chip resistors.