1. Failure modes of typical components

 

In order to obtain the sensitive environment of electronic components, the typical failure modes related to the environment are analyzed, as shown in Table 1.

Table 1 Environmental-related failure modes of electronic components and analysis of the sensitive environment involved

Serial number

Electronic component name

Environment-related failure modes

Environmental stress

1

Electromechanical components

Vibration caused the coil to break due to fatigue and the cable to loosen.

Vibration, shock

2

Semiconductor microwave device

High temperature and temperature shock cause delamination at the interface between the packaging material and the chip of the plastic-encapsulated microwave monolith, and the interface between the packaging material and the chip holder.

High temperature, temperature shock

3

Hybrid integrated circuit

The impact causes the ceramic substrate to crack, the temperature impact causes the capacitor terminal electrode to crack, and the temperature cycle causes the welding failure.

Shock, gentle cycle

4

Discrete devices and integrated circuits

Thermal breakdown, chip soldering failure, inner lead bonding failure, and impact causing the passivation layer to rupture.

High temperature, shock, vibration

5

RC element

The core substrate is broken, the resistance film is broken, and the lead is broken.

Impact, high and low temperature

6

Board level circuit

The solder joints are cracked and the hole copper is broken.

high temperature

7

Electric vacuum

The hot wire breaks due to fatigue.

vibration

 

2. Analysis of failure mechanism of typical components

 

The failure modes of electronic components are not single, and only representative parts of typical components are analyzed for the tolerance limit of the sensitive environment in order to obtain a more general conclusion.

 

2.1 Electromechanical components

 

Typical electromechanical components include electrical connectors, relays, and so on. The failure modes of the two types of components are analyzed in depth.

 

1) Electrical connector

 

The electrical connector is composed of three basic units: a shell, an insulator and a contact body, and its failure modes can be summarized as contact failure, insulation failure and mechanical connection failure. The main failure form of the electrical connector is contact failure, and its failure performance is: the contact pair is instantaneously broken and the contact resistance increases.

 

For electrical connectors, due to the existence of contact resistance and material conductor resistance, when a current flows through the electrical connector, the contact resistance and metal material conductor resistance will generate Joule heat. The increase in Joule heat will increase the heat, resulting in The temperature of the contact point increases. An excessively high temperature of the contact point will soften, melt or even boil the metal on the contact surface, and will also increase the contact resistance, thereby causing contact failure.

 

Under the action of a high temperature environment, the contact parts will also experience creep phenomenon, which makes the contact pressure between the contact parts continue to decrease. When the contact pressure is reduced to a certain level, the contact resistance will increase sharply, resulting in poor electrical contact and contact failure.

 

On the other hand, the electrical connector will be subjected to various vibration loads and impact forces during storage, transportation and work. When the excitation frequency of the external vibration load is close to the natural frequency of the electrical connector, the electrical connector will resonate. This phenomenon causes the gap of the contact piece to become larger, and the gap increases to a certain extent, and the contact pressure will disappear instantaneously, resulting in the "instant break" of the electrical contact.

 

Under the action of vibration and impact load, stress will be generated inside the electrical connector. When the stress exceeds the yield strength of the material, the material will be damaged and fractured; under the action of this long-term stress, the material will also experience fatigue damage, and finally Cause failure.

 

2) Relay

 

Electromagnetic relays are generally composed of iron cores, coils, armatures, contacts, reeds, etc. As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, which will produce electromagnetic effects. Under the action of electromagnetic force, the armature will overcome the pull force of the return spring and attract to the core, thereby driving the armature. The moving contact and the static contact (normally open contact) are pulled together.

 

When the coil is de-energized, the electromagnetic attraction will also disappear, and the armature will return to its original position under the reaction force of the spring, so that the moving contact and the original static contact (normally closed contact) are attracted. This pulls in and releases, so as to achieve the purpose of conducting and cutting off in the circuit.

 

The main modes of the overall failure of electromagnetic relays are: relay normally open, relay normally closed, relay spring action does not meet the requirements, and the electrical parameters of the relay are out of tolerance after the contacts are closed. Due to the insufficiency of the electromagnetic relay production process, many electromagnetic relay failures have hidden quality hazards during the production process. For example, the mechanical stress release period is too short to cause deformation of the parts after the mechanical structure is formed, and the incomplete removal of residues leads to unqualified or even failure of the PIND test. , The factory inspection and use screening are not strict, so that the failed devices are put into use.

 

The impact environment can easily cause plastic deformation of the metal contacts, leading to failure of the relay. When designing a device containing a relay, it is necessary to focus on its adaptability to the impact environment.

 

2.2 Semiconductor microwave components

 

Microwave semiconductor devices refer to components made of Ge, Si and III~V compound semiconductor materials that work in the microwave band. Used in electronic equipment such as radars, electronic warfare systems, and microwave communication systems. In addition to providing electrical connection and mechanical and chemical protection for the die and pin, the packaging of microwave discrete devices must also consider the impact of the package parameters on the microwave transmission characteristics of the device in the design and selection of the package. The microwave shell is also a part of the circuit, and it constitutes a complete input and output circuit.

 

Therefore, the shape and structure, size, dielectric material, conductor configuration, etc. of the package must match the microwave characteristics of the components and circuit applications. These factors determine the capacitance, electrical lead resistance, characteristic impedance and loss of conductors and media.

 

The environment-related failure modes and mechanisms of microwave semiconductor components mainly include gate metal sinking and resistance degradation. The sinking of the gate metal is due to the accelerated thermal diffusion of the gate metal (Au) into GaAs, so this failure mechanism mainly occurs during accelerated life testing or extremely high temperature operation. The rate at which the gate metal (Au) diffuses into GaAs is a function of the gate metal material’s diffusion coefficient, temperature and material concentration gradient. For a perfect lattice structure, the diffusion rate is very slow at normal operating temperature and will not affect the device’s performance. Performance, however, the diffusion rate will be significant when the grain boundaries are large or the surface defects are many.

 

Resistors are usually used in the feedback circuit of microwave monolithic integrated circuits, setting the bias point of active devices, isolation, power synthesis or coupling ends. There are two types of resistors: metal film resistors (TaN, NiCr) and lightly doped Doped GaAs sheet resistance. Tests show that the degradation of NiCr resistance caused by humidity is the main mechanism of its failure.

 

2.3 Hybrid integrated circuits

 

Traditional hybrid integrated circuits are divided into thick film hybrid integrated circuits and thin film hybrid integrated circuits according to the thick film conduction band and thin film conduction band technology of the substrate surface: some small printed circuit board (PCB) circuits, Since the printed circuit forms a conductive pattern on the surface of the flat board in the form of a film, it is also classified as a hybrid integrated circuit.

 

With the emergence of the advanced hybrid integrated circuit of multi-chip components, the unique multilayer wiring structure and through-hole technology of the substrate have made the components synonymous with a high-density interconnection structure in hybrid integrated circuits. The substrates used also include: thin-film multilayer, thick-film multilayer, high-temperature co-firing, low-temperature co-firing, silicon-based, and PCB multilayer substrates.

 

The environmental stress failure modes of hybrid integrated circuits mainly include electrical open circuit failure caused by substrate cracking, and welding failure between components and thick film conductors, components and thin film conductors, and substrates and housings. The mechanical impact force caused by the product drop, the thermal impact caused by the soldering operation, the additional stress caused by the uneven substrate warping, the transverse tensile stress caused by the thermal mismatch between the substrate and the metal shell and the bonding material, the internal substrate The potential damage caused by the mechanical stress or thermal stress concentration caused by defects, the substrate drilling and the local microcracks in the substrate cutting, eventually leads to the external mechanical stress being greater than the inherent mechanical strength of the ceramic substrate, resulting in failure.

 

The welding structure is prone to repeated action of the temperature cycle stress, which will cause thermal fatigue of the solder layer, resulting in a decrease in bonding strength and an increase in thermal resistance. For tin-based ductile solders, the thermal fatigue of the solder layer caused by temperature cycling stress is due to the inconsistency of the thermal expansion coefficients of the two structures connected by the solder, which is the displacement or shear deformation of the solder. After repeated repetitions, the solder layer will follow Fatigue crack propagation and extension will eventually lead to fatigue failure of the welded layer.

 

 

2.4 Discrete devices and integrated circuits

 

Semiconductor discrete devices are classified into diodes, bipolar transistors, MOS field effect transistors, thyristors and insulated gate bipolar transistors. Integrated circuits have a wide range of applications and can be divided into three categories according to their functions, namely, digital integrated circuits, analog integrated circuits, and digital-analog hybrid integrated circuits.

 

1) Discrete devices

 

There are many types of discrete devices, and due to their different functions and processes, the failure performance is quite different and has its particularity. However, as a basic device formed by a semiconductor process, its failure physics has certain similarities. The failures related to external mechanics and natural environment mainly include thermal breakdown, dynamic avalanche, chip bonding failure and inner lead bonding failure.

 

Thermal breakdown: Thermal breakdown or secondary breakdown is the main failure mechanism that affects semiconductor power components. Most of the damage during use is related to secondary breakdown. Secondary breakdown is divided into forward-biased secondary breakdown and reverse-biased secondary breakdown. The former is mainly related to the thermal performance of the device itself, such as the doping concentration and intrinsic concentration of the device, and the latter is related to the carrier avalanche multiplication in the space charge region (such as near the collector). Both are always accompanied by the internal device The current is concentrated. In the application of such components, special attention should be paid to heat protection and heat dissipation.

 

Dynamic avalanche: During the dynamic turn-off process caused by external or internal force, the impact ionization phenomenon that occurs inside the device controlled by the current and affected by the free carrier concentration causes dynamic avalanches. This phenomenon occurs in bipolar devices and diodes. It can happen in both IGBT and IGBT.

 

Chip soldering failure: The main reason is that the chip and the solder are different materials and have different thermal expansion coefficients, so there is a thermal mismatch problem at high temperatures. In addition, the existence of solder voids will increase the thermal resistance of the device, worsen heat dissipation, form hot spots in local areas, increase the junction temperature, and cause temperature-related failures such as electromigration.

 

Inner lead bonding failure: the main reason is the corrosion failure of the bonding point, which is caused by the corrosion of aluminum caused by the action of water vapor and chlorine in the humid and hot salt spray environment. Thermal cycling or vibration causes fatigue fracture of the aluminum bonding wire. The IGBT packaged in the module has a large volume. If the installation method is improper, it is easy to cause stress concentration and lead to fatigue fracture of the internal leads of the module.

 

 

2) Integrated circuit

 

The failure mechanism of integrated circuits is closely related to the use environment. The damage caused by moisture, static electricity or electrical surges in a humid environment, excessive use of graphics, and the use of integrated circuits without radiation reinforcement in a radiation environment are also It will cause the failure of the device.

 

Interface effects related to aluminum: In electronic devices using silicon as a material, SiO2 layer is widely used as a dielectric film, and aluminum is often used as a material for interconnecting wires. SiO2 and aluminum will chemically react at high temperatures, causing The aluminum layer becomes thinner, and if the SiO2 layer is exhausted due to reaction consumption, it will cause direct contact between aluminum and silicon.

 

In addition, the Au-Al interface contact occurs at the bonding point between the gold lead wire and the aluminum interconnection wire or the aluminum bonding wire and the gold-plated lead wire of the package. Due to the different chemical potentials of these two metals, a variety of intermetallic compounds will be produced after long-term use or high temperature storage above 200 ℃, and due to their different lattice constants and thermal expansion coefficients, large stresses are generated in the bonding points, and electrical conductivity The rate becomes smaller.

 

Metallization corrosion: The aluminum connecting wires on the chip are susceptible to corrosion by water vapor in a hot and humid environment. Due to price deviation and easy mass production, many integrated circuits are encapsulated with resin. However, water vapor can pass through the resin to reach the aluminum interconnection line. The impurities brought in from the outside or the impurities in the dissolved resin interact with the metal aluminum. Corrosion occurs in the aluminum interconnection line.

 

Delamination effect caused by water vapor: Plastic packaged IC refers to an integrated circuit encapsulated with resin polymer materials such as plastic. In addition to the layering effect (commonly known as the "popcorn" effect) between the plastic package material, the metal frame and the chip, due to the resin type The material has the characteristic of adsorbing water vapor, and the delamination effect caused by water vapor adsorption will also make the device invalid. The failure mechanism is that the moisture in the plastic packaging compound expands rapidly at high temperature, causing the plastic to separate from other materials attached to it, and in severe cases, the plastic packaging body will burst.

 

2.5 RC components

 

1) Resistors

 

Common non-wired resistors can be divided into four types according to the different materials used in the resistor body: alloy type, thin film type, thick film type and composite type. For fixed resistors, the main failure modes are open circuit, electrical parameter drift, etc.; for potentiometers, the main failure modes are open circuit, electrical parameter drift, noise increase, etc. The use environment will also cause the aging of the resistor, which has a great impact on the life of electronic equipment.

 

Oxidation: The oxidation of the resistor body of the resistor will increase the resistance value, which is the most important factor causing the aging of the resistor. Except for resistors made of precious metals and alloys, other materials will be destroyed by oxygen in the air. Oxidation is a long-term effect. When the influence of other factors gradually weakens, the oxidation will become the main factor. The high temperature and high humidity environment will accelerate the oxidation of the resistor. For precision resistors and high-resistance resistors, the fundamental measure to prevent oxidation is sealing protection. Sealing materials should be made of inorganic materials, such as metals, ceramics, and glass. The organic protective layer cannot completely prevent moisture and air permeability, and can only delay oxidation and adsorption.

 

The aging of the bonding agent: For organic synthetic resistors, the aging of the organic bonding agent is the main factor affecting the stability of the resistor. The organic bonding agent is mainly synthetic resin. In the manufacturing process of the resistor, the synthetic resin is converted to high Thermosetting polymer with degree of polymerization. The main factor causing polymer aging is oxidation. The free radicals generated by oxidation cause the hinges of the polymer molecular bonds, so that the polymer further solidifies, becomes brittle, and then loses elasticity and mechanical damage occurs. The curing of the adhesive causes the volume of the resistor to shrink, the contact pressure between the conductive particles increases, the contact resistance becomes smaller, and the resistance value decreases, but the mechanical damage of the adhesive also increases the resistance value.

 

Usually the curing of the bonding agent occurs before, and the mechanical damage occurs later, so the resistance value of organic synthetic resistors shows the following law: some decline in the initial stage, and then turn to increase, and there is a tendency to increase. Because the aging of the polymer is closely related to temperature and light, the composite resistor will accelerate the aging under the high temperature environment and strong light.

 

Aging under electrical load: Applying a load to a resistor will accelerate its aging process. Under DC load, electrolysis will damage the thin film resistors. Electrolysis occurs between the grooves of the grooved resistor. If the resistor matrix is ​​a ceramic or glass material containing alkali metal ions, the ions move under the action of the electric field between the grooves. In a humid environment, this process takes place more violently.

 

2) Capacitor

 

The failure modes of capacitors include short circuit, open circuit, degradation of electrical parameters (including changes in capacitance, increase in loss tangent and decrease in insulation resistance), leakage and lead corrosion fracture.

 

Short circuit: Arcing at the edge of the electrode under high temperature and low pressure will cause a short circuit of the capacitor. In addition, under the action of mechanical stress such as external impact, it will also cause an instantaneous short circuit of the dielectric.

 

Open circuit: due to the oxidation of the lead wire and the electrode contact caused by the hot and humid environment, the low level is blocked and the anode lead foil is corroded and broken.

 

Degradation of electrical parameters: Degradation of electrical parameters due to the influence of the humid environment.

 

2.6 Board level circuit

 

The printed circuit board is mainly composed of an insulating base material, metal wiring, wires connecting different layers, and "pads" for welding components. Its main function is to provide a carrier for electronic components