Brief introduction and failure analysis of inductor

Source:Kyotoku

1. Essence of inductance

We usually say inductance refers to inductive devices, which are made of insulated wires (such as enameled wire, sandbag wire, etc.).

In a circuit, when the current flows through a conductor, it will produce an electromagnetic field. The size of the electromagnetic field divided by the size of the current is the inductance.

Inductance is a physical quantity to measure the ability of a coil to produce electromagnetic induction. When a current is applied to a coil, a magnetic field will be generated around the coil, and the magnetic flux will pass through the coil. The larger the power supply into the coil, the stronger the magnetic field and the greater the magnetic flux through the coil. Experiments show that the magnetic flux through the coil is directly proportional to the current, and their ratio is called self inductance coefficient, also called inductance.

Brief introduction and failure analysis of inductor

1.1 inductance classification

According to the type of inductance: fixed inductance, variable inductance.

According to the nature of the magnet: air core coil, ferrite coil, iron core coil, copper core coil.

According to the nature of work: antenna coil, oscillation coil, choke coil, notch coil, deflection coil.

According to the winding structure classification: single layer coil, multi-layer coil, honeycomb coil.

According to the working frequency classification: high frequency coil, low frequency coil.

According to the structure characteristics: core coil, variable inductance coil, color code inductance coil, non core coil, etc.

Hollow inductors, magnetic core inductors and copper core inductors are generally medium frequency or high frequency inductors, while core inductors are mostly low frequency inductors

1.2 material and process of inductor

Inductor is generally composed of frame, winding, shield, packaging material and magnetic core.

1) Skeleton: generally refers to the bracket of winding coil. It is usually made of plastics, bakelite and ceramics, and can be made into different shapes according to actual needs. Small inductors generally do not use the skeleton, but directly wind the enameled wire on the magnetic core. Instead of magnetic core, frame and shield cover, the hollow inductor is wound on the mold first, then the mold is removed, and a certain distance is drawn between the coils.

Brief introduction and failure analysis of inductor

2) Winding: refers to a group of coils with specified functions, which can be divided into single layer and multi-layer. There are two types of single layer: close winding and inter winding; There are many kinds of multi-layer winding, such as layered flat winding, random winding, honeycomb winding and so on.

Brief introduction and failure analysis of inductor

3) Magnetic core: nickel zinc ferrite or manganese zinc ferrite and other materials are generally used. It has "I" shape, column shape, cap shape, "e" shape, can shape and other shapes.

Brief introduction and failure analysis of inductor

Core: mainly silicon steel sheet, permalloy, etc., its shape is mostly "e" type.

Brief introduction and failure analysis of inductor

4) Shield cover: used to avoid the magnetic field produced by some inductors during operation affecting the normal operation of other circuits and components. The inductor with shield will increase the loss of coil and reduce the Q value.

Brief introduction and failure analysis of inductor

5) Packaging materials: some inductors (such as color code inductor, color ring inductor, etc.) are wound and sealed with packaging materials. The packaging material is plastic or epoxy resin.

Brief introduction and failure analysis of inductor

Brief introduction and failure analysis of inductor

Brief introduction and failure analysis of inductor

1.3 main parameters of inductance

1) Inductance: also known as self inductance coefficient, is a physical quantity that represents the self induction ability of an inductor.

The size of inductance mainly depends on the number of coils, winding mode, whether there is a magnetic core and the material of the magnetic core, etc. Generally, the more the number of coils and the denser the coils, the greater the inductance. The inductance of the coil with magnetic core is larger than that of the coil without magnetic core; The greater the permeability of the core, the greater the inductance.

Brief introduction and failure analysis of inductor

The higher the working frequency, the smaller the size of the inductor

For the same impedance, the higher the frequency, the smaller the sensitivity

If the inductance is small, the number of turns can be reduced, and the size of the inductor can be reduced

The sensitivity is small, and the permeability of the material should not be too high

(the higher the permeability of the material, the less suitable it is for high frequency operation.)

Brief introduction and failure analysis of inductor

2) Allowable deviation: refers to the allowable error between the nominal inductance and the actual inductance. Generally used in oscillation or filter circuit, the inductance requires high accuracy, and the allowable deviation is ± 0.2%~ ± 0.5%; However, the precision of the coils used for coupling and high frequency current blocking is not high; The allowable deviation is ± 10%~15%。

3) Natural frequency: the equivalent circuit of the inductor is actually the parallel resonant circuit of the inductor and the capacitor, and its oscillation frequency

F0 = is the natural frequency.

It is also defined as the corresponding self resonant frequency when inductive reactance and capacitive reactance are equal. When using inductance coil, in order to ensure the stability of inductance, the working frequency of the coil should be far lower than the natural frequency.

4) Distributed capacitance: refers to the capacitance between turns of coils and between coils and cores. The smaller the distributed capacitance of the inductor, the better its stability.

Methods to reduce the distributed capacitance:

1) If magnetism is a conductor, use a material with a low dielectric constant

2) The starting end and the ending end are far away (the included angle is 40 °)

   3) Single layer winding as far as possible, and increase the turn to turn distance

4) In multi-layer winding, progressive winding is adopted to avoid back and forth winding

5) DC resistance RDC: refers to the resistance value of the device under DC state, which is DC resistance, indicating the quality of the internal coil of the device.

6) Impedance Z: represents the total resistance of the component to the AC current flowing through it at a given frequency.

7) Quality factor: also known as Q value, is the main parameter to measure the quality of inductance. It refers to the ratio of inductance to equivalent loss resistance when the inductor works under AC voltage of a certain frequency.

The higher the Q value of the inductor, the smaller the loss and the higher the efficiency. In practice, q is not only related to the DC resistance of the coil, but also to the dielectric loss of the coil framework, the loss of the iron core and shield, and the skin effect when working at high frequency. It is not easy to improve the Q value of the coil.

The choice of practical inductor must take into account the smaller inductance fluctuation and higher Q value at the same time.

Brief introduction and failure analysis of inductor

8) Rated current IR: refers to the maximum current allowed when the inductor works normally. If the working current exceeds the rated current, the performance parameters of the inductor will be changed due to heating, or even burned due to over-current.

Failure analysis of inductor

Inductor failure mode: out of tolerance, open circuit and short circuit of inductance and other performance

The failure mechanism of die wound chip inductor is as follows

1. The mechanical stress of the magnetic core in the process of processing is large and has not been released

2. There are impurities or holes in the core, and the material of the core is not uniform, which affects the magnetic field of the core and makes the permeability of the core deviate;

3. Because of the sintering crack after sintering;

4. When the copper wire is connected with the copper strip by immersion welding, the coil part is splashed with liquid tin, which melts the insulating layer of the enameled wire and causes a short circuit;

5. The thin copper wire causes false soldering and open circuit failure when it is connected with the copper strip

1. Welding resistance

After reflow soldering, the sensitivity of low frequency film increases by 20%

Because the temperature of reflow soldering exceeds the Curie temperature of low-frequency sheet sensing material, demagnetization occurs. After demagnetization, the permeability of the sheet sensing material returns to the maximum value, and the sensitivity increases. The control range of general requirement is that the increase range of sensing is less than 20% after welding heat resistance.

The possible problem of solderability is that sometimes when small batch of manual welding, the circuit performance is all qualified (at this time, the chip sense is not heated as a whole, and the sense rise is small). However, it is found that the performance of some circuits is degraded when a large number of chips are mounted. This may be due to the increase of chip sensitivity after over reflow soldering, which affects the performance of the circuit. In the places where the accuracy of chip sensing is strictly required (such as signal receiving and transmitting circuits), more attention should be paid to the solder resistance of chip sensing.

Detection method: first measure the sensitivity value of the chip sensor at room temperature, then immerse the chip sensor in the molten tin can for about 10 seconds, and take it out. After the film sense is completely cooled, measure the new sensitivity value of the film sense. The percentage of the increase of the sensitivity is the welding resistance of the chip

2. Solderability

Brief introduction of electroplating

When the reflow soldering temperature is reached, silver (Ag) will react with tin (SN) to form eutectic, so tin can not be directly plated on the silver end of the sheet. Instead, nickel plating (about 2 UM) was used to form an insulating layer on the silver end, and then tin plating (4-8 UM) was used.

Weldability test

Clean the end of the chip sensor to be detected with alcohol, immerse the chip sensor in the molten tin can for about 4 seconds, and take it out. The solderability is qualified if the solder coverage of the chip sensing end is more than 90%.

Poor solderability

1) End oxidation: when the tablet feels the influence of high temperature, humidity, chemicals and oxidizing gases (SO2, NO2, etc.), or the storage time is too long, the metal Sn on the sensing end is oxidized to SnO2, and the sensing end becomes dark. Because SnO2 does not form eutectic with Sn, Ag, Cu and so on, the solderability of sheet sensing decreases. Shelf life: half a year. If the chip sensing end is contaminated, such as oily substances, solvents, etc., the solderability will also be reduced

2) Nickel coating is too thin, eat silver: if nickel, nickel coating is too thin can not play a role in isolation. During reflow soldering, the Sn on the chip sensing end reacts with its own Ag first, which affects the eutectic of Sn on the chip sensing end and the solder paste on the pad, resulting in the phenomenon of eating silver and the decrease of solderability of the chip sensing.

Judgment method: immerse the chip sensor in the molten tin can for a few seconds and take it out. If it is found that there are pits at the end, or even porcelain body exposed, it can be judged that there is silver eating phenomenon.

3. Poor welding

internal stress

If there is a large internal stress in the process of manufacturing and no measures are taken to eliminate the stress, in the process of reflow soldering, the attached film will be erect due to the influence of internal stress, commonly known as stele effect.

Brief introduction and failure analysis of inductor

A simple method can be used to judge whether there is a large internal stress in the sense of film

Take a few hundred pieces and put them into an ordinary oven or a low-temperature oven. Raise the temperature to about 230 ℃, keep the temperature and observe the situation in the oven. If you hear the sound of crackling, or even the sound of the film jumping up, it indicates that the product has greater internal stress.

Component deformation

If the film sensing product has bending deformation, there will be amplification effect during welding.

Poor welding and false welding

Welding is normal

Brief introduction and failure analysis of inductor

Improper pad design

Brief introduction and failure analysis of inductor

a. Both ends of the pad should be designed symmetrically to avoid different sizes, otherwise the melting time and wetting force of both ends will be different

b. The welding length is more than 0.3 mm (i.e. the overlap length between the metal end of the chip and the pad)

c. The length of pad space should be as small as possible, generally not more than 0.5mm.

d. The width of pad itself should not be too wide, and its reasonable width should not exceed 0.25 mm compared with MLCI width

When the chip is pasted, the chip sense is offset due to uneven pad or slip of solder paste θ Angle. Due to the wetting force generated during the melting of the pad, the above three situations may be formed, in which self straightening is the main one, but sometimes the pulling is more inclined or single point straightening occurs, and the sheet feeling is pulled to a pad, or even pulled up, inclined or upright (stele phenomenon). Current belt θ This kind of failure can be reduced by the mounter with angle offset visual inspection

Brief introduction and failure analysis of inductor

welding temperature 

The welding temperature curve of reflow welder should be set according to the requirements of solder, and the solder at both ends of the chip sensor should be fused at the same time as far as possible, so as to avoid the displacement of the chip sensor in the welding process due to the different time of generating wetting force at both ends. In case of bad welding, confirm whether the reflow welder temperature is abnormal or the solder is changed.

The inductor is easy to be damaged in the case of rapid cooling, rapid heating or local heating. Therefore, special attention should be paid to the control of welding temperature during welding, and the welding contact time should be shortened as much as possible

Brief introduction and failure analysis of inductor

Recommended temperature curve for reflow soldering

Brief introduction and failure analysis of inductor

Recommended temperature curve for manual welding

4. Open circuit on the computer

False welding and poor welding contact

Remove the chip sense from the circuit board to test whether the chip sense performance is normal

Current burn through

If the rated current of the selected chip inductor and magnetic bead is small, or there is a large impulse current in the circuit, the current will burn through, and the chip inductor or magnetic bead will fail, resulting in an open circuit. When the chip sense is removed from the circuit board for testing, the chip sense fails and sometimes burns out. If the current burn through occurs, the number of failed products will be more, and the failed products in the same batch generally reach more than 100 grades.

Welding open circuit

The rapid cooling and heating during reflow soldering will cause the stress in the inner part of the chip inductor, which will lead to a small part of the defect of the chip inductor with hidden trouble of open circuit. After the chip sense test is removed from the circuit board, the chip sense fails. In case of welding open circuit, the number of failed products is generally less than 1000 in the same batch.

5. Broken magnet

Magnet strength

The porcelain body is not strong enough and brittle due to poor sintering or other reasons. The porcelain body is damaged due to the impact of external force during the placement

adhesion

If the adhesion of the silver layer on the chip tip is poor, during reflow soldering, the chip tip and the porcelain body may separate and fall off due to the rapid cooling and heating of the chip tip, the stress caused by thermal expansion and contraction, and the impact of external force on the porcelain body; Or if the pad is too large, the wetting force of solder paste melting and end reaction is greater than the end adhesion during reflow soldering, resulting in end failure.

During the manufacturing process, micro cracks are produced inside the chip. The rapid cooling and heating during reflow welding will cause stress in the chip, crystal crack or microcrack expansion, which will damage the porcelain body.