Analysis of common failure causes of inductance third party detection mechanism

Date：2022-05-20 17:28:25Views：962

Inductance is a physical quantity that measures the ability of the coil to produce electromagnetic induction. When a current is applied to a coil, a magnetic field will be generated around the coil, and the coil will have magnetic flux to pass through. The greater the power supply to the coil, the stronger the magnetic field and the greater the magnetic flux through the coil. Experiments show that the magnetic flux passing through the coil is directly proportional to the incoming current. Their ratio is called self inductance, also known as inductance.

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

Failure mechanism of molded wound chip inductor:

1. The mechanical stress generated by the magnetic core during processing is large and has not been released

2. There are impurities or holes in the magnetic core, and the magnetic core material itself is uneven, which affects the magnetic field condition of the magnetic core and makes the permeability of the magnetic core deviate;

3. Sintering cracks after sintering;

4. When the copper wire is connected with the copper strip by immersion welding, the tin liquid splashes on the coil part, melting the insulating layer of the enameled wire, resulting in a short circuit;

5. The copper wire is thin, causing false welding and open circuit failure when connected with the copper strip

1. Welding resistance

The sensitivity of low-frequency film increases < 20% after reflow soldering

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

The possible problem caused by welding resistance is that sometimes when small batch manual welding is carried out, the circuit performance is all qualified (at this time, the chip sensor is not heated as a whole, and the rise of sensor is small). However, when a large number of chips are pasted, it is found that the performance of some circuits decreases. This may be due to the increase of chip inductance after reflow soldering, which affects the performance of the circuit. In places with strict requirements on the accuracy of chip sensing (such as signal receiving and transmitting circuit), we should pay more attention to the welding resistance of chip sensing.

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

2. Weldability

Introduction to electroplating

When the temperature of reflow soldering is reached, the metal silver (Ag) will react with the metal tin (SN) to form a eutectic, so tin cannot be directly plated on the silver end of the sheet feeling. Instead, nickel (about 2um) is plated on the silver end to form an insulating layer, and then tin (4-8um) is plated.

Weldability test

Clean the end of the film sensor to be tested with alcohol, immerse the film sensor in the molten solder can for about 4 seconds, and take it out. If the solder coverage of the chip sensing end reaches more than 90%, the weldability is qualified.

Poor weldability

1) End oxidation: when the film 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 film sensing end is oxidized to SnO2, and the film sensing end becomes dark. Since SnO2 does not form eutectic with Sn, Ag and Cu, the sheet sense weldability decreases. Shelf life of film feeling products: half a year. If the sensing end is polluted, such as oily substances and solvents, it will also reduce the weldability

2) Nickel plating layer is too thin, eat silver: if nickel plating, the nickel layer is too thin to play an isolation role. During reflow soldering, Sn on the chip sensing end reacts with its own Ag first, which affects the co melting of Sn on the chip sensing end and solder paste on the pad, resulting in silver eating phenomenon and the decline of the solderability of the chip sensing.

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

3. Poor welding

internal stress

If the film sense produces large internal stress in the manufacturing process and no measures are taken to eliminate the stress, the pasted film sense will produce vertical film due to the influence of internal stress during reflow soldering, commonly known as monument effect.

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

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

Improper pad design

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

b. The length of welding shall be more than 0.3mm (i.e. the overlapping length between the metal end of sheet feeling and the pad)

c. The length of pad space shall be as small as possible, generally no more than 0.5mm.

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

Poor patch

When pasting, the film feeling is offset due to the uneven welding pad or the sliding of solder paste θ Horn. Due to the wetting force generated by the melting of the pad, the above three situations may be formed, in which self correction is the main, but sometimes it will be pulled more obliquely or at a single point. The sheet feeling will be pulled onto a pad, or even pulled up, inclined or upright (monument phenomenon). Current band θ The placement machine with visual detection of angular offset can reduce the occurrence of such failure.

welding temperature

The welding temperature curve of the reflow welder must be set according to the requirements of solder. Try to ensure that the solder at both ends of the sheet sense melts at the same time, so as to avoid the displacement of the sheet sense during the welding process due to the different time of generating wetting force at both ends. In case of poor welding, first confirm whether the temperature of the reflow welder is abnormal or the solder is changed.

The inductor is easy to be damaged in case of rapid cooling, 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.

Recommended temperature curve for reflow soldering

Recommended temperature curve for manual welding

4. Open circuit on the machine

Faulty welding and poor welding contact

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

Burn through current

For example, the rated current of the magnetic bead is small, or there is a large impulse current in the circuit, which will cause the current to burn through, and the chip inductance or magnetic bead will fail, resulting in an open circuit in the circuit. When the chip sense is removed from the circuit board for test, the chip sense is invalid and sometimes there are signs of burning. If 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

There is little hidden danger of open circuit in the inner part of the cold sensing sheet, resulting in large open circuit stress in the inner part of the cold sensing sheet. Remove the chip sense test from the circuit board, and the chip sense fails. In case of open circuit in welding, the number of failed products is generally small, and the number of failed products in the same batch is generally less than 1000.

5. Magnet damage

Magnet strength

The porcelain body is not strong enough and brittle due to poor sintering or other reasons. When pasting, or the product is impacted by external force, the porcelain body is damaged

adhesion

If the adhesion of the silver layer of the sheet sensing end is poor, during reflow welding, the sheet sensing end may be cooled and heated rapidly, the stress caused by thermal expansion and contraction, and the porcelain body may be impacted by external forces, which may cause the separation and falling off of the sheet sensing end and the porcelain body; Or the pad is too large. During reflow soldering, the wetting force generated by solder paste melting and end reaction is greater than the end adhesion, resulting in end damage.

The sheet is over burnt or raw burnt, or microcracks are generated inside during the manufacturing process. In case of rapid cooling, cracks or micro cracks will be generated in the porcelain body, resulting in rapid cooling and thermal stress.

The above is the related content of inductance failure analysis compiled by the core detection team. I hope it will be helpful to you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing organization for electronic components in China. It has three standardized laboratories with an area of more than 1000 square meters. The scope of testing services covers: testing and verification of electronic components, identification of IC authenticity, product design, material selection, failure analysis, function testing, factory incoming material inspection, X-ray testing of components, tape braiding and other testing items.