PCB Technical - Share patch inductor failure cause summary(PCB Assembly Announcements)

Share patch inductor failure cause summary(PCB Assembly announcements)

The failure causes of chip inductors are mainly reflected in five aspects, namely, the failure caused by welding resistance, weldability, poor welding, open circuit on the machine, magnetic circuit damage, etc,

Before that, let's first understand the failure mode of PCBA inductor and the failure mechanism of chip inductor.

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

Failure reason of chip power 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 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 enamelled wire, resulting in short circuit;

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

1, Welding resistance

After reflow soldering, the inductance of low-frequency patch power inductor increases by < 20%.

Because the temperature of reflow soldering exceeds the Curie temperature of low-frequency chip inductor, demagnetization occurs. After the chip inductor demagnetizes, the permeability of the chip inductor material returns to the maximum value and the inductance increases. Generally, the control range is that after the chip inductor 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, the circuit performance is all qualified (at this time, the chip inductor is not heated as a whole, and the inductance rise 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 inductance accuracy of chip inductors (such as signal receiving and transmitting circuits), more attention should be paid to the soldering resistance of chip inductors.

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

2, Weldability

When the reflow temperature 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 chip inductor. 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 chip inductor to be tested with alcohol, immerse the chip inductor in the molten solder can for about 4 seconds, and take it out. The solderability is qualified if the solder coverage of the chip inductor end reaches more than 90%.

Poor weldability

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

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

Judgment method: immerse the chip inductor 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 silver eating occurs.

3. Poor welding

internal stress

If the SMD inductor produces large internal stress in the manufacturing process and no measures are taken to eliminate the stress, the SMD inductor will stand up due to the influence of internal stress during reflow soldering, commonly known as monument effect.

A simple method can be used to judge whether the chip inductor has large internal stress:

Take hundreds of chip inductors, put them into a general oven or low-temperature furnace, raise the temperature to about 230 degree Celsius, 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.

Element deformation

If the chip inductor has bending deformation, there will be amplification effect during welding.

Poor welding and faulty welding

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 welding length shall be more than 0.3mm (i.e. the overlapping length of the metal end of the patch inductor 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 the chip inductance is offset due to the unevenness of the solder pad or the sliding of the solder paste θ Angular time. Due to the wetting force generated during pad melting, 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 patch inductor will be pulled onto a pad, or even pulled up, inclined or upright (monument phenomenon). Current band θ The placement machine with angle offset visual detection 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 chip inductor melts at the same time, so as to avoid the displacement of the chip inductor 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, rapid heating or local heating. Therefore, special attention shall be paid to the control of welding temperature during welding, and the welding contact time shall be shortened as much as possible.

4, Open circuit, faulty welding and poor welding contact on the machine

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

Current burn through

If the rated current of the selected chip inductor magnetic bead is small, or there is a large impulse current in the circuit, the current will burn through, the chip inductor or magnetic bead will fail, resulting in an open circuit. Remove the chip inductance from the circuit board for test. The chip inductance fails and sometimes burns out. 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

The rapid cooling and heating during reflow soldering will cause stress in the chip inductor, resulting in the defect of a small part of the chip inductor with open circuit potential, resulting in the open circuit of the chip inductor. Remove the chip inductance test from the circuit board, and the chip inductance fails. In case of welding open circuit, 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

Poor sintering of the chip inductor or or other reasons cause insufficient overall strength and high brittleness of the porcelain body. When the chip is pasted, or the product is impacted by external force, the porcelain body is damaged.

adhesion

If the adhesion of the silver layer at the tip of the SMD inductor is poor, during reflow welding, the SMD inductor is rapidly cooled and heated, the stress caused by thermal expansion and contraction, and the porcelain body is impacted by external force, it may cause the separation and falling off of the SMD inductor tip 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 chip inductor is burned or burnt, or microcracks are generated in the manufacturing process. Rapid cooling and heating during reflow soldering will cause stress in the chip inductor, crystal crack or micro crack expansion, resulting in magnet damage, etc.