The reasons for the failure of the chip inductor are mainly manifested in five aspects, namely, solder resistance, solderability, poor soldering, open circuit, magnetic circuit damage, etc., the following will be made on the five points. Explanation.
Before we do that, let's take a look at the inductor failure mode and the mechanism of chip inductor failure.
Inductor failure mode: Out of tolerance, open circuit, short circuit of inductance and other performance.
Chip power inductor failure causes:
1. The mechanical stress generated by the magnetic core during processing is relatively large and is not released;
2. There are impurities or voids in the core. The core material itself is not uniform, which affects the magnetic field of the core and causes the magnetic permeability of the core to deviate.
3. Sintering cracks due to sintering;
4. When the copper wire is soldered to the copper strip, the coil part is splashed with tin liquid, melting the insulating layer of the enameled wire, causing a short circuit;
5. The copper wire is slender, causing false welding when the copper tape is connected, and the open circuit is invalid.
First, solder resistance
The low-frequency chip power inductor has a sensitivity increase of <20% after reflow soldering.
Demagnetization occurs because the temperature of the reflow soldering exceeds the Curie temperature of the low frequency patch inductor material. After the chip inductor is demagnetized, the magnetic permeability of the chip inductor material returns to the maximum value, and the sense of inductance increases. The general control range is that after the chip inductor is resistant to soldering heat, the sensitivity increases by less than 20%.
The problem that the solder resistance may cause is that sometimes the circuit performance is all qualified when the small batch is manually soldered (the chip inductor is not heated as a whole, and the sensitivity is small). However, when the mass was patched, some circuit performance was found to be degraded. This may be due to the increase in the inductance of the chip after reflow soldering, which affects the performance of the line. In places where the sensitivity of the chip inductor is more stringent (such as signal receiving and transmitting circuits), attention should be paid to the solder resistance of the chip inductor.
Detection method: first measure the inductance value of the chip inductor at normal temperature, and then immerse the chip inductor in the molten solder pot for about 10 seconds, and take it out. After the chip inductor is completely cooled, the new inductance value of the chip inductor is measured. The percentage increase in sensitivity is both the solder resistance of the chip inductor.
Second, solderability
When the reflow temperature is reached, the metallic silver (Ag) reacts with the metallic tin (Sn) to form a eutectic, so that it cannot be directly tinned on the silver end of the chip inductor. Instead, nickel is plated on the silver tip (about 2um) to form an insulation layer, which is then tinned (4-8um).
Solderability test
The end of the chip inductor to be inspected is cleaned with alcohol, and the chip inductor is immersed in a molten solder can for about 4 seconds and taken out. If the solder coverage of the chip inductor tip reaches 90% or more, the solderability is acceptable.
Poor solderability
1. End oxidation: When the patch senses the influence of high temperature, humidity, chemicals, oxidizing gas (SO2, NO2, etc.), or the storage time is too long, the metal Sn on the end of the chip inductor is oxidized to SnO2. The chip inductor tip is dimmed. Since SnO2 does not form a eutectic with Sn, Ag, Cu, etc., the solderability of the chip inductor is degraded. Patch inductor product shelf life: half a year. If the tip of the chip inductor is contaminated, such as oily substances, solvents, etc., it will also cause a decrease in solderability.
2. The nickel plating layer is too thin: if the nickel layer is too thin, the nickel layer cannot be isolated. During reflow soldering, Sn on the inductor end of the chip reacts with its own Ag first, which affects the Sn on the inductor end of the chip and the solder paste on the pad, resulting in silver eating and chip inductor. The weldability is degraded.
Judgment method: Dip the chip inductor into the molten solder pot for a few seconds and take it out. If you find that there is a pothole at the end, and even if the porcelain is exposed, you can judge that it is eating silver.
3, poor welding
Internal stress
If the chip inductor generates large internal stress during the manufacturing process, and no measures are taken to eliminate the stress, in the reflow process, the attached chip inductor will produce a vertical piece due to the influence of internal stress, commonly known as the tombstoning effect.
To determine whether there is a large internal stress in the chip inductor, a simpler method can be adopted:
Take hundreds of chip inductors and put them into a general oven or low-temperature furnace. Warm up to about 230 °C, keep warm, and observe the inside of the furnace. If you hear the sound of a slap, and even the sound of the film jumping, the product has a large internal stress.
Component deformation
If the chip inductor product has a bending deformation, there will be an amplification effect when soldering.
Poor welding, cold welding
Welding is normal
Improper pad design
a. Both ends of the pad should be designed symmetrically to avoid different sizes, otherwise the melting time and wetting force of the two ends will be different.
b. The length of the solder joint is above 0.3 mm (ie, the coincidence length of the metal end of the chip inductor and the pad).
c. The length of the pad space is as small as possible, generally not exceeding 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 width of MLCI.
Poor patch
When the patch is offset by the angle θ due to the unevenness of the pad or the sliding of the solder paste. Due to the wetting force generated when the pad is melted, the above three conditions may be formed, in which the self-reduction is dominant, but sometimes the pulling is more oblique, or the single point is pulled, the chip inductor is pulled to a welding. On the plate, even pulled up, erected or erected (steep phenomenon). At present, a placement machine with θ angle offset visual inspection can reduce the occurrence of such failures.
Welding temperature
The soldering temperature curve of the reflow soldering machine must be set according to the requirements of the solder. The solder at both ends of the chip inductor should be melted at the same time to avoid the time of the wetting force at both ends, which causes the chip inductor to shift during the soldering process. . In the event of poor soldering, check to see if the reflow soldering temperature is abnormal or the solder has changed.
The inductor is easily damaged in the case of rapid cooling, rapid heat or local heating. Therefore, special attention should be paid to the welding temperature control during welding, and the welding contact time should be shortened as much as possible.
Fourth, the open circuit of the open circuit, poor soldering contact
Remove the chip inductor test from the board and check whether the chip inductor performance is normal.
Current burn through
If the rated current of the selected chip inductor bead is small, or there is a large inrush current in the circuit, the current will burn through, and the chip inductor or the magnetic bead will fail, resulting in an open circuit. Remove the chip inductor test from the board, the chip inductor fails, and sometimes there is burnt marks. If there is a current burn-through, the number of failed products will be more, and the failed products in the same batch generally reach above the percentage.
Open welding
During the reflow soldering, the quenching heat is generated, causing stress inside the chip inductor, resulting in a defect in the chip inductor having a small part of the internal open circuit, which causes the chip inductor to open. Remove the chip inductor test from the board and the chip inductor fails. In the event of an open weld, the number of failed products is generally small, and the number of failed products in the same batch is generally less than a thousand.
Jinhao Technology Integrated Inductor
Five, the magnet is damaged
Magnet strength
If the chip inductor is not sintered well or other reasons, the overall strength of the ceramic body is insufficient, and the brittleness is large. When the patch is applied, the product is damaged by the impact of the external force.
Adhesion
If the adhesion of the silver layer at the end of the chip inductor is poor, when the reflow is performed, the chip inductor is quenched and quenched, the thermal expansion and contraction generate stress, and the porcelain body is impacted by external force, which may cause the chip inductor tip and porcelain. The body is separated and detached; or the pad is too large. When reflowing, the wetting force generated by the melting of the solder paste and the reaction of the tip is greater than the adhesion of the tip, causing damage to the tip.
The chip inductor is over-fired or burnt, or micro-cracks are generated inside during the manufacturing process. During reflow soldering, it is quenched and hot, causing stress inside the chip inductor, crystal cracking, or micro crack growth, causing damage to the magnet.
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