The failure reasons of patch inductance are mainly manifested in five aspects: weldability, weldability, poor welding, on-line open circuit, magnetic circuit breakage and so on. The following five points will be explained.
Before that, we will first understand the failure modes of inductors and the mechanism of the failure of patch inductors.
Inductor failure mode: inductance, and other performance of overshoot, open circuit and short circuit.
The reason for the failure of the power inductor is:
The mechanical stress produced by the 1. core during machining is large and has not been released.
2. There are impurities in the core or the material of the hollow core itself is not uniform, which affects the magnetic field of the core and makes the permeability of the core deviate.
3. sintering cracks due to sintering.
4. When copper wire and copper strip are dipped and welded, the coil part is splashed with tin liquid, which melts the insulating layer of enamelled wire and causes short circuit.
5. the copper wire is slender. When it is connected with the copper strip, it causes false welding and open circuit failure.
1. Soldering resistance
The inductance of low-frequency patch power is increased by <20% after reflow soldering.
Because the temperature of reflow soldering exceeds the Curie temperature of low frequency chip inductor material, demagnetization occurs. After the demagnetization of the patch inductor, the permeability of the patch inductor material reaches the maximum value and the inductance increases. The general requirement is that the chip inductor is resistant to welding heat and the sensitivity increases less than 20%.
The problem that solderability may cause is that sometimes when small batches of manual soldering, the circuit performance is all qualified (at this time, the patch inductance is not heated as a whole, and the inductance rise is small). However, when a large number of patches were posted, the performance of some circuits decreased. This may be due to the increase of inductance inductance after reflow, which affects the performance of the circuit. Where the precision of inductance is strictly required (such as signal receiving and transmitting circuit), more attention should be paid to the solderability of patch inductance.
Detection method: First, the inductance of the patch is measured at room temperature, and then the patch inductance is immersed in the molten tin can for about 10 seconds, and removed. After the chip inductor is completely cooled, the new inductance value of the patch inductor is measured. The percentage of increase in sensitivity is the size of the solder resistance of the chip inductor.
Two. Solderability
When the reflow temperature is reached, silver (Ag) reacts with tin (Sn) to form a eutectic, so it can not be directly tin plated on the silver end of the patch inductor. Instead, the silver end is first plated with nickel (about 2um), forming an insulating layer, and then tin (4-8um).
Weldability test
The end of the patch inductor to be tested is cleaned with alcohol, and the patch inductor is immersed in the molten tin can for about 4 seconds and removed. If the solder coverage rate of the chip inductor reaches more than 90%, the weldability is qualified.
Poor weldability
1. End oxidation: When the patch inductance is affected by high temperature, humidity, chemicals, oxidizing gases (SO2, NO2, etc.), or the storage time is too long, the metal Sn on the patch inductance end is oxidized to SnO 2, and the patch inductance end becomes dark. Because SnO2 does not generate co melts with Sn, Ag and Cu, the solder inductance of the chip inductor decreases. The shelf life of patch inductor is half a year. If the chip inductor ends are contaminated, such as oil substances, solvents, etc., it will also cause a decrease in weldability.
2. Nickel plating is too thin: if nickel is plated, the nickel layer is too thin to act as an isolating agent. In reflow soldering, Sn on the inductor end of the patch reacts with Ag first, which affects the co-melting of Sn on the inductor end of the patch and solder paste on the pad, resulting in the phenomenon of eating silver, and the solderability of the patch inductor decreases.
Method of judgement: dip the chip inductor into the molten tin tank for a few seconds. If a pit is found at the end, or even the porcelain body is exposed, it can be judged that the phenomenon of eating silver occurs.
3. Poor welding
internal stress
If the patch inductor produces large internal stress in the process of fabrication and does not take measures to eliminate the stress, in the reflow welding process, the patch inductor will produce the standpoint effect because of the influence of internal stress, commonly known as the standpoint effect.
To judge whether the chip inductor has large internal stress, a simpler method can be adopted:
Take hundreds of patch inductors and put them into a general oven or low temperature oven. The temperature is raised to about 230 C. The temperature is kept warm and the situation in the oven is observed. If you hear the loudspeaker, or even the sound of a piece jumping up, it shows that the product has a greater internal stress.
Three. Component deformation
If the chip inductor product has bending deformation, there will be magnification effect when welding.
Poor welding and false welding
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