The failure reason of SMD inductance is mainly shown in five aspects, which are the failure caused by weldability, weldability, poor welding, open circuit on the machine, magnetic circuit damage, etc.
Inductor failure mode: inductance and other performance of the out-of-tolerance, open, short.
Failure reason of SMT power inductance:
1. The mechanical stress generated by the magnetic core in the process of processing is relatively large and has not been released;
2. There are impurities in the magnetic core or uneven materials of the hollow magnetic core, which affect the magnetic field of the magnetic core and cause the permeability deviation of the magnetic core;
3. Sintering cracks caused by sintering;
4. When the copper wire is connected with the copper strip by dip welding, the part of the coil splashes tin liquid, which melts the insulating layer of enameled wire and causes a short circuit;
5. The copper wire is fine, causing false welding and open circuit failure when connected with the copper strip.
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I. welding resistance
The inductance of low frequency patch power increases by < 20% after reflow soldering.
Demagnetization occurs because the reflow soldering temperature exceeds the Curie temperature of the low frequency SMT inductor material.After demagnetization of SMT inductance, the permeability of SMT inductance material returns to the maximum value and the inductance increases.General requirements of the control range is the SMT inductance resistance to welding heat, sensitivity rise less than 20%.
The problem that welding resistance may cause is that sometimes when manual welding is done in small batch, the circuit performance is all qualified (at this time, SMT inductance is not heated as a whole, and the inductance increases little).But when a large number of patches, it is found that the performance of some circuits decreases.
This may be because the inductance of SMT increases after reflow soldering, which affects the line performance.More attention should be paid to the weldability of SMT inductance where the inductance precision is strictly required (such as signal receiving and transmitting circuit).
Detection method: measure the inductance of SMT at room temperature, then dip SMT inductance into the melting tin pot for about 10 seconds and take it out.After the SMT inductance is cooled thoroughly, the new inductance value of SMT inductance is measured.The increase percentage of inductance is the welding resistance of SMT inductance.
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When the reflow soldering temperature is reached, the metallic silver (Ag) reacts with the metallic tin (Sn) to form a eutectic, so it is not possible to deposit tin directly on the silver end of SMT inductor.Instead, the silver ends are nickel plated (about 2um) to form an insulating layer, followed by tin plated (4-8um).
Clean the end of the SMT inductor to be tested with alcohol. Immerse the SMT inductor in the melting tin tin for about 4 seconds and remove.If the solder coverage of SMT inductance terminal is above 90%, the weldability is qualified.
1. End oxidation: when the SMT is electrically affected by high temperature, humidity, chemicals and oxidizing gas (SO2, NO2, etc.), or the storage time is too long, the metal Sn on the end of SMT inductance will oxidize to SnO2 and the end of SMT inductance will become dark.Because SnO2 does not generate eutectic materials such as Sn, Ag and Cu, the weldability of SMT inductor decreases.Shelf life of SMD inductor: half a year.If SMT inductor end is contaminated, such as oily substances, solvents, etc., will also cause a decrease in weldability.
2, nickel plating layer is too thin: if the nickel plating, the nickel layer is too thin to play a role in isolation.In reflow welding, Sn on the end of SMT inductance and its Ag react first, which affects the co-fusion of Sn on the end of SMT inductance and solder paste on the solder disc, causing the phenomenon of eating silver and decreasing the weldability of SMT inductance.
Judge method: dip SMT inductance into the melting tin can for a few seconds, take out.If it is found that the end of the pothole, or even the appearance of porcelain outside the body, it can be judged that the phenomenon of eating silver.
3. Poor welding
If the SMT inductor produces a large internal stress in the process of making, and no measures are taken to eliminate the stress, in the process of reflow welding, the pasted SMT inductor will produce a piece due to the influence of internal stress, commonly known as the stele effect.
A simple method can be adopted to determine whether there is a large internal stress in SMT inductor:
Take hundreds of SMT inductance, put into the general oven or low temperature furnace, temperature rise to about 230℃, insulation, observe the situation in the furnace.If hear the sound of beep beep beep, even the sound of the film jump up, that the product has greater internal stress.
If the SMD inductor has bending deformation, there will be amplification effect when welding.
Poor welding, virtual welding
When the SMT due to the uneven pad or paste sliding, resulting in the SMT inductance deviation theta.As a result of the wetting force produced when welding pad melt, may form the above three cases, which is self-correcting, but sometimes will appear more inclined to pull, or single point to pull the situation, SMT inductance is pulled to a welding pad, even be pulled up, oblique or upright (stele phenomenon).At present, the SMT machine with theta Angle deviation can reduce the occurrence of such failures.
The welding temperature curve of the reflow welder should be set according to the requirements of the solder. The solder at both ends of the SMT inductance should be melted at the same time as possible to avoid the time difference of wetting force at both ends, which may cause the SMT inductance to shift during the welding process.In case of defective welding, please first confirm whether the reflow welder temperature is abnormal or the solder has been changed.
Inductance in the case of rapid cooling, rapid heating or local heating is easy to damage, so when welding should pay special attention to the control of welding temperature, and shorten the welding contact time as far as possible.
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Four, open machine welding, welding contact is bad
Remove SMT inductance from circuit board and test whether SMT inductance performance is normal.
Current burn through
If the rated current of the selected SMD inductance magnetic bead is small, or there is a large impact current in the circuit will cause current burn through, SMD inductance or magnetic bead failure, resulting in open circuit.Remove SMT inductance test from circuit board. SMT inductance fails and sometimes burns out.If the current burns through, the number of failed products will be large, and the failed products in the same batch generally reach over 100 grade.
The welding way
When the reflow welding hot and cold, make the SMT inductor internal stress, resulting in a very small part of the internal open circuit hidden trouble SMT inductor defects become large, resulting in SMT inductor open circuit.Remove SMT inductance test from circuit board. SMT inductance fails.If open welding occurs, the number of failed products is generally small, the same batch of failed products are generally less than 1000 grade.
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5. Damaged magnets
The intensity of magnets
The SMT inductance sintering is not good or other reasons, resulting in the overall strength of the porcelain body is not enough, brittleness, in the SMT, or the product by the impact of external forces caused damage to the porcelain body.
If the adhesion of the silver layer at the end of SMT inductance is poor, during reflow welding, the SMT inductance will be rapidly cooled and heated, heat expansion and contraction will cause stress, and the ceramic body will be impacted by external forces, which may cause the SMT inductance end and ceramic body to separate and fall off.Or if the pad is too large, during reflow welding, the wetting force generated by the melt of solder paste and the reaction of the end end is greater than the adhesion of the end end, resulting in the end end damage.
The SMT inductor is overfired or uncooked, or during the manufacturing process, micro-cracks are generated inside.In reflow welding, the quick cooling and hot make the SMT inductor generate stress, crystal crack or micro-crack expansion, causing damage to the magnet and so on.
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Post time: Oct-30-2019