After dissolving the sample, it was found that the gold wire was well welded and there was evidence of black burning near the electrode.
By zooming in, each failed LED has a blackout phenomenon, and the burned position is near the electrode.
Possible conditions for this short circuit:
1. The voltage is too large and breaks through the PN junction.
2. The surface roughening treatment of the chip is not completed in the electrode area, resulting in poor ohmic contact.
3, epitaxial defects, in the roughening time to enlarge the defects.
So, what are the fatal injuries of the vertical chip?
I just saw an article by Jinjian Testing:
A customer uses a silicone package, a vertical flip-chip light source bonded by a conductive silver paste, and a leakage phenomenon occurs. Through the analysis of the bad lamp beads, Jinjian detected abnormal silver elements on the side of the chip, and observed that the silver particles gradually diffused from the bottom positive silver paste region to the vicinity of the upper PN junction side of the chip. It is very likely that the failure of the lamp bead leakage is caused by the ion migration of silver ions from the solid silver paste on the side of the chip. Silver ion migration phenomenon is gradually formed during the use of the product. As the migration phenomenon increases, the final silver ion will turn on the chip PN junction, resulting in a low resistance path on the side of the chip, resulting in abnormal leakage current of the chip. Even the chip is shorted. The reason for silver migration is multi-faceted, but the main reason is that the silver-based material is damp. After the silver glue is wet, the invading water molecules ionize the silver and migrate along the side of the chip under the action of the electric field from the bottom to the top. Therefore, Jinjian Testing recommends that customers carefully use the silicone package, silver glue to bond the vertical flip chip lamp beads, use the gold tin eutectic soldering method to fix the chip on the bracket, and strengthen the waterproof performance detection of the lamp.
Based on the summary of a large number of LED failure cases, it was found that vertical flip-chips bonded with silica gel and silver glue are prone to leakage. This is because the silica gel has the physical properties of water absorption and gas permeability, and it is easy to make the conductive silver glue damp. After the water molecules invade, the hydrogen ions and hydroxide ions are electrolyzed on the surface of the silver-containing conductor, and the silver in the silver gel is in the DC electric field and the hydroxide. Ionized by ions to produce positive valence silver ions. At the same time, because the anode of the vertical flip chip is on the bottom surface and the cathode is on the top, the current flows through the chip almost from bottom to top, and the positive-valent silver ions migrate in the upper direction of the chip, forming a dendritic migration on the side of the chip. The path, and eventually bridging the PN junction of the LED active area, causes the chip to be in an ion-conducting state, causing leakage or even short-circuit faults.
I still remember that when I used the RGB lamp beads for the display, the red light was easy to open the dead light, causing the customer to complain. Red light is a vertical chip that is fixed with silver glue. After the damp, there will be signs of cracking of the silver glue and the stent.
The way the display industry circumvents this problem is to add a wire bond to the silver-glued attachment area. This method is simple and effective.
A brief summary:
For the packaging factory:
1. Control the amount of glue and the shape of the gel.
2. Add a bottom line.
3. Eutectic treatment (traditional ICs have many such operations).
For the application plant: 1, the ripple of the power supply must be strictly controlled.
2. The inspection of the lamp bead should be strengthened.
3. Pay attention to dehumidification before using the lamp bead.
Any kind of chip form has its advantages and certainty, don't blindly worship. The most suitable is the best. The inspection of the lamp bead is not just a luminous flux, voltage, or light effect. Pay attention to the conditions of use and the environment of use.
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