Jingke Electronics: Chip-level LED package solution for flip-chip gold-free

[Source: Gaogong LED 's " LED Good Products" magazine March issue reporter / Long Zonghui]

At present, the common problems in LED lighting applications are still dead lights and light decay. The dead light can be roughly divided into two cases, one is not bright at all, and the other is hot, not bright, cold, or flickering. The main reason for the lack of light is the open circuit of the electrical circuit. The reason for the flashing is that the gold wire is weakly soldered or poorly contacted. The culprit is the conductive channel used in the traditional packaging process - the gold wire.

There are three specific reasons for the disconnection of the gold wire. First, the diameter of the gold wire commonly used in the industry is 25-30 μm, which is about the thickness of the hair. The mechanical tension that it can withstand generally does not exceed 10 g, and there is a great risk of disconnection. The difference in thermal expansion coefficient of the encapsulating material is very large. For example, the general silica gel is on the order of 10-4, while the gold wire and the chip are on the order of 10-5. The internal stress caused by the difference in thermal expansion coefficient also causes the gold wire to be broken. Risk; in addition, surges can also cause the gold wire to blow.

At present, there are three ways to solve the gold wire breakage in the industry, namely, updating the white light encapsulation material; increasing the mechanical strength of the gold wire; and adopting a gold-free wire package form.

At present, the use of the first two methods to solve the gold line problem is difficult to achieve due to the constraints of specific process and technical conditions. Therefore, in recent years, high-power packaging device manufacturers represented by Jingke Electronics (Guangzhou) Co., Ltd. (hereinafter referred to as "Jingke Electronics") have invested in the development of gold-free packaging solutions.

Dr. Chen Haiying, Director of Electronic Application Development of Jingke Electronics, said in an interview with Gaogong LED's "LED Good Products" that Jingke Electronics' "Easy Series" and ceramic-based COB products are all based on patented technology - flip-chip welding technology. The chip-free and multi-chip module has no gold wire or solid-state adhesive package, and has many advantages such as high brightness, high light efficiency, high reliability, low thermal resistance, and good color consistency.

From the aspect of reliability, the packaged chip package form is mainly electrically connected by a gold wire, and the impact of a large current is extremely easy to burn the gold wire. The flip-chip package used to connect the chip and the ceramic substrate accounts for about 60% of the chip area. Compared with the gold wire, its electrical contact surface is more than doubled, and the current density can be increased by 10,000 times. The chip thrust will increase to more than 2000g, so the reliability of the entire packaged device will be higher.

From the aspect of thermal resistance, for the positive package chip packaging method, most of the current industry uses silver glue to solidify, the thermal conductivity of silver glue is generally 2.5-(m·k), in addition, the sapphire layer between the chip and the substrate The thermal conductivity is also only (m·k), and the heat generated by the LED chip is difficult to export downward through sapphire and silver paste.

From the material point of view, the chip and the substrate are mainly inorganic components, and the silver paste is an organic component. The organic component and the inorganic component are greatly different in thermal expansion coefficient, and in the case of alternating hot and cold, the chip is easily peeled off from the substrate.

The stripped chip will be more affected in terms of heat dissipation, which directly leads to an increase in light decay. The method of flip-chip bonding, the chip and the substrate are in direct contact, the heat conduction area is significantly increased, and the thermal conductivity of the metal alloy is substantially above 200 W/(m·k), so the thermal resistance of the packaged device is greatly reduced, and the heat dissipation is better.

From the viewpoint of light color uniformity, when the positive-load chip package is coated with the phosphor, the gold wire is likely to have a sticky powder phenomenon. Since the position of the gold wire may be different, when the position of the gold wire changes within a range, and the phosphor is on the gold wire, it is easy to appear that the final package device has uneven light color distribution. Flip-chip technology allows the phosphor to be applied directly on a flat surface, with better process operation and a more uniform light color distribution.

Unfinished , please refer to the March issue of Gaogong LED 's " LED Good Products" magazine.

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