[LED Knowledge] Quality Control of LED Gold Wire Bonding Process
Abstract: The LED chip lead of light-emitting diodes is a gold ball thermosonic bonding process, which uses thermal energy, pressure, and ultrasound to bond the chip electrode and the bonding area on the bracket with AU wires to end the connection of the inner and outer leads operation.
1. The LED chip leads of light-emitting diodes are made of gold ball thermosonic bonding technology, which uses thermal energy, pressure, and ultrasound to bond the chip electrodes and the bonding area on the bracket with AU wires to end the connection of the inner and outer leads Work .
2.1 Bonding orientation and solder joint shape requirements
(1) The gold ball of the first bonding point must not have more than 1/4 outside the chip electrode, and cannot touch the boundary between the P-type layer and the N-type layer. As shown in Figure 1, the GaAs single electrode chip wire gold ball Pass and fail comparison pictures.
Figure 1 Orientation criteria for the first bond point
(2) The second solder joint must not exceed the planning of the bracket bonding area, as shown in Figure 2.
Figure 2 Orientation criteria for the second bond point
(3) The ball diameter A of the first welding point is about 3.5 times the wire diameter Ф, the sphere becomes uniform and excellent, and the wire and the ball are concentric; the shape of the second welding point is wedge-shaped, and the width D is about 4 times the wire diameter Φ Right and left, the spherical thickness H is 0.6-0.8 times of Ф. The root of the gold ball must not have obvious damage or thinning, and there must be no obvious crack at the wedge of the second solder joint.
Figure 3 First solder joint shape
(4) Other apparent requirements after bonding: no solder wire, no chip, no damaged chip, no pressure electrode. The surface of the chip must not have metal slag, broken wires and other components that cannot be cleaned due to bonding. Pollutants. There is no short circuit, no broken wire, no hook wire.
Figure 4 Second solder joint shape
2.2 Pull test
Tension test is widely used in thermosonic welding wire. It is a kind of damage test. It can detect the thinnest break point. The characteristics of the test point and the arch wire directly affect the measurement value. The requirements for the pulling force and breakpoint orientation of the LED work key alloy wire are generally: when pulling the wire, the first point of the gold ball cannot be separated from the electrode, and the second point of the wedge cannot be separated from the bracket bonding area, that is, no matter what the pulling force F is Both are judged to be unqualified; if disconnected from other points, a gold wire diameter Ф25um pulling force value F> 5g is a pass, and a gold wire diameter Φ32um pulling force value F> 7g is a pass.
3. Bonding process conditions
3.1 Bonding temperature
The bonding temperature can help remove surface contaminants, such as moisture, oil, water vapor, etc .; increasing the vividness of the molecules is beneficial to the composition of the alloy. However, too high temperature will not only cause excessive oxides to affect the bonding quality, but also due to the effects of thermal stress and strain, the accuracy of image monitoring and the reliability of the device will also decline. In practice, the temperature control system will increase the preheating zone and cooling zone, and the stability of the forward control. At present, the bonding temperature of LED chip bonding machine (eg Eagle60 type) is generally set at 180-250 ° C.
3.2 Bonding machine pressure / power
Ultrasonic power makes the welding wire and the welding surface soft, heat energy is generated, and the constituent molecules interfit with each other, changing the spherical standard. Ultrasound power has the greatest impact on the quality and appearance of the bond, as it plays a dominant role in the deformation of the bond ball. Too little power can cause too narrow, unshaped bonds or tail wires to rise; too much power can cause root cracks, bond depressions, or pad splits.
Ultrasonic power and bonding pressure are parameters that are related to each other. Increasing the ultrasonic power generally requires increasing the bonding force so that the ultrasonic energy is transmitted to the bonding point through the bonding thing. Therefore, when setting the bonding machine pressure and power parameters in the production process, it is necessary to close the two to a good summary. Actively set according to the type of machine and the practical situation encountered in the production process, and strive to find the best distribution combination .
3.3 Bonding time
Bonding time refers to the time for controlling the effect of ultrasonic energy. Generally, the time for LED chip bonding machine (eg Eagle60 type) is set at 8-20ms. Generally speaking, too short wire bonding time does not constitute an excellent alloy, and too long wire bonding time is the cause of poor tensile force or chip electrode damage. The longer the bonding time, the more energy the lead ball absorbs, the larger the diameter of the bonding point, the stronger the interface and the lower the neck, which will cause the bonding point to exceed the pad distance and increase the probability of void generation. Therefore, it is very important to set a suitable bonding time.
4, bonding things and raw materials
The choice and application of the bond splitter has an important influence on the quality of the solder joint. The splitter life is generally 2 million points. With the increase of the number of welding points used by the splitter, the wear of the splitter is also getting more severe. Figure 7 is a comparison of the appearance of the new splitter from the beginning to the 3500K solder joint. With the wear of the splitter, the quality of the solder joints gradually deteriorated. Figure 8 is a comparison picture of the appearance of the first and second solder joints as the number of splitter welds increases.
Figure 6 The number of solder joints and the appearance of the solder joint
4.2 Gold Wire
As an important raw material, gold wire must have several important characteristics: excellent mechanical function and conductive function, suitable breaking force, selection of the correct standard, clean surface without pollution and no harm. At present, most of the gold wire diameter for LED packaging work is 1mil or 1.25mil, the purity is 99.99%, the elongation is generally 2% -8%, and the crack load is generally required: 1mil gold wire is greater than 10g, and 1.25mil gold wire is greater than 16g.
Too soft gold wire can cause the following:
(1) Sagging arch wires;
(2) spherical instability;
(3) The neck of the ball is shortened briefly;
(4) The gold wire is easy to crack.
Too hard gold wire can cause the following disadvantages:
(1) punch out the chip electrode or epitaxy;
(2) The neck of the golden ball is cracked;
(3) Difficulties in forming alloys;
(4) Arch wire arc control is difficult.
4.3 Influence of lead bracket quality on solder joints
(1) Generally affect the second solder joint;
(2) The unevenness of the surface causes the energy to be lost during the welding process, and the solder joints are asymmetric;
(3) Poor adhesion results in poor plating quality, which is directly reflected in poor tensile strength after welding;
(4) The surface is bad (cleanliness, oxidation). When several parameters of the welding wire cannot damage the surface pollution layer, it will affect the quality of the welding wire.
4.4 Effect of chip electrodes on solder joints
(1) The chip electrode itself is unreliable in vapor deposition, which causes the electrode to fall or be damaged after the bonding wire.
(2) The chip electrode itself has poor solderability, which may cause solder balls to be soldered.
(3) The surface of the chip electrode is dirty, such as the glue on the rack, other objects contact the surface of the chip; improper storage of the chip causes the electrode surface to oxidize, etc. Because the standards of metal melting balls and pads are small, the cleanliness of the bonding surface is very active, and the fine contamination of the bonding surface may cause the metal atoms between the two to fail to conduct, which constitutes failure or virtual soldering.
The bonding process is the most important and crucial process technology in the LED packaging industry, which affects the electrical functions and reliability of the packaged products. This article introduces the quality control standards for bonding wires and balls, introduces parameters such as bonding time, pressure, and power that affect the quality of bonding. Others also talk about things such as cleavers, gold wires, and raw materials for bonding. The impact of quality.
Editor: Xiao Yan
Origin: fine metal special materials