**Prevention and improvement of cracks in copper wire bonded aluminum pads.
Meng Xingmei. Tianshui Huatian Technology Co., Ltd. ***
Abstract: This paper briefly describes the potential hazards of aluminum pad cracks. The causes of aluminum pad cracks were analyzed, and the negative effects of the inherent characteristics of copper wires on bonding reliability during the bonding process of copper wires were studied [1]. Specific measures to improve the cracks of aluminum pads are described. Through a series of hardware and software improvement measures, the low yield of the product caused by the crack problem of the aluminum pad has been reduced, and the transformation of the product from the verification batch to the risk batch and then to the mass production batch has been realized, and the abnormal proportion of aluminum gasket cracks caused by improper parameter setting has shown a straight downward trend, and finally the yield of the product has reached 99More than 9%, in line with production and quality requirements, to achieve the expected purpose.
1 Introduction. The destruction of the aluminum pad, aluminum layer and bottom layer on the surface of the chip will lead to cracks, and the reliability and stability of the chip with cracks will be poor, which will cause the early failure of the product function. Figure 1 shows an aluminum pad crack on the surface of the chip under a high-magnification microscope (>500x).
With the development of integrated circuit chips to miniaturization and multi-functionalization, chips are now multi-layer wiring in design, and there are more and more products with devices and circuits under the aluminum pad in the chip packaging process. In order to improve product reliability and prevent the occurrence of aluminum gasket cracks in integrated circuits, it is becoming more and more important to prevent aluminum gasket cracks in the early stage. Therefore, only if the crack problem of aluminum pads is improved, can copper wire bonding technology develop in the direction of multi-layer wiring.
2 Aluminum pad cracks have a negative impact on production.
Aluminum pad cracks are not so obvious in appearance, for slight small cracks, often need to be seen through destructive experiments, but it has a great impact on the performance of the product, and then affects the yield and reliability of the product, and at the same time leads to serious economic losses. In 2014, a packaging company was returned by the customer due to the abnormal crack problem of aluminum gasket in a product, which brought millions of economic losses to the customer and the packaging company. In the third quarter of 2015, the test yield of a product with a relatively stable package suddenly dropped sharply, and it was seriously not up to standard, and after a lot of investigation, DOE verification and analysis, it was confirmed in November of the same year that there was a problem with the quality of the incoming aluminum pad of the wafer. Because this crack is often slight and does not completely damage the circuit, it is not easy to screen for terminal testing, and eventually it becomes a defective product that flows out to the client and has adverse effects.
Therefore, copper wire, as a bonding wire, although it saves a lot of cost to a certain extent, the problem of aluminum gasket crack has always been a quality problem faced by integrated circuit packaging companies. Due to the high hardness and high mechanical strength of copper wire, it is easy to hit non-stick if the pressure is too small, and the aluminum layer on the surface of the chip is damaged by too much pressure, and the aluminum pad crack phenomenon is easy to occur.
3. Hardware measures to improve aluminum pad cracks.
Due to the different size of the chip bonding area and the different structure of the aluminum pad of the solder joint, it is necessary to choose different types of capillary and bonding wires with different wire diameters to deal with it. Copper wire bonding has requirements for the structure of the chip solder joint pad [2], the chip solder joint probe impression, the chip flatness, etc., and these hard bonding process requirements need to be clearly defined and recommended for application to the fab.
3. 1.1 Bonding equipment stability and consistency.
The quality control of the product depends on the consistency and stability of the bonding equipment, and the consistency and stability of the bonding equipment also directly determine the stability of the bonding process.
3. 1.2 Selection of wire type.
From the analysis of physical properties, pure copper wire is harder than gold wire, and palladium-plated copper wire is harder than pure copper wire, so when using palladium-plated copper wire bonding ball welding, the chip is more susceptible to damage and cracks appear on the surface. In order to solve this problem, the hardness of the bonding ball is adjusted to reduce the damage to the aluminum layer by combining the quality of the aluminum layer of the solder joint, and the wire with different hardness and strength is selected [3]. At the same time, when determining the type of bonding wire, it is also necessary to consider the impact of other aspects, such as the impact on cost, product electrical performance, arc shape of bonding wire, etc.
3. 1.3. Structural requirements for chip solder joints and pads.
The bond strength of the solder ball to the underlying silicon compound, and the thickness of the aluminum layer at the solder joint. These two points directly determine the maximum pressure that the aluminum pad can withstand, and for high-power products, the diameter of the solder ball is large enough, so the thickness of the aluminum layer of the solder joint needs to meet the requirements [4] . The physical properties of copper (hardness, toughness, etc.) are higher than those of gold wire, and it needs to withstand greater bonding pressure and ultrasonic energy during the bonding process, so the thickness of the aluminum layer of the chip solder joint should be large enough.
3. 1.4 Chip flatness requirements.
Chip flatness is also a key factor affecting aluminum pad cracks, and the difference in flatness includes: large height difference at different positions of the same chip;The height of different chips in the same window varies greatly;There is a large difference in height in different areas of the same frame. The surface of the chip is uneven, and any kind of height difference may cause cracks in the high area of the chip, and the part in the low area is not sticky. To solve this problem, it is necessary to prescribe the right medicine: the amount and strength of the glue are strictly controlled when gluing the chips to ensure that there is no difference in the height of the chips [5] .
3.2. Verification of prevention of oxidation of copper balls.
As a result, the hardness of the oxidized copper ball will increase, and a greater bonding pressure needs to be applied during bonding, and when the applied bonding pressure exceeds the bearing capacity of the aluminum pad, the surface of the aluminum pad will be cracked. The prevention of copper ball oxidation requires process validation and result confirmation to obtain the best control method, so relevant verification and parameter optimization work has been carried out, as follows:
3. 2.1. Comparative verification of shielding gas device.
As shown in Figure 2, it played an important role in the packaging of large aluminum pad spacing (70 m) chips in the early stage of copper wire technology, but with the maturity of copper wire technology, the development of small aluminum pad spacing (45 m) and chip interconnection technology gradually showed shortcomings such as poor burnout consistency, easy oxidation, and susceptibility to the size of the gas flow.
The new shielding gas device provides annular gas protection, as shown in Figure 3, using a machined ceramic or glass tube, through which the mixed gas of nitrogen and hydrogen is sent to the place where the solder ball is formed, the purpose is to make the gas flow be sprayed around the solder ball at a uniform speed, reduce the influence of the gas flow and reduce the amount of gas, and completely isolate oxygen.
The burn-out experiments under the unidirectional blowing protection device and the annular blowing protection device were divided into five groups, and the content components of nitrogen and hydrogen filled into the devices were consistent in the two groups. The measured data of the solder ball diameter were recorded one by one, and the standard deviation corresponding to the diameter of each group of solder balls was calculated.
From the results of Table 1, it is concluded that the standard deviation of the solder ball size is the smallest when the annular gas shielding device is used, indicating that the uniformity and consistency of the solder ball shape can be better ensured by using this device.
3. 2.2. Experimental verification of shielding gas composition and flow.
The proportion and flow rate of nitrogen and hydrogen in the shielding gas have clear requirements in the copper wire process. H2 concentration: generally between 5% and 10%, the purpose is to add nitrogen to prevent oxygen from reacting with copper during bonding, and then add hydrogen as a reducing gas to remove the oxide layer on the copper surface.
On the ASMEAGLE60 equipment, the flow verification experiment of nitrogen-hydrogen mixed gas was carried out with a copper wire with a wire diameter of 25 m, and the nitrogen-hydrogen mixed gas was filled into the annular gas protection device, and the protective device was filled into the protective device in strict accordance with the ratio of 5%N2 +95%H2 for burning ball [6], the gas around the copper ball expanded violently due to the sudden change in temperature and caused the aura field in the area to be disordered, and if the flow rate of the protective gas was insufficient, the surrounding oxygen would be involved in the burning ball environment. Therefore, the size of the flow rate should be determined according to the formation process of the ball, the appearance quality and the degree of oxidation, otherwise it is easy to cause the tail of the line to be blown crooked, the appearance of "golf ball", or the oxidation protection is not sufficient, and the oxidation ball or pointed ball appears, as shown in Figure 4. Table 2 shows the experimental results for different gas flows.
The flow rate of the gas mixture should generally be controlled at 04 l/min～1.0l/min。If the gas flow is too small, the protective effect will be affected, the burnt bulb is easily oxidized by the oxygen in the surrounding air, and the diameter of the burnt bulb is seriously beyond the standard required range, as shown in Figure 4(a).If the gas flow is too large, the tail line will be blown by the air current, and the roast ball will appear like a golf ball, as shown in Figure 4(b) and (c). The above practice shows that the gas flow rate has little influence on the diameter of the burnt ball, but it is directly related to the appearance quality of the burnt ball and determines whether the spherical shape is normal.
3. 2.3. Optimization and verification of burnout parameters.
The parameters mainly include: burn current, burn voltage, and burn time. In order to prevent the oxidation of copper balls, it can be adjusted by increasing the electron ignition (EFO) current and shortening the electron ignition time, but each has its own advantages and disadvantages. It should be noted that large currents will increase the hardness of copper balls, and small currents are easy to oxidize. As shown in Figure 5, the influence of ball burning current and ball burning time on the spherical size increases with the gradual increase of current and time, and the diameter of the ball size also increases correspondingly, but the change caused by the ball burning time is more significant in the process of parameter change.
According to the verification results of the above different methods and parameters, when the hardware of the equipment is set properly, the burnt ball parameter is the key process parameter that directly affects the copper ball forming, oxidation prevention and poor consistency.
4 Software implementation plan to improve aluminum pad cracks.
The calibration of the parameters of the bonding equipment is the primary task to prevent and improve the cracks of aluminum pads, and only when the parameters of the equipment are calibrated properly, can the product have a suitable processing environment. At the same time, the addition of bonding parameters is also a necessary link, so the calibration of the equipment is the first problem to prevent aluminum gasket cracks.
4.1. Calibration of equipment parameters and addition of process card parameters.
The equipment setting is the first condition for bonding, and only after a series of verification and calibration of the equipment parameters to find the best parameters for the equipment, and the same equipment is set on the same level, can it be put into production.
The parameters of the same model of equipment may be different, and they all need to be verified by DOE before the equipment is put into use, and the parameters of the same model of equipment are calibrated at the same parameter level, so the parameters of the new equipment must be calibrated before the implementation of production, and the corresponding parameters should be added to the process card at the same time. The addition of process card parameters is the process of using the parameters on the basis of equipment calibration, once the equipment parameters are calibrated accurately, the process card parameters become fixed values.
4. 1.1 Control measures for procedures.
Through DOE verification to obtain the best parameters, and the parameters are called to the host device, the parameters are networked and integrated, and the rest of the same model equipment only needs to call the host parameters during bonding, without reprogramming and setting parameters. All equipment of the same type and model are operated according to the parameters provided by this host, so as to reduce the occurrence of aluminum gasket cracks and other abnormalities due to improper parameter setting, realize the effective and unified control of parameter programs, and reduce the occurrence of aluminum gasket cracks and other abnormalities.
4. 1.2. Realize the control of the upper and lower limits of parameters.
Engineering and technical personnel are responsible for setting the upper and lower limits of parameters, modifying parameters and authority control, and realizing the integration of bonding parameters. The parameter range is reflected on the host to avoid product scrapping due to incorrect parameter values.
4. 1.3. Implement special product control.
Establish an engineering verification process for abnormal products, and after passing DOE verification, when an abnormality is found, the engineering personnel will disable the previous parameters as soon as possible, continue to do verification, and do not carry out batch processing until the best parameters are obtained. The disabling of parameters is to disable the corresponding drawing parameters of abnormal products through the ERP system authority management system, and inform the process specification conversion personnel, and mark the corresponding BD data one by one in the system, and the subsequent customer incoming materials will be re-verified according to the corresponding BD one by one, which plays a good role in monitoring and prevention.
4. 1.4. Develop a product management process.
In order to reduce unnecessary losses in production and improve the yield of products, usually there will be abnormal chips or initial processing products, engineering verification, after the engineering verification is qualified, the product will be converted into a small batch tracking, the verification of qualified parameters and procedures will be handed over by the corresponding engineering team, and the risk batch processing will be carried out after the system processingIf the verification fails, the DOE secondary parameter verification is redone. The specific embodiment is shown in Figure 6.
4. 1.5. Realize the special management and control of dual chips.
A dual-chip package consists of a main chip and a secondary chip (MOS die), which are connected in parallel to each other. The structure of MOS chips is fundamentally different from other ordinary chips, there is no passivation layer on the surface of MOS chips, and the general passivation layer plays the role of preventing oxidation and protecting the surface of the chip, so the MOS die is more prone to oxidation and damage. Therefore, if the bonding pressure is too large, the structure of the MOS chip is easy to be damaged and cracks appear. Table 3 shows the MOS chip product management solution. Therefore, the MOS chip cannot be wired by conventional wire bonding methods, and must use the BSOB process [7] to pre-protect the chip by planting balls before wirering to prevent cracks in the aluminum pad on the surface of the chip.
Bond Stitch on Ball (BSOB), as shown in Figure 7, is a schematic diagram of copper wire BSOB, that is, a solder ball is first planted on the chip (or pin position), and then the normal wire is welded, and the second solder joint of the normal wire is welded to the ball planting point. BSOB is mainly used in multi-chip module (MCM) or stack die products [8] .
In the process of completing the BSOB arc, because the first solder joint of the BSOB line is not firmly bonded with the chip pad [9] (it should be noted that it is also possible that the aluminum loss is formed by overwelding, and the debugging method will be different), the line arc pulls up the first solder joint and causes the first solder joint to fall offThe bond strength can be improved by adjusting the parameters of the first solder joint ball of the BSOB wire [10].
Through the above methods, various abnormal problems in the production process are effectively solvedReferring to the improvement method and debugging optimization results, the management scheme of MOS dual-chip products is summarized, as shown in Table 3.
The above from the process technology, management methods of improvement and optimization, will not cause the increase in production costs, for the follow-up mass production has played a key role, the application of these technologies and management methods, so that the proportion of copper wire in 2D (double chip plane D (double chip stack) packaging gradually increased, the technology has been improved, bringing more profits.
The series of improvement measures in Table 3 have fundamentally prevented and improved the occurrence of aluminum gasket cracks, and these measures have been fixed and standardized, and the corresponding documents have been implemented in actual production. Fundamentally eliminate and prevent abnormalities, and comprehensively improve product quality.
5 Actual results after improvement.
After a series of product verification and parameter optimization, it is confirmed that the packaging yield of the product has been significantly improved, and the probability of aluminum gasket cracks has been greatly reduced. The product can smoothly transfer from the verification batch to the mass production batch and enter the normal mass production and packaging stage.
Table 4 shows the improvement of aluminum gasket cracks in the production line after a series of parameter improvements.
Through the data analysis of 1-8 months, it is concluded that the crack problem of aluminum pad has been gradually improved and reduced, and the defect rate has gradually decreased with the increase of the number of processing work orders, so it has been proved that through the above measures, the crack problem of aluminum pad has been well prevented and controlled, and the purpose of improvement has been achieved.
6 Conclusion. The best parameters of copper wire bonding and measures to improve the crack of aluminum pads obtained are summarized, which have been applied in production practice to improve the yield of the product. Avoiding a large number of subsequent parameter verification processes can save manpower, financial and material resources, and also lay the foundation for the mass production of subsequent products and improve economic benefits. With the deepening of research, various problems will be gradually solved, and it is believed that copper wire bonding will be well used in the electronic packaging industry.