Ball Placement Process for SiP Packaging Products - Solder Paste

The development of chips is no longer just about reducing power consumption and improving performance (Moore's Law), but more pragmatically meeting market needs (beyond Moore's Law). In order to maximize chip efficiency, minimize the volume after packaging and customize, SiP packaging technology has become one of the most important technologies in the semiconductor industry. Referring to the development trend chart of SiP packaging technology, it can be seen that its integration and complexity are increasing day by day.

Ball planting process

·       Ball Terminal Type According to the industry standard IPC-7095 "Design and Implementation of BGA Assembly Process", there are three types of package ball terminals, and each company can choose the corresponding terminal type according to the process requirements of its own SiP products.

·       Ball planting method At present, there are three main ball planting methods in the industry, and the whole SiP product field mainly adopts the ball planting method.

·       Solder paste printing balling The solder paste printing ball planting method is to print an appropriate amount of solder paste directly onto a preset pad, and after reflow soldering, it forms a bump terminal. The advantage is that the equipment used is the same as the existing SMT line, and the existing SMT solder paste printing equipment can be utilized. When using paste solder to print the planting ball, there are two main considerations in the design of the stencil opening: a) In order to ensure that a specific shape of the solder bump is formed on the pad after reflow soldering, it is necessary to print a sufficient amount of solder paste, and the method of increasing the thickness of the stencil and expanding the stencil opening is generally adopted, focusing on the shape, height and consistency of the bump after the furnace; b) In order to reduce the voiding rate of the solder bump after reflow soldering, it is recommended to increase the bridging method for the stencil opening (for example, the 0.6mm diameter terminal is recommended to use the bridge of 0.15mm or 0.2mm in the middle of the stencil), which is used as the exhaust channel during welding to reduce the voiding problem, and at the same time, it is also necessary to focus on the demoulding effect and the quality of the stencil.

·       SIP solder paste : Large SIP package solder paste (water-washable type, water-free type), especially suitable for fine pitch printing of SiP package; When the minimum opening of the stencil is 55μm, the printing release performance is excellent, the solder paste molding effect is good, the anti-collapse property is good, and the continuous printing stability is very high. It can maintain high viscosity for a long time and solve the problem of chips that are easy to fall in long-term production; The stencil has a long service life (≥ 10 hours) and a long working time (≥ 10 hours) after printing; It has excellent wetting performance, and the solder joints can be evenly tiled; High oxidation resistance, no tin beads, no color residue; Excellent cold and thermal collapse resistance; The voiding rate is low, and the reflux curve process window is wide.

·        At present, there are three methods of ball placement and ball planting in the industry: "solder paste" + "solder ball", "flux" + "solder ball" and "solder paste printing". The specific operation is to first apply solder paste or flux to the pad of the solder ball, and then place the solder ball on the pad by means of ball planting machine or screen printing, and form a ball terminal after passing through the reflow oven.

·       The automatic ball planting machine places the ball and plants the ball

·       a. Use a fixture that matches the solder ball pad to dip the flux dip and dispense it on the solder ball pad. Before adding flux for the first time, check and clean the template of the ball planting equipment to ensure that there are no other foreign objects or different types of flux in the template. To select the appropriate thickness of the flux scraper according to the diameter of the solder ball (generally recommended scraper thickness is 1/4 - 1/3 of the diameter of the solder ball), manually run the equipment flux scraper back and forth 8 - 10 times after adding the flux to stir and scrape the flux on the template.

·       b. The PIN pin of the flux fixture is evenly dipped in flux on the horizontal flux template and spot-coated onto the solder ball pad. It is necessary to control the thickness of the flux on the template and the dispensing time of the PIN pin to ensure that the flux can be completely wetted to cover the pads after dispensing, otherwise it may lead to the problem of solder ball drift after ball planting or poor solder ball wetting and soldering after the furnace. Note that flux addition follows the principle of "small quantities, many times". b. Vacuum suck up the solder balls through the Ball attach tool and transfer them to the pads stained with flux; Release the vacuum switch, and the solder ball sticks to the substrate pad under the action of flux stickiness;

·        c. The substrate planted with solder balls is reflowed by hot air reflow soldering, the solder balls are melted at high temperatures, and with the help of flux, they are infiltrated, diffused, dissolved, and metallurgically combined with the substrate pads to form a bonding layer (IMC), and after cooling, the solder balls are welded together with the substrate pads. In order to reduce the high-temperature oxidation of solder balls, it is recommended to solder in a nitrogen atmosphere;

·       d. The substrate soldered with solder balls should be cleaned, and the excess flux and dirt on the substrate should be cleaned off, and finally dried.

·       Steel mesh ball planting

·       a. Printing paste solderor printing flux: The hole of the flux stencil and the substrate pad need to be completely consistent (aligned), and there can be no deviation, use a 45° - 60° scraper to evenly leak the evenly stirred solder paste or flux onto the pad, then lower the substrate workbench, and then slowly lift the stencil and take out the substrate. Observe whether the solder paste or flux that is missing from the substrate is uniform, has deviations, or other printing defects. Note that too much or too little flux can cause ball planting to fail. If there is too much flux, the excess flux will spill through the small holes of the stencil, affecting the solder ball arrangement; If there is too little flux, it will affect the quality of the solder ball solder.

·       b. Place the solder ball: After the solder paste or flux is printed, place the substrate on the workbench, fix the ball planting steel mesh according to the correct position, adjust the height of the workbench, and keep a certain distance between the substrate and the ball planting steel mesh, which is about 1/2 - 2/3 of the solder ball diameter. After the adjustment is completed, take a certain amount of solder balls and pour them on the planting steel mesh, use a brush to fill the solder balls into the corresponding mesh, and put the excess solder balls next to it with a brush. Then check that the mesh is filled with solder balls to ensure that there is only one solder ball in each mesh. Then lower the table, remove the ball mesh, and remove the substrate. Be careful not to remove the substrate with too much force or too fast, as this will cause the solder balls to drift.

·       Laser Ball Planting Laser ball planting is a method that uses a laser device to melt solder balls by laser and spray them onto the corresponding pad to complete the welding to form a spherical terminal. In the laser welding system, the solder balls are conveyed from the solder ball box to the nozzle, and after being heated and melted by the laser, they are sprayed from the special nozzle, directly covering the pads, and no additional flux is required. It has the advantages of non-contact, no solder, low heat, and precise and controllable solder. Compared with the ordinary solder ball injection method, it has the characteristics of impact deformation and instantaneous solidification, reflecting the unique process characteristics.

2.2.4 Selection of SiP ball planting method

Factors influencing the quality of ball planting and key points of control In  the ball planting process of SiP products, the factors affecting the quality of ball planting mainly include ball planting materials, ball planting methods and process conditions. Ball planting materials mainly include solder balls, flux or solder paste, and substrates; The process conditions are mainly the ball planting process method, reflux temperature curve, protective atmosphere, etc. For the planting material, the solder balls should be kept clean and prevent oxidation; The flux should maintain a certain viscosity and good flux properties; The substrate should be kept clean and flat. In the process of ball planting, the corresponding process test and control are mainly carried out through the control of flux dose, welding temperature curve and protective atmosphere. In the context of the continuous development of SiP packaging technology and its increasing integration and complexity, the ball planting process is particularly critical.

The factors affecting the quality of the balls include the materials of the balls, the methods of the balls, and the process conditions. The flux in the ball planting material plays a variety of roles, there are different types and storage requirements; Solder balls come in a variety of compositions and diameters, and storage requires specific environments; The substrate should be selected according to the product requirements, such as the appropriate plate and thickness, and the flatness should be ensured.

In the ball planting process, there are three types of ball terminals according to industry standards, and each company can choose according to their needs. There are three main methods of ball planting: solder paste printing ball planting, ball placement ball planting, and laser ball planting. Solder paste printing ball planting using the existing SMT equipment, stencil opening design should consider the amount of solder paste to ensure the shape of the solder bump and reduce the void rate. SIP solder paste has superior performance and is suitable for fine pitch printing. There are three ways to operate the ball planting, when the ball is planted in the automatic ball planting machine, there are many key points in the operation of the flux, such as the use of the fixture dipped in the flux, PIN pin spot coating, etc., and the whole process should pay attention to avoid problems such as solder ball offset or poor soldering; When the steel mesh is placed on the ball, the paste solder or flux printing process should be guaranteed to be uniform and without deviation, and the filling of each mesh should be paid attention to when placing the solder ball. Laser ball planting uses laser equipment, which has unique advantages.

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·       Bulb planting material

·       Flux Flux mainly plays the role of flux, one is to isolate the air to prevent oxidation; The second is to remove oxides and contaminants on the surface of the PCB pad and the solder ball soldering part, increase the capillary effect, improve the wettability, and prevent virtual soldering; Another important role is the adhesion and fixation of solder balls. There are two types of flux: water-washed flux and no-clean flux, which can be selected and used according to the process requirements of the product. Flux storage requirements: For example, the flux of Senju is required to be stored in a sealed state in an environment with a temperature of ≤ 30 °C and a relative humidity of 40 - 60%RH; Storage period: within 6 months from the manufacturer's manufacturing date; For details, please refer to the requirements of the flux product specification.

·       Solder balls Solder balls are divided into leaded solder balls and lead-free solder balls, and leaded solder balls include Sn63Pb37, Sn62Pb36Ag2, Sn10Pb90, Sn5Pb95; Lead-free solder balls include Sn100, Sn96.5Ag3.5, Sn96.5Ag3Cu0.5. The diameter specifications of solder balls are generally 0.1mm, 0.15mm, 0.2mm, 0.25mm, 0.3mm, 0.35mm, 0.4mm, 0.45mm, 0.5mm, 0.55mm, 0.6mm, 0.65mm, 0.76mm. Specifically, it can be selected and used according to the process requirements of the product. Solder balls are very easy to oxidize, storage requirements: stored in a sealed state in an environment with a temperature of 25±10 °C and a relative humidity of ≤ 60%RH, and the unused solder balls should be placed in a moisture-proof cabinet or stored in a drying cabinet with nitrogen; Storage period: within 12 months from the manufacturer's manufacturing date; For details, please refer to the requirements of the product specification.

·       Substrate (PCB) Substrate (PCB): For PCB of substrate-level SiP products, it is recommended to choose a board with a high Tg (Tg≥170°C); After ball planting, it needs to be cleaned, and ENIG (chemical Ni/Au) is preferred for the surface treatment of the substrate pad to prevent oxidation after cleaning; PCB thickness specifications are 1.6mm, 1.4mm, 1.2mm, 1.0mm, 0.8mm, etc. Specifically, it can be selected and used according to the process requirements of the product, and the focus is to ensure the flatness of the PCB.

Conclusion SiP has been widely used in consumer electronics, Internet of Things, intelligent driving, HPC and 5G networks, and has a very broad application and market prospect. As a key process in the production of SiP products, the ball planting process will directly affect the performance and reliability of the device and circuit conduction. In order to ensure the quality of the company's SiP products, the company has introduced automatic ball planting technology and advanced production equipment, adopted fully automated ball planting equipment with good stability, high repeatability and self-feedback of ball planting quality, and built an industry-advanced, high-precision, high-stability fully automated SiP line, thereby improving product quality and efficiency. Combined with the characteristics of the company's SiP products, this paper briefly introduces and analyzes the main factors affecting the quality of planting balls, and through the control of the manufacturability design and process of SiP products, the straight-through rate of planting balls of our SiP products can reach more than 99.98%, and the defective rate is less than 100PPM. In the context of the continuous development of SiP packaging technology and its increasing integration and complexity, the ball planting process is particularly critical.

In the ball planting process, there are three types of ball terminals according to industry standards, and each company can choose according to their needs. There are three main methods of ball planting: paste solderprinting ball planting, ball placement ball planting, and laser ball planting. Solder paste printing ball planting using the existing SMT equipment, stencil opening design should consider the amount of solder paste to ensure the shape of the solder bump and reduce the void rate. SIP solder paste has superior performance and is suitable for fine pitch printing. There are three ways to operate the ball planting, when the ball is planted in the automatic ball planting machine, there are many key points in the operation of the flux, such as the use of the fixture dipped in the flux, PIN pin spot coating, etc., and the whole process should pay attention to avoid problems such as solder ball offset or poor soldering; When the steel mesh is placed on the ball, the solder paste or flux printing process should be guaranteed to be uniform and without deviation, and the filling of each mesh should be paid attention to when placing the solder ball. Laser ball planting uses laser equipment, which has unique advantages.

The factors affecting the quality of the balls include the materials of the balls, the methods of the balls, and the process conditions. The flux in the ball planting material plays a variety of roles, there are different types and storage requirements; Solder balls come in a variety of compositions and diameters, and storage requires specific environments; The substrate should be selected according to the product requirements, such as the appropriate plate and thickness, and the flatness should be ensured.

The conclusion emphasizes that SiP is widely used, and the balling process has a significant impact on its performance and reliability. In order to ensure the quality of SiP products, the company introduces advanced technology and equipment, builds a fully automated line, and controls the process from the main factors affecting the quality of planting balls, so that the products have a high straight-through rate and a low defective rate. This shows that in the development process of SiP packaging technology, it is an effective way to improve product quality and competitiveness by paying attention to all aspects of the ball planting process and optimizing all aspects from materials to processes to equipment.

The conclusion emphasizes that SiP is widely used, and the balling process has a significant impact on its performance and reliability. In order to ensure the quality of SiP products, the company introduces advanced technology and equipment, builds a fully automated line, and controls the process from the main factors affecting the quality of planting balls, so that the products have a high straight-through rate and a low defective rate. This shows that in the development process of SiP packaging technology, it is an effective way to improve product quality and competitiveness by paying attention to all aspects of the ball planting process and optimizing all aspects from materials to processes to equipment.