Board chip-cob

Chip on board (COB) Thermal conductive epoxy resin (Generally, silver-doped epoxy resin is used) to cover the placement point of the silicon wafer. Then, the silicon wafer is directly placed on the substrate surface and heat treated until the silicon wafer is firmly fixed to the substrate, then, the SI welding method is used to directly establish an electrical connection between the silicon wafer and the substrate. The bare chip technology has two main forms: COB technology and flip chip ). On-board chip encapsulation (COB), the semiconductor chip is directly mounted on the printed circuit board. The electrical connection between the chip and the substrate is achieved by wire stitching, and the electrical connection between the chip and the substrate is achieved by wire stitching, covered with resin to ensure reliability. Although CoB is the simplest bare chip mounting technology, its encapsulation density is far inferior to that of tab and reverse chip welding. Main Welding Methods 1: Hot Press Welding The wire is welded together with the welding zone by heating and processing pressure. The principle is that by heating and adding pressure, plastic deformation occurs in the welding zone (such as AI) and the oxide layer on the press welding interface is damaged, so that the attractiveness between atoms can be "bonded, in addition, when the two metal interfaces are not flat and heated and pressurized, the upper and lower metals can be embedded with each other. This technology is generally used as a cog chip on a glass board. 2: Ultrasonic Welding Ultrasonic Welding uses the energy produced by the ultrasonic generator. The transducer rapidly scales to produce elastic vibration, causing the corresponding vibration of the cutter, and at the same time, applies certain pressure on the cutter, therefore, with the combination of these two forces, the cutting tool will drive the rapid friction of the AI wire on the metal layer of the welded area, such as the surface of the (AI film, it causes plastic deformation on the surface of the AI wire and the AI film, which also destroys the oxide layer on the UI of the AI layer, and makes the two pure metal surfaces closely in contact with each other to form welding. The main welding material is aluminum wire welding head, generally wedge. 3: Golden Wire Welding Ball welding is the most representative welding technology in wire bonding, because the current semiconductor encapsulation II and transistor encapsulation all adopt Au wire ball welding. It is easy to operate, flexible, and the solder joints are solid (the Welding Strength of the Au wire with a diameter of 25 um is generally 0.07 ~ 0.09n/point), no direction, welding speed up to 15 points/second or more. Golden wire welding is also called Hot (pressure) (Super) acoustic welding. The main bonding material is Gold (Au) wire welding head is spherical, so it is ball welding. Cob encapsulation process Step 1: Expand the crystal. The expansion machine is used to uniformly expand the entire led wafer film provided by the manufacturer, so that the LED grains closely arranged on the film surface can be opened, making it easy to penetrate the crystal. Step 2: adhesive tape. Place the expansion ring of the expanded crystal on the adhesive surface of the scraped silver paste layer, and carry the silver paste on the back. Click silver paste. Suitable for bulk LED Chips. Use a dispenser to place the right amount of silver paste on the PCB. Step 3: place the expanded ring of the silver paste into the thorn frame, and the operator stabbed the LED wafer to the PCB with a thorn pen under the microscope. Step 4: Put the thorn crystal PCB printed circuit board into the thermal cycle oven for a period of constant temperature and static, after the silver paste is solidified out (not for a long time, otherwise the LED chip coating will bake yellow, that is, oxidation, which makes it difficult to establish the State ). If an LED chip is attached, the preceding steps are required. If only the IC chip is attached, the preceding steps are canceled. Step 5: Stick the chip. Apply a proper amount of red glue (or black glue) to the IC position of the PCB printed circuit board, and then use anti-static equipment (vacuum suction pen or sub-) to correctly place the bare IC film on the red glue or black glue. Step 6: drying. Place the pasted bare slice in the thermal cycle oven and place it on the large plane heating board for a period of constant temperature and static time. It can also be naturally solidified (For A Long Time ). Step 7: binding (hitting ). The aluminum wire bonding machine is used to bridge the chip (LED grain or IC Chip) with the corresponding pad aluminum wire on the PCB, that is, COB internal lead welding. Step 8: pre-test. Use a dedicated detection tool (COB has different devices for different purposes, simply high-precision regulated power supply) to detect the COB board and repair the unqualified board again. Step 9: Click glue. Use the dispenser to place the appropriate amount of AB glue on the fixed led grain, IC is packaged with black glue, and then the appearance is encapsulated according to customer requirements. Step 10: curing. Place the PCB printed circuit board with sealant in the thermal cycle oven for constant temperature and static setting. Different drying times can be set as required. Step 2: post-test. The encapsulated PCB printed circuit board is used with a dedicated testing tool for electrical performance testing to distinguish between good and bad. Compared with other packaging technologies, cob is cheaper (only about 1/3 of the same chip), saves space, and has mature technology. However, any new technology cannot be perfect when it first appeared. Cob technology also has shortcomings such as the need for additional welding machines and sealing machines, sometimes unable to keep up with the speed, and stricter environment requirements and inability to maintain PCB patches. The layout of some board chips (COB) can improve the ic signal performance, because they remove most or all of the encapsulation, that is, most or all of the parasitic devices. However, with these technologies, there may be some performance problems. In all these designs, the substrate may not be well connected to VCC or ground due to a lead frame or BGA sign. Possible problems include thermal expansion coefficient (CTE) problems and poor substrate connections. Cob Process Clean PCB --- droppers glue --- chip paste --- test --- seal black glue heated and solidified --- test --- warehouse receiving 1. clean PCB

Cleaned PCB The board is still contaminated with oil or oxide layer, and try it with skin. Help locate or test the needle position,The PCB to be wiped should be cleaned with a brush or blown with an air gun to flow into the next process. Use anti-static products Ion Dust blower. The purpose of cleaning is to clean the dust and oil stains on the PCB bonding pad to Improve the Quality of bonding. 2. The purpose of dropping gel is to prevent the product from being transferred to the leased line. Die. Usually used in CoB operations Needle transfer and stress Injection . Dot Matrix Transfer Method: use a needle to take a small drop of adhesive points from the container and apply them to the PCB. This is a very rapid dispensing method. Stress injection method: load the glue into the syringe, apply a certain amount of pressure to squeeze out the glue. The size of the glue point is determined by the size of the nozzle diameter, pressurization time, and pressure, and is related to the viscosity. This process is generally used on the glue dropper or die bond automatic device. The size and height of the glue dropper depend on the type, size, distance from the pad, and weight of the chip. The size and weight of the chip glue drops a large amount, should not be too large to ensure enough viscosity, at the same time, not contaminated state wire pad. If there are any standards, they can only be determined by different products. Hard to what cannot exceed the chip 1/3 height can not expose the amount of glue as a standard, it is not necessary. 3. Chip pasting Chip pasting is also called Die bond, Bondie, Bondie, and bonic. Different companies have different names. Requirements for Chip pasting Vacuum suction pen (Suction tip) Material hardness is small (some companies also use cotton swab paste ). The diameter of the nozzle depends on the size of the chip. The tip must be flat to avoid scratching the die surface. When pasting, check the die and PCB model. If the paste direction is correct, the die towel must be" Stable and positive "," Flat "means that the die and the PCB are attached in parallel without virtual space." stable "means that the batch die and the PCB are not easy to fall off throughout the process, "positive" refers to the positive placement of the die and the reserved position of the PCB. Be sure to note that the chip (die) must not be reversed. 4. State wire (wire bonding) Bonding Wire Bond Bonding The connection names are different. Here, we use bonding as an example. The two solder joints of each state line are connected according to the position specified in the bonding diagram to achieve electrical and mechanical connection. Customized PCB Bonding tensile testing The tension is required to comply with the company's standards (refer to the 1.0 line greater than or equal to 3.5G 1.25 line greater than or equal to 4.5g ). Aluminum wire solder joints are in an elliptical shape, and gold wire solder joints are in a spherical shape. . Standard aluminum wire with fixed melting point: the end of a wire is greater than or equal to 0.3 times the diameter is less than or equal to 1.5 times the diameter of a solder joint is greater than or equal to 1.5 times the diameter is less than or equal to 5.0 times the width of a solder joint is greater than or equal to 1.2 times the diameter is less than or equal the height of the 3.0 times wire diameter line arc is equal to the parabolic height of the circle (not too high, not too low, depending on the product) golden wire: Generally, the welding ball is about 2.6-2.7 times of the wire diameter. In the state line process, the ball should be put light and light. The point should be accurate. The operator should observe the state line process with a microscope to see if there is any broken line or rolling line, eccentric, hot and cold welding, starting from aluminum and wait until the phenomenon, if any, immediately notify the management staff or technical staff. Before the formal production, you must have a designated person to perform the first inspection to check whether there is any state error, less state, or broken state tension. A dedicated person should be assigned every two hours to verify its correctness. 5. Glue sealing is mainly used for testing the PCB with OK. Note that the black glue should be completely covered during dispensing. PCB sun Ring And bonding chip aluminum wire, there cannot be Rose phenomenon, black glue can not seal out of the sun ring and other places with black glue, such as leaking glue application cloth instantly wipe off. In the whole dispensing process, needle Tsui or hair signs cannot touch die and fixed lines. The black rubber surface after drying cannot have Pore Size And black glue uncured. It is recommended that the black glue height not exceed 8MM The special requirements should be less than 1.5mm. The temperature of the preheated board and the drying temperature should be strictly controlled during gluing. (Vibrating the be-08 black rubber fr4pcb board is used as an example: the preheating temperature is 120 + 15 degrees, the drying temperature is 1.5-3.0 minutes, and the drying temperature is 140 + 15 degrees, and the drying time is 40-60 minutes) the injection method usually adopts the needle transfer method and the pressure injection method. Some companies also use drop glue machine, but the cost is relatively low efficiency. Generally, cotton swab and needle dropper are used. However, the operator must be skillful in operation and strict technical requirements. It will be very difficult if the chip is damaged and then repaired. Therefore, the process management personnel and engineering personnel must strictly control the process. 6. Test the chip failure caused by poor phenomena such as broken wires, coils, and false welding during the bonding process. Therefore, chip-level encapsulation must perform performance testing according to the detection method. Non-contact Detection (inspection) and contact Detection (testing) Two main categories: Non-contact Detection Manual Visual Testing Developed Automatic Optical Image Analysis (AOI) X-ray analysis From the inspection of the appearance circuit graphics to the inspection of the quality of the inner solder joints, and from a separate inspection to the combination of quality monitoring and defect repair. Although the bonding machine is equipped Automatic Wire Quality Inspection (BQM ), Because the automatic welding line quality inspection of the bonding machine mainly uses the design rule detection (DRC) and graphic recognition methods. DRC checks the quality of welding wires according to some given rules, such as the number of set standards whose melting point is smaller than the wire diameter or greater than the wire diameter. The graphic recognition method compares the stored digital images with the actual work. However, this is affected by the process control, process procedures, parameter changes, and so on. The specific method should be determined based on the production line conditions of each unit and the product. However, no matter what conditions are met, visual inspection is a basic inspection method and must be mastered by cob process personnel and inspection personnel. They should complement each other and cannot be replaced by each other. Assembly Technology 1. The trend of hybrid Integration Technology in today's electronic products is constantly integrating more and more devices and functions in a small component or whole machine. Hybrid integration has become one of the key technologies to increase the encapsulation density of both active and passive devices. In the hybrid integration of various manufacturing steps, the interconnection between devices and circuits, some passive devices, such as resistors, are directly produced on the substrate using thick film or thin film process. There are many important parameters in the layout and wiring design of the Hybrid IC substrate. wire width, wiring of the closest connection between the wire and the bonding disk, bonding strength, and the height of the arc ring of the bonding lead, heat Dissipation and so on must be considered. Thick Film integrated circuit technology, device and circuit interconnection, wire and resistance are on the substrate, using various functional Paste Printing and sintering. The Film Integrated Circuit process, the interconnection and wire are deposited on the Ceramic base plate by electroplating or other method, and the conductive drawing is prepared by photoetching. The resistance can be printed or welded to other passive devices. After all the passive table-mounted devices on the substrate are connected, the chip-mounted device pastes the circuit chip to the given position of the substrate, and then uses the bonding device to bond the gold or aluminum wire, realize the electrical connection between the chip and the substrate circuit, and finally encapsulate it. Hybrid integration technology can integrate a large number of Circuit chips and small passive devices on a very small substrate area. If the standard SMT Surface Mount process is adopted, it will occupy up to 20 times the area of the hybrid integration technology. The Hybrid IC manufacturing process requires a comprehensive understanding of the semiconductor wafer manufacturing process, as well as the chipset assembly and bonding process. Some small companies do not have these conditions, and the cost of creating hybrid circuit components in small batches is relatively expensive. However, the application of hybrid integrated circuits involves medical, aerospace, military, and automotive and communication fields. In these fields, hybrid integrated circuit technology is indispensable. 2. Cob chip board assembly technology for many years, the industry has been committed to developing the advantages of hybrid integrated circuit technology, but has not made a breakthrough in manufacturing costs. Therefore, the PCB assembly process is still the best choice for complex circuit assembly. It is easy and reliable to simply improve some aspects and directly mount and bond on the bare chip board. The CoB chip assembly technology is first used for digital clocks and watches. Each printed and circuit board is equipped with a chip. It is now widely used in digital cameras, calculators, phone cards and various smart cards. COB has expanded its application in complex circuit components, such as a printer module with a combination of 5,000 LEDs and IC drivers, and 32-bit hp9000 computer motherboard With 22 ICS and a modem circuit. Today, the multi-chip process for assembling more than 100 chips on a single Printed Board has also been successful. Japanese entertainment devices and almost all electronic components have adopted cob technology, in some application fields, cob is likely to replace Smt. Cost analysis shows that the cost of DIP encapsulation is often three times higher than that of its embedded chips. Using cob technology, saving packaging costs can be significantly reduced, especially in mass production. Cob technology started late in Europe and is expanding its application field. So far, it is still unable to compare with Japan and the United States, especially in terms of high assembly density and thin encapsulation. 3. Cob assembly process the manufacturing process of the module directly assembled on the chip board is very similar to that of the Hybrid IC. The main difference is the basic material and encapsulation form used by the two. The substrate used by CoB is an organic printed circuit board, and the latter is a ceramic substrate. The CoB bare chip is encapsulated by Polymer organic resin or spherical plastic, and the Hybrid IC is finally encapsulated using a metal shell. Compared with the standard SMT assembly process, cob and hybrid integrated assembly process have fewer process steps. Printed boards or PCB are made from many different materials, such as phenolic resin, poly amino formic acid resin, polyamide resin, silicone, fluorine plastics, etc. fluorine plastics (polyptfe) have high resistance in High Temperature environments, poly amino formic acid resin can adapt to extremely large temperature changes, such as automotive electronics, in very high temperature conditions, requires a very small thermal expansion coefficient, at this time fluorine plastic is the most competent. Generally, the wire used by the COB Printed Board is copper-based wire, and the bonding disk needs to be surface processed. The copper substrate is coated with 2-4 μm nickel, next, plating 0.1-0.2 μm gold (cuniau) with silver-containing epoxy conductive adhesive to bond the chip to the printing board installation position, at 250 ℃ curing. The heat dissipation problem of power devices is formed by the copper layer of the printed Board attached to the back of the chip. When the chip is assembled, the cooling board is fixed and installed on the cooling device or package body. The circuit connection between the chip and the printed board is made of aluminum wire or gold wire. The biggest advantage of aluminum wire bonding is that bonding can be performed at room temperature. When the product is subject to high temperature or large temperature changes, the ultrasonic bonding display of aluminum wire is highly reliable. To achieve the Bonding Reliability, the bonding temperature of the gold wire must be above 120 ℃. Many materials used in printed boards become soft at a high temperature, and even the bonded disks are pulled out from the Printed Board substrate. If the final product is required to withstand a high operating temperature when it is used for bonding with the aluminum layer bonding disk on the chip, the bonding disk may be damaged. This damage mechanism is caused by the Kirkendall pore which causes the bond disk to be pulled out. The choice of aluminum wire or gold wire mainly depends on the product application requirements and working environment temperature. After the wire bonding is completed, the chip can use a variety of techniques for encapsulation protection, silicone can be cured at room temperature, there are also black epoxy or other materials. The chip can also be covered with plastic or metal shells. Finally, the COB unit is loaded into the encapsulation cavity, and the circuit connection is achieved through welding or bonding. 4. when high bonding quality is required for the aluminum wire ultrasonic bonding process, the aluminum wire ultrasonic bonding process is usually used. The bonding speed is much slower than that of the Golden Ball welding process, the final product using the aluminum wire bonding process is also of low price because the material surface processing cost is not expensive. Aluminum wire ultrasonic bonding is actually a grinding welding process. The ultrasonic vibration of the two pure metals under the preset pressure is mutually pressurized by the ultrasonic transducer until the bonding is completed. The amplitude of ultrasonic vibration is 1-2 μm. The welding process can be divided into three parts: cleaning the surface, cleaning the oxide layer, and connecting the two pure metals. The two metal surfaces are under mutual pressure, and the distance between them is smaller than one atom. The welding result is of high quality and high reliability. The chip metal layer is usually made of pure aluminum or aluminum alloy with a thickness of 0.8-2 μm. It is particularly suitable for ultrasonic bonding with aluminum wires. The printed board bonding disc is a copper-nickel-gold composite metal layer (Cu/Ni2-4 μm/au0.1-0.2 μm). The surface of the gold layer is protected by impurities and chemicals during processing. When cleaning the surface, the gold layer is removed, but this does not affect the bonding process. The friction welding occurs between the aluminum wire and the nickel layer. It has been tested and evaluated that aluminum and nickel bonding is the best in terms of stability, reliability, conductivity, and special high operating temperature. In the layout and wiring design of printed boards, there are many parameters such as the size of the bonding disk, and the spacing must be considered. In order to avoid some problems during bonding, the printing board must have a High smoothness and cannot be deformed. Aluminum wire bonding is a room temperature ultrasonic welding process. During the welding process, the printing board within the bonding range should be moved or vibrated. Therefore, when bonding, the printing board must be fixed by vacuum negative pressure clamping. The bonding force of copper wires in the adjacent area of the bonding disk is also a basic factor. Even if the vibration of 1 μm exists, the bonding will be adversely affected. The uniformity of the surface of the printed board is another factor. For example, if the thickness of the nickel layer changes or falls below 0.5 μm, the bonding quality is unstable and the bonding force may be reduced to another factor. The roughness of the copper layer in the bonding zone should be controlled less than 2 μm. This is the maximum deviation compensated by ultrasonic vibration. 5. Unlike the ultrasonic bonding process of aluminum wire, the welding process of golden wire cannot be carried out at room temperature, and the welding quality can be obtained only at least 120 ℃. As with aluminum wire bonding, Canary ball welding must be flat during the welding process to avoid changes in surface temperature and loss of ultrasonic power. The surface of the bonding disk is metallic. The nickel layer thickness is 1 μm, and the gold layer is 1.5-2 μm (cuniau ). Because the processing cost of the printed board is higher than that of the aluminum wire bonding board, the speed of the Canary ball welding is three times faster than that of the aluminum wire bonding board. Because the aluminum wire is a low-temperature welding process, a larger ultrasonic power and precision bonding workbench are required to hold the fixed Printed Board, thus affecting the overall production capacity. The capillary tubes used for ultrasonic bonding using a cutting tool and a canary ball welding also affect the processing speed. 6. In addition to isolating the chip from the outside, cob and encapsulation process package also have circuit connections. The standard Encapsulation Format is limited, and the number of pins is also standard. This means that if an additional electrical connection is required, a large package must be selected, which will inevitably increase the package size and cost. Chips with more than 100 pins generally require high-price encapsulation. Sometimes the encapsulation geometric size makes the bonding difficult and damages the chip. Specialized ic asic is usually produced in small batches, increasing the difficulty of selecting the appropriate encapsulation. But the biggest difficulty is how to meet the requirements as high as possible for users with dedicated Number of pins. The current technology requires a lot of interconnection with printed boards in a very short time. Today, COB technology can provide satisfactory solutions for them. A large number of chip connections and interconnections can be properly solved at very low production or low processing costs. After the wire bonding is completed, the circuit chip and all the bonding leads are encapsulated using the above process. Small Batch ASIC components that require intellectual property protection cannot be easily copied. This encapsulation provides advantages. In addition, passive devices and other chips can be integrated into the same package. The advantage of the CoB encapsulation method is that the encapsulation volume is small, and the standard encapsulation size is usually 10 times that of the chip. Second, the cost of standard encapsulation of High-pin ASIC is often much larger than that of the chip itself. 7. cob Wire Bonding equipment uses cob technology to process 90% of the products from experience. 100 × 100mm printed boards are required, and each printed board chip is smaller than 100. Therefore, COB bonding equipment must meet the following minimum requirements; the minimum size of the printed board is 100x100mm. For Image Recognition, more than 200 reference images need to be stored. Multi-chip height, programmable focus. For chips with different contrast processing, there are more than four programmable light sources. Refined landing mode, applicable to non-flat surfaces. Lead control device for detecting lead loss. The large Z axis travel is suitable for processing large-sized capacitors. Knife bonding, with the ability to bend 60 degrees. Vacuum clamping Printed Board. The bonding tool should have enough space to meet the bonding requirements of Deep Cavity encapsulation. The main functions of CoB Chip Bonding equipment are as follows: the minimum size of the printed board is 100 × 100mm. Point or edge recognition, image recognition, more than 100 reference images need to be stored. Multi-chip height, programmable focus. Suitable for wafer, waffle plate or gel-packed chip feeding devices. Supports four programmable dispensing devices with at least four chips of different sizes. Under program control, you can select or replace the capability that requires printing or dispensing. The bonding tool should have enough space to meet the bonding requirements of Deep Cavity encapsulation. Flexible Transmission System with 25-15-mm length Printed Board 9. Conclusion Article The process parameters of the CoB Printed Board must be considered during design. To master these basic requirements, COB has the advantages of loading, encapsulation, assembly density, and reliability under various variable factors under the control conditions. Compared with the standard SMT process, reduces the manufacturing cost of the product.