Emerging wafer level packaging process technology
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There are currently five mature process technologies available for wafer bumps, each with its own advantages and disadvantages. Among them, gold stud bumps and electrolytic or electroless gold solder bumps are mainly used for packages with fewer pins (generally less than 40). Applications include COG and COF ) and RF module. Due to the high cost and long processing time of such technology materials, it is not suitable for packages with many I/O pins. Another technique is to place solder balls first, and then reflow the pre-formed solder balls. This technique is suitable for packages with up to 300 pins. The two most used wafer bumping processes that are currently used are electroplating or electroless plating of solder and printed solder paste using a high-precision imprinting platform.
One of the advantages of printed solder paste is its low investment in equipment, which allows many wafer bumping manufacturers to enter the market and serve semiconductor manufacturers. As WLP is gradually accepted by the commercial market, the demand for professional processing services for new wafer bumps continues to grow rapidly. Indeed, most wafer bumping plants are premised on print functionality and provide one or more other technologies. Many people in the industry believe that solder paste printing technology will dominate most wafer bump applications.
Practical process development
Many emerging applications (such as WLP for stand-alone power MOSFET devices used in mobile phones and portable devices) require fewer solder bumps. The key requirement for this type of application is that the solder bump must have a large cross-section to reduce the number of bare die The package resistance (DFPR), because the device's on-resistance R(DS)on mainly comes from this kind of DFPR, it will eventually affect the end products (such as mobile phones, PDAs, media players or consumer electronics products and industrial, scientific and medical instruments. ) The efficiency and battery life. Using a solder ball cementation process with a maximum ball diameter of 0.5 mm, a large volume of solder can be applied under the premise that the solder bump spacing permits to form a maximum connection cross section.
Recently, Europe has initiated a project to develop solder ball adhesion technology that enables WLP to be applied to vertical power MOSFETs, which can reduce package footprint by 30% and DFPR by 90% compared to standard TO packages. The program is funded by the European Union and aims to develop next-generation WLP packaged devices for wireless portable device applications (ie Blue Whale Projects) in local area networks, including SoCs (system-level chips) needed to implement highly compact short-range wireless networking baseband devices. ) and power amplifiers.
WLP technology has been successfully used in horizontal devices, but the vertical structure has a smaller footprint and is more suitable for mobile applications. The difficulty in implementing a vertical WLP power device is that before the solder bumps are generated, the connection contacts on the back side need to be re-routed to the front of the wafer to complete the WLP package. Rerouting is accomplished by plating and filling tiny vias, which requires very thin wafers to achieve the right aspect ratio. If the via diameter is 300 μm, a 150 μm thick wafer must be used to maintain a 2:1 aspect ratio.
Therefore, Blue Whale plans to pose two challenges to the solder ball adhesion process. One is that high process repeatability and yield must be achieved with lead-free solder balls and solder bump spacing of 500 μm; the second is to ensure that solder balls can be attached to very thin (150 μm thick) wafers, and In the subsequent reflow process, the wafer is not damaged due to deflection of the wafer or the alignment of the solder balls is destroyed.
In order to complete the development of the WLP packaging process required for MOSFET power amplifiers, DEK, a member of the Blue Whale Program Alliance, and the Technical University of Berlin (TUB) have developed a solder ball adhesion process with 500 μm diameter on a 6-inch diameter wafer. The solder balls with a diameter of 300 μm±10 μm are adhered to the spacer. The process was initially verified using a 680 μm thick wafer before successful solder ball adhesion was achieved on thinner 150 μm wafers.
The solder ball adhesion process requires two linear printers. The first station applies flux to the wafer pad; the second station is responsible for placing the ball and completing all processes before the reflow process.
The first machine loads the wafers, aligns the position with a video recognition system, and imprints the flux on the underside of the solder bumps. The wafers are then transported to a second machine, where the flux-embossed wafers are loaded onto the machine and aligned so that they adhere to the steel plate. The solder ball placement head is then moved to the top of the steel plate, while the solder balls are separated to form a single layer, and a slight forward force is then applied to push the solder ball into the opening. This action can ensure that the solder ball and the flux are in close contact, which helps to reduce the movement of the solder ball during subsequent processing.
Ball placement can be done multiple times to ensure that all openings are filled. The Blue Whale Program has determined that two placement operations at a moving speed of 10 mm per second will result in more than 99.9% of ball placement yields. After the ball is mounted, the wafer will be moved back at the preset speed under the control of the machine, separated from the steel plate, and transferred to the reflow oven.
In the Blue Whale program, the flux is coated with a screen, so the coating can be very thin, and screen-flux cleaning techniques are used between each process cycle. A two-layer "hybrid" steel plate is used during the ball placement phase to form a gap to prevent the hole from being contaminated by the flux left by the previous printing. The main layer of the steel plate contains through-holes for the solder balls, and the other layer (ie, the isolation layer) is bonded to the bottom side of the steel plate.
The process developed specifically for the Blue Whale program uses a 50 μm thick screen with a perforation diameter of 200 μm for flux printing, plus a hybrid plate with a total thickness of 300 μm for solder ball placement.
In addition to the ability to coat large volumes of solder, another advantage of the solder ball adhesion process is that the volume of the solder balls that are coated will not shrink after reflow processing. This will result in higher repeatability, resulting in final package finishes. The solder bump height is more uniform.
Process boundaries become blurred
The Blue Whale Program has demonstrated that solder ball bonding is a WLP bump technology with high practicality and high yield. With the popularity of such packaging, the solder ball adhesion process will be adopted by more and more manufacturers. This type of technology is particularly beneficial for wafer solder bumpers already equipped with solder paste printing because most printers used for wafer bumps are easily turned to solder ball and can be converted back. . Therefore, manufacturers can invest in wafer-level power device packaging and other types of packaging services at a low cost. Ultra-CSP, for example, is most suitable for solder ball adhesion technology adoption; it can also accelerate the return on initial investment in large-scale equipment.
DEK's solder paste printing and ball bonding technology can achieve this interchangeability. DEK presses are based on the same technology for both transfer head designs, ie ProFlow DirEKt imprinting, and therefore all use the same press interface. A suitable high-precision batch stamper (such as DEK Galaxy, which can perform solder bumping using solder paste printing or ball bonding technology) can change from one production process to another within 10 minutes. However, it is of course necessary to insert a flux application process before the solder balls are stuck, as described in the Blue Whale Plan.
As more and more wafer solder bump manufacturers use the solder paste printing process for WLP packaging, batch stamping technology has begun to be widely used in the field of semiconductor packaging. However, large EMS companies have also entered the WLP field. The boundaries between packages and boards, as well as the boundaries between packaging and assembly processes, are increasingly blurred, forcing companies to have wafer-level and chip-level process technologies to serve their customers. Of course, these companies are already familiar with the precision screen printing process, and this process technology has been used for many years to perform solder paste coating prior to device placement. Therefore, it is relatively easy to shift the printing technology to the WLP process.