Brief introduction of wafer bump packaging technology

Wafer level packaging is a well recognized process, and component vendors are seeking to use WLP in more applications, and the technology to support WLP is rapidly maturing. As component vendors are actively turning to WLP applications, their range of use is expanding.

At present, there are 5 kinds of mature process technology which can be used for wafer bumping. Each technique has its own advantages and disadvantages. The gold stud welding bumps and electrolytic or electroless gold welding bumps are mainly used for encapsulation with less pin number. The application areas include glass flip chip package, soft film flip chip package and RF module. Because of the high cost and long process time of this kind of technology, it is not suitable for the package with more I/O pins. Another technique is to solder the preform before reflow soldering the preformed solder ball, which is suitable for packages with pins up to 300. The most widely used two kinds of wafer bumping processes are electrolytic or electroless solder and printed solder paste using high-precision lithography platforms.

One of the advantages of printed paste is that equipment investment is low, which allows many wafer manufacturers to enter the market and serve the semiconductor manufacturers. With the gradual acceptance of WLP for the commercial market, the demand for new wafer projection professional processing services continues to grow rapidly.

Practical process development

Many emerging applications required bump less in quantity, the key requirements of the application is the solder bump must have large cross section, in order to reduce the resistance of no bare chip package, because the on resistance of the device is mainly from the DFPR RON, it will ultimately affect the product efficiency and battery life.

Recently, Europe launched a development plan of solder ball stick planting technology, can make the application of WLP in vertical power MOSFET, compared with the standard TO package can reduce 30% of the package and occupying area of 90% DFPR. It is funded by the European Union, the goal is to develop the next generation of WLP packaging devices wireless portable applications in local area network, including SoC power amplifier and a short-range wireless networking baseband equipment required to achieve a high degree of miniaturization.

WLP technology has been successfully used in horizontal devices, but the upright structure occupies less space and is more suitable for mobile applications. The difficulty of implementing an upright WLP power device is that it is necessary to re connect the contacts on the backside before generating the solder bump, and then to the front of the wafer to complete the WLP package. Re routing is achieved by plating and filling tiny vias, which requires very thin wafers to achieve the appropriate aspect ratio. If the diameter of the through hole is 300 m, a wafer with a thickness of 150 m must be used to maintain the aspect ratio of the 2:1.

In order to complete the development of the MOSFET power amplifier for WLP packaging technology, DEK and the Technical University of Berlin developed a solder ball bonding process with members of alliance in the blue whale plan, 6 inches in diameter on the wafer with 500 m spacing planting stick ball with diameter of 300 m + 10 m.

The process was initially validated by using a wafer of 680 m thickness, and then solder bonding was successfully achieved on a thinner 150 m wafer.

The bonding process requires two linear presses. The first one applies a soldering flux to the wafer pad; the second is responsible for placing solder balls and completing all processes prior to reflow soldering.

The first machine loads the wafer and uses the video recognition system to calibrate the position and then solder the flux to the metal pad at the bump. The wafer is then delivered to the second machine, which loads the impression wafer onto the machine and positions it against the plate. The ball holder is then moved to the top of the plate to separate the solder ball into a single layer and to push the solder ball into the opening with a little positive force.

Solder ball placement can be repeated several times to ensure all openings are filled. The blue whale project has determined that two positions will be operated at a speed of 10mm per second for more than 99.9% of the solder ball placement rate. After the ball has been implanted, the wafer will be moved back at the preset speed under the control of the machine, separated from the steel plate and transmitted to the reflow furnace.

In the blue whale project, the flux is coated with wire mesh, so the coating can be very thin, and the wire flux cleaning technique is used between each process cycle. In the solder ball placement stage, a "two" layer of "mixed" steel plate is used to form an interval to prevent the hole from being contaminated by the flux left by the previous printing.

The process developed for the blue whale project uses a 50 m thick wire with a diameter of 200 m to print the flux, plus a mixed steel plate with a total thickness of 300 m, used for solder ball placement.

In addition to the large volume of solder coated, another advantage of the solder ball is not sticky and process the coated solder ball volume reduction in the reflow process, so the repeatability will be higher, so to the final completion of welding convex package height is more uniform.

The process boundaries are blurring

The blue whale project proves that the bonding process of welding balls is a practical and high rate WLP welding technology. With the popularity of such packages, bonding process will be adopted by more and more manufacturers. The wafer bump processing factory has been equipped with solder paste printing function, using this technology especially benefit, because the majority of printing machine for wafer solder bumps are easy to turn the solder ball stick plant processing, and can convert it back.

DEK solder paste printing and bonding technology can achieve this function swap. DEK printing presses are designed for two transfer heads based on the same technology, ProFlowDirEKt stamping, so they all use the same printing machine interface. A suitable high precision batch impression function in 10 minutes from a raw

Change from one manufacturing process to another. But before soldering, it is necessary to plug in the flux coating process, as described in the blue whale project.

With more and more wafer bonding manufacturers applying solder paste printing technology to WLP packaging, batch imprinting technology has been widely used in the field of semiconductor packaging. However, large EMS enterprises have also entered the WLP field. The boundaries between packages and boards, as well as the boundaries between packaging and assembly processes, are becoming increasingly blurred, forcing companies to have wafer level and chip level process technology to serve customers.