Fujitsu Develops Ultrafine-Pitch Solder Bumping and Flip-Chip Bonding

12/16/2003 - Fujitsu Limited announced that it has developed new technology that enables formation of ultrafine pitch 35 micron (center-to-center distance of bumps) solder bumps (*1), and high-precision flip-chip bonding (*2) interconnection. This breakthrough technology increases connection density by approximately 50 times compared to conventional flip chip interconnections and actualizes downsizing of LSIs and LSI packaging.

With the continuing strive toward downsizing and higher functionality of a variety of devices such as mobile phones and digital cameras, demand is increasing for both finer bump pitch and high-precision interconnection technologies, which enable downscaling of LSIs and LSI packaging.

Flip-chip bonding, which is widely utilized as ideal for reducing the mounting area of LSIs and LSI packaging, entails interconnection of molten solder bumps. In order to avoid short-circuiting between solder bumps, currently bump pitches are in general between about 200 micron to 250 micron.

To enable further downsizing of LSIs and LSI packaging, both finer pitch solder bumping and high-precision flip-chip bonding capable of avoiding short-circuiting between solder bumps are indispensable.

Fujitsu's New Technologies
Fujitsu has succeeded in developing the world's first-ever ultrafine-pitch solder bumping technology with 35 micron pitch, and high-precision flip-chip bonding technology that ensures interconnections. Features of the new technologies are as follows:

1. Ultrafine-pitch solder bumping technology
This new technology adopts solder bump formation using a plating method (*3). To narrow down the bump pitch, it is necessary to refine the photo-resist resolution used for bump formation, raise the bump height, and have even formation. However in the past, the smaller the bump pitch became, the more increasingly difficult it was to have even formation of bumps, due to the fact that the plating solution did not sufficiently reach photo-resist openings. This resulted in uneven bump formation.

With Fujitsu's new technology, by improving the photo-resist material, optimizing the parameters for exposure and developing of photo-resist patterns, and precisely controlling the electric current used during plating, the even formation of ultrafine-pitch bumps with height is now possible. Furthermore, as part of Fujitsu's eco-friendly mission, lead-free solder bumps have been used.

2. High-precision flip-chip bonding technology
With conventional flip-chip bonding, by heating the bumps while a calibrated force is applied to them, the bumps are melted and then pressure-welded. However, with ultrafine-pitch bumps, short-circuiting frequently occurred when this method was employed due to neighboring bumps coming into contact with each other. Furthermore, in order to ensure connection to the connection surface, a unique process to mechanically planarize bumps was necessary to eliminate any bump height deviations.

With Fujitsu's new technology, by employing precision-control of temperature and force during flip-chip bonding, high-precision flip-chip bonding is now possible without the need to employ a bump planarization process. Through these new techniques, short-circuiting can be avoided and connections can be ensured even during flip-chip bonding of LSIs with ultrafine-pitch solder bumps in area-array configuration.

Utilizing this technology, Fujitsu has begun to supply to Value Added Technologies (VATECH) (*5) of South Korea a chip-on-chip Multi-Chip Module (MCM) (*4), consisting of 4 units of detection signal-processing CMOS devices for which the entire connection surface is covered with 160,000 ultrafine-pitch solder bumps in full area-array configuration, that have been flip-chip bonded to an X-ray detection device. VATECH has succeeded in installing this chip-on-chip MCM and verifying operation of a dental X-ray image sensor.

The chip-on-chip MCM for the dental X-ray imaging sensor has been manufactured by Fujitsu Integrated Microtechnology Ltd. with assistance from Fujitsu Laboratories Ltd.

Future Developments
In order to meet demands for further miniaturization and higher functionality of mobile devices and others, Fujitsu is committed to the continued development of downscaled LSIs and LSI packaging utilizing this new technology. Fujitsu is also exploring the applicability of this technology for stud bumps (*6) and gold bumps (*7), which were not alternatives with conventional solder bump pitches.


  1. Bump
    A protruding electrode on the surface of a semiconductor chip, formed through an evaporation method, plating method, or printing method.
  2. Flip-chip bonding
    An interconnection technique whereby a chip with bumps is flipped over and attached onto a board (or in the case of a chip-on-chip MCM, onto another chip).
  3. Plating Method
    A common method for forming solder bumps. A seed layer is applied employing a sputtering method, photo-resist is coated on, and then openings are created only in the bump area through a photolithographic process. The wafer is then dipped in plating solution, and the solder is deposited.
  4. Chip-on-chip MCM
    Of the various multi-chip modules in which several chips are combined within one package, a module in which the upper "daughter" chip is flip-chip mounted on the lower "mother" chip.
  5. Value Added Technologies Headquarters
    7511 Seokwoori Dongtanmyun Hwaseongsi Kyunggido 445-811 South Korea
    President & CEO: Chang Joon Ro.
    Specialized developer and producer of X-ray imaging systems for medical and dental application, and TFT-LCD test systems, founded in 1992.
  6. Stud bump
    Chiefly used in System-in-Package (SiP) applications. Gold balls are made of gold wire and formed into bumps using cutting techniques. Only applied on the peripheral configuration of a chip pad.
  7. Gold bump
    Chiefly used in chips for LCDs, or in TAB (tape-automated bonding) - BGA. Bumps are formed by gold plating. Only applied on the peripheral configuration of a chip pad.

About Fujitsu
Fujitsu is a leading provider of customer-focused IT and communications solutions for the global marketplace. Pace-setting technologies, highly reliable computing and telecommunications platforms, and a worldwide corps of systems and services experts uniquely position Fujitsu to deliver comprehensive solutions that open up infinite possibilities for its customers' success. Headquartered in Tokyo, Fujitsu Limited (TSE:6702) reported consolidated revenues of 4.6 trillion yen (US$38 billion) for the fiscal year ended March 31, 2003. For more information, please see :

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