Tutorial #104
George Riley
May 2010

Spheretek has developed a low cost non-contact wafer bumping approach that can deposit highly-uniform solder bumps with 40µm to 1,250µm diameters. Spheretek’s patented liquid transfer bumping method and associated tool set are compatible with all solderable materials, wafers, and substrates. This non-contact process has many advantages over older solder bumping methods.

The process includes three steps: depositing a proprietary under-bump metallization on the wafer, filling and aligning the non-contact transfer mask, and transferring liquid solder from the mask across a controlled gap to form uniform spherical bumps on the wafer.

Underbump Metallization

Because a bump is only as reliable as its underbump metallization, Spheretek first developed a proprietary sputtered tri-metal UBM as the foundation of its bumping process. Dry film photomasking defines the wafer areas to be sputtered.

Aqueous-based dry film photomasking is low cost, and it eliminates the pre-conditioning bake required by liquid photoresist. After bumping, aqueous chemistries lift off the residual dry film without corrosive metal etchants and their related hazardous waste disposal.

The tri-metal UBM, including a pre-cleaning sputter etch, is deposited in a single vacuum pump-down. This eliminates intermetallic oxidation, a potential source of delamination. The low cost tri-metal UBM is compatible with all bond pad metallizations. It may also be deposited as redistribution layer.

Bumping

Liquid transfer bumping requires a metal solder mask and a related heater assembly. The metal solder mask has a fully open aperture for each bump. Aperture size determines the amount of solder each bump receives. During filling, the mask is kept in close contact with the face-up, unpowered heater assembly. A precision double-blade design prevents solder paste voiding or disturbance as the blade deposits a controlled volume of solder into each opening.

Fig.1 Double blade filling aperture to controlled depth as solder mask rests on the heater.

The heater assembly and filled mask are inverted and aligned close to, but not touching, the wafer. This controlled gap is essential for proper liquid transfer. An added benefit of the gap is that gasses can vent during the liquefaction of the solder, preventing solder voids in the completed bumps.

Figure 2. Interference gap between mask and UBM.

Heating melts the solder. Gravity flow and surface tension transfer the solder as a liquid, crossing the gap from the mask to the UBM pad. Cooling creates uniform solder bumps to complete the process.

Figure 3. Gap vents gasses between mask and wafer.

The liquid transfer method will not deposit an undersized bump, nor will a bump form on flawed UBM. A low-power visual inspection will identify any such missing bumps. They can be reworked if needed by replacing the stencil over the good bumps and repeating the transfer process.

Figure 4. Bumps after transfer from mask to wafer.

Results

The smooth, uniform fine-pitch bumps are well suited for flip chip, wafer-level packaging, large arrays, and similar precise, fine pitch applications. Bumps down to 40µm diameter are controlled to +/- 1% height variation.

In one challenging application the liquid transfer process repeatedly produced 10,000 bumps of 40µm diameter on 90µm pitch with less than 0.05% missing bumps.

Advantages

Major advantages of this patented method reported by Spheretek include:

• Low cost, reliable UBM.
• Bump uniformity, size, and smoothness suited to high-density arrays and challenging applications.
• Compatible with all bond pad metallizations.
• Reliable and predictable adhesion with known UBM film thickness.
• Pre-deposition sputter etch and subsequent tri-metal depositions in the same system pump-down.
• Single pump-down deposition eliminates intermetallic oxidation and delamination.
• Sputter-etch removes pad surface oxidation before depositing UBM.
• Low cost dry film photomasking does not require preconditioning bake cycles.
• Dry film lift-off with low cost proprietary aqueous-based chemistries.
• No unreliable corrosive metal etch chemistries and associated disposal problems.
• Tri-metal UBM is also suitable for forming Redistribution Layers (RDL).

FOR MORE INFORMATION

Spheretek LLC, a Division of MVM Technologies Inc.      
www.spheretek.org
gary@mvmtech.com