Electroless Nickel Plating    

PAC TECH offers subcontractor electroless nickel plating services for wafer bumping, pad resurfacing (wirebonding) and ACF/ACA applications. PAC TECH has four manufacturing sites around the world which offer these services. Our goal is to be customer-oriented and flexible enough to meet all the customers' bumping needs. We support customers with prototyping, engineering, and R&D projects as well as bumping wafer in high volumes. Each PAC TECH facility has a throughput of 600,000 wafer per year.

PAC TECH offers several different pad finshes and layer thicknesses using electroless nickel plating in combination with electroless palladium and immersion gold process:

1)   e-Ni/Au
2)   e-Ni/Pd
3)   e-Ni/Pd/Au

The nickel thickness is between 2 and 25µm depending on the application. Palladium is deposited in the range of 1000-3000A, and gold is typically deposited between 300 and 1000A.
PAC TECH can customize any of these thicknesses to meet your applications' needs.

Electroless Ni/Au Plating Overview:

The Under-Bump-Metallization (UBM) is an integral part of all bumping processes. This layer is typically deposited by physical vapor deposition (PVD), electroplating, or electroless plating. The choice between these three UBM technologies is often dictated by cost and reliability. PVD and electroplating techniques require both high vacuum and photolithography steps and are therefore considered high cost operations. The electroless nickel/gold process technology is a simple wet chemical process that is self-patterning and therefore involves lower costs in relation to its total capital investment and operational costs.

Automated Plating Line (PacLine 300 A50)	Aluminum Bond pad	Ni/Au Plated Bond Pad

Process Flow: Plating on Aluminum based semiconductors


1) Pad Cleaning: Cleaning the pad off organics, silicon oxides, and/or nitrides (solvent and acid based solvation)

2) Aluminum Etching: Removes aluminum oxide (alkaline based etch)

3) Zincation 1: Activating the aluminum

4) Zinc Stripping: Removes zinc layer which is often rough (strong acid)

5) Zincation 2: Activates the aluminum with smooth zinc layer

6) e-Nickel Plating: Deposits nickel selectively on activated Al bond pad (sulfate based chemistries with phosphorous reducing agent)

7) Au and/or Pd Plating: Deposits Au and/or Pd on top of nickel layer (cyanide or sulfate based chemistries)

For aluminum based integrated circuits, the chemical sequence for depositing the electroless nickel layer comprises following steps: 

1. Cleaning the pads off any organic or silicon based contaminates which may occur due to wafer handling, storage, or variations in the manufacturing process.

2. Removing any native oxide that may have built up on the aluminum pad surface. This is typically performed using caustic based etching chemicals.

3. Activation of the surface of the aluminum. The most commonly used wet chemical system for this is “zincation”, where a zinc oxide solution is used to replace some of the pad aluminum with zinc in an electrochemical reaction. Empirical research has shown that by stripping this zinc layer off and then reforming it in a second zincation step, a higher quality layer of zinc is created. This is often referred to as “double zincation”. This zinc layer changes the electric potential of the aluminum pad, and when immersed in a nickel sulfate solution, nickel replaces this zinc and an autocatalytic nickel reaction continues. By adjusting the time, temperature, pH, and chemical concentrations of the nickel plating bath, nickel structures between 1 and 25 microns tall can be created.

4. For most applications the deposition of solder does not immediately follow the nickel deposition process. The nickel surface will oxidize fairly quickly, and therefore a thin layer of a noble metal is typically deposited on top of the nickel to protect the surface from oxidation. There are two common metals (Pd and Au) which are compatible with the electroless nickel process and can be deposited sequentially within the same plating line using either immersion or electroless based processes.

Process Flow: Plating on Copper based semiconductors

1) Pad Cleaning: Cleaning the pad off organics, silicon oxides, and/or nitrides (solvent and acid based solvation)

2) Copper Oxide Etching: Removing copper oxide (acid based chemistries)

3) Copper Etching: Etching the Copper (acid based chemistries)

4) Pd Catalyst: Catalyzing the copper surface (Pd chemistries)

5) Passivation Cleaning: Removing residual Pd from all surfaces but Cu

6) e-Nickel Plating: Depositing nickel selectively on activated Al bond pad (sulfate based chemistries with phosphorous reducing agent)

7) Au and/or Pd Plating: Depositing Au and/or Pd on top of nickel layer (cyanide or sulfate based chemistries)

For copper based semiconductors, the nickel and gold plating baths are the same as those for aluminum based semiconductors. Several acid based cleaning steps are typically used to clean off contaminates and to remove copper oxide from the surface of the I/O pads. The activation step for copper is similar to that used in the laminate board plating industry, and usually uses a palladium based catalyst. The know-how for plating Cu semiconductors is the ability to selectively catalyze the copper I/O pads without activating the surrounding passivation.

This electroless plating processes are inherently low cost, and can be used for a variety of different applications in addition to Flip Chip and WLCSP Bumping, including:

1. Polymer flip chip (1-5um of Ni/Au + conductive epoxies)
2. Anisotropic conductive adhesives (10-25um of tall Ni/Au + ACF or ACA materials)
3. Pad resurfacing of copper and aluminum for wire bonding (2-5um of Ni/Au, Ni/Pd, or Ni/Pd/Au)
4. Pad resurfacing of copper pads for probe testing (2-5 um Ni/Au, Ni/Pd, or Ni/Pd/Au)

High throughput, and consequently low cost, is accomplished by batch processing cassettes of wafer through an automated electroless plating line. The fact that the nickel plating process is highly selective, and will only plate on the exposed metal surfaces (aluminum or copper), translates into a major cost advantage for this UBM deposition technique. Compared to conventional techniques for depositing the UBM, the use of electroless nickel has the following advantages:

• There are no processing steps necessary to define the solderable area (such as vacuum metal deposition, photolithography, and mask etching).
• One system handles all wafer sizes without change over (3" to 12").
• The capital investment for plating technologies is relatively small.
• The operational costs (labor and overhead) are reduced.

Electroless plating on integrated circuits can, however, be challenging because of the fab-specific variations in materials and processes involved in creating the circuits. Aluminum (or copper) alloy composition, sub-structures under the pad metal, passivation material and quality, pad electrical potential, and energy sensitivity (radiation and grounding effects) all play a role in the plating rates, uniformity, and adhesion of the nickel.

Because the process details (inherent tricks of the trade) are not generally regarded as patentable, developers treat their processes as proprietary. Hence, particulars of electroless nickel plating are not readily available.

The first three steps in the process are critical in determining the overall selectivity of the plating process, nickel morphology, and the adhesion of the nickel to the aluminum (or copper) pad. In general, a process that produces fine grained, uniform, thin layers of the catalyst (zinc or palladium) will produce the best nickel plated structures. The specific chemistries and absolute component ratios are critical in producing this desired structure. In addition to selecting the appropriate plating chemistries, one must also consider availability, place of origin, price, toxicology, bath life, waste treatment/disposal, and environmental issues related to the chemicals when implementing a process in a manufacturing setting.