The rapid growing demand for handheld devices, such as cell phones, personal digital assistants, camcorders, etc., pushes technology to produce smaller, faster, lighter, and more functional devices. It increases the number of features without changing, or even reducing, the device sizes. On IC packaging, the use of system-in-package (SiP) is increasing to fulfill the technology and market needs. It is characterized by any combination of more than one active electronic component of different functionality, plus optionally passives and other devices like microelectromechanical systems (MEMS) or optical components assembled, preferably, into a single standard package that provides multiple functions associated with a system or sub-system.

It may be required to apply wire bonding, flip chip, and surface-mount processes on a substrate. This change impacts the substrate of the IC package--to provide a platform for the assembly of different kinds of components using different assembly techniques.

Use of Nickel-Palladium-Gold Finishing

Palladium plating film has good gold wire bondability and solderability. It has been used and proven on IC lead frames for years. The typical plating thickness of palladium and gold is 0.1 – 0.15 micron and 0.005 micron respectively. The first purpose for applying palladium plating to lead frames was to reduce package assembly time by omitting the multiple plating processes of solder plating on the outer-lead and silver plating on the bonding area at the package process. Now, it has also been recognized for fulfilling the lead free requirement of the RoHS directive.

The electroless nickel, electroless palladium, and immersion gold (ENEPIG) process is a suitable candidate for the IC package PCB substrates--especially for SiP products. As electrolytic nickel palladium gold plating, ENEPIG is also good for both soldering and wire bonding. A layer of 0.2 micron electroless palladium and 0.03 micron immersion gold on electroless nickel could provide wire bonding and soldering capabilities. Unlike the electrolytic process, the ENEPIG process does not need bussing lines, which provide extra flexibility on the circuitry design and enable high-density design for current advanced products and future needs.

Unlike the electroless nickel immersion gold (ENIG) process, ENEPIG does not have a “black pad” issue. The palladium is plated on the electroless nickel by chemical reduction instead of displacement reaction, therefore there is no attack on the electroless nickel layer.

Enlarge this picture Figure 2. Auger analysis of the plating interface of ENIG (left) and ENEPIG (right).

By Auger analysis (Figure 2), it is proven that there is a phosphorus rich layer between the nickel and palladium layer and also no phosphorus rich layer between palladium and gold. There is no “corrosion” of the nickel layer by the palladium plating process and also no excessive corrosion of the palladium layer by the immersion gold plating process--because of the low phosphorus content of the electroless palladium layer (~3%P) and low thickness of immersion gold.

Figure 3. SEM analysis of the ENEPIG surface before (left) and after (right) gold stripping.

Using scanning electron microscope (Figure 3), it is clearly shown that there is no hyper-corrosion observed on the electroless nickel surface and grain boundaries. Functionally, ENEPIG has solderability and bondability similar to ENIG and electrolytic plated nickel gold, respectively.

On material cost, the ENEPIG process is cheaper than either electrolytic bondable gold or electroless bondable gold process--assuming the gold metal and palladium metal prices are $18/g and $9.6/g, respectively. Since the density of gold (19g/cm³) is about two times that of palladium (10g/cm³), with the same plating thickness, the metal cost of palladium is only about one forth of the metal cost of gold. There is up to an 80% savings on the final finishing processing cost by using ENEPIG instead of electrolytic nickel gold process. The typical saving is about $5/sq.ft. board area.

Technical Concerns

Enlarge this picture Figure 4. Soldering results – As received and after aging to 1,000h at 150oC.

Assembly Parameters Re-Optimization

For both wire bonding and soldering, parameters re-optimization might be required for the best results.

Ternary (Cu,Ni)₆Sn₅ IMC Problem

There is concern on the use of nickel for lead-free solder. The bulk SnAgCu lead-free solder microstructure as reflowed is comprised of a dispersed eutectic phase which consists of small Ag₃Sn and Ni₃Sn₄ particles in a tin matrix. During soldering, palladium and gold rapidly dissolve into the melted solder, which causes the nickel underlay to contact the solder and forms intermetallic compounds. Nickel from the pad, together with tin and copper, form the solder participate in the reaction to form a ternary intermetallic (Cu,Ni)₆Sn₅ at the interface, which is observed on top of the Ni₃Sn₄ (formed on a nickel surface). There is high dislocation density from differences in crystal structure and lattice (between Ni₃Sn₄ and (Cu,Ni)₆Sn₅). Voids will grow at the Ni₃Sn₄/(Cu,Ni)₆Sn₅ interface and increase the probability of failure. However, for IC packaging applications, as the solder joints are fixed by the underfill, this potential risk is minimized or eliminated.

Conclusion

Enlarge this picture Figure 5. ENEPIG with SnAgCu, 10 times reflowed, 500 thermal cycles.

ENEPIG is a universal finish for both soldering and wire bonding applications. It is also a potential final finish candidate for next-generation and advanced IC packages.

This article was presented at the TPCA Conference 2006 and is reprinted with TPCA’s permission.