Aluminum wire-aluminum die pads. Al-Al bonding is an extremely reliable system. Being a mono metal bond, it is neither prone to IMC formation nor corrosion. Al-Al bonding is best performed ultrasonically. Al-Al thermocompression bonding is possible by high deformation. Al-Al wedge bonding is predominant in COB applications, the mainstay of the low-cost, disposable product range and also often used in midrange products.¹

Aluminum wire-nickel PCB pads. Al-Ni bonds are typically made with large (greater than 75 mm) diameter wires. Large wires bonds are less prone to Kirkendahl voids and galvanic corrosion. Nickel bonds are more reliable than Al-Ag or Al-Au. Electroless nickel on pads from boride or sulfamate base systems give reliable bonds. Aluminum to nickel pad bonding remains very popular in COB applications, low-cost yet reliable.

Bondability is affected by nickel surface oxidation, and measures to improve nickel pad bondability include the (short) time between plating and bonding; PCB storage in an inert atmosphere; and chemical cleaning before bonding.

Soldering with nickel pads. Aforementioned measures to improve bondability also apply to improve solderability. Electroless nickel coated components have a short shelf life (fewer than 24 hr., to meet MIL-893C solderability standards) unless protected over by gold. Issues also relate to the phosphorus content in EnP plating. Storage conditions are important, as is first in, first out). Use within six months from date of manufacture remains a related concern with nickel.

Copper bond wire-aluminum die pads. The Cu-Al system is prone to various IMC failures similar to Au-Al systems. IMC growth in Cu-Al is lower than in Au-Al systems and no Kirkendahl voiding is seen in Cu -Al IMC. Brittle CuAl₂ in IMC layer lowers shear strength at 150-200°C; at 300-500°C, excessive IMC growth significantly reduces bond strength. Cu-Al IMC is impacted by atmospheric composition. Presence of O₂ creates copper oxides, which inhibit bondability by growth of void-like grooves under the bonds. Chlorine contamination and H2O (moisture) can corrode the aluminum and Cu-Al IMC layer.

Metalization and Product Positioning

Figures 7 to 12 show several diverse applications of COB-SMT. Figure 7 is an assembly from a handheld toy fan with motion activated flashing lights. It uses a single-sided BT resin PCB with OSP over copper pads and nickel over copper bond pad metalization. At the next price point, Figure 9 shows the COB in a PDA using a multilayer FR-4 PCB with uniform nickel metalization at both soldering and wire bond sites with AlSi wire bonding process. Figure 10 is the display and control module from a microwave oven and Figure 11 shows the main and remote control assemblies from a mini-disk player. Both products use ENIG pads for both soldering and wire bonding. Figure 12 is the assembly from a high-end heart rate monitor using ENIG at the solder sites and selectively plated soft pure gold at the COB sites. The common factor in each of the shown products is COB, yet metalization choices are dictated by product positioning, life, reliability expectation and price point. The cost-sensitive low end uses metalization that leans in favor of the soldering process.


Conclusion

Wire bonding is not out of steam, given improvements in the latest generation of bonders.⁵ The metalization choice can help provide the right balance between wire bond reliability, soldering yields, and solder joint reliability. At the low-to-mid cost spectrum, the metalization tends to lean toward alignment with the soldering process. Despite this compromise, the wire bonding process for this product class proves sufficiently robust for the application.

The required metalization for reliable wire bonding is not necessarily the same as that needed for high quality solder joints. ENIG – or its derivative, ENEPIC – provided the nickel and gold plating control is carefully maintained^4,6,7 when combined with selective pure thick gold on the wire bond sites, provides the highest reliability for both soldering and wire bonding but also incurs the highest cost.

At the next lower level, ENIG is an option for both soldering and wire bonding without selective plating on the wire bonding sites. ENIG metalization yields an excellent solder joint quality and while not “the” best for wire bonding, proves adequate for midrange product needs.

Where cost sensitivity is acute, either OSP over copper for solderability with selective nickel or gold plating on the bond sites, or simply a uniform nickel as a common soldering and bond site finish together with aluminum wedge bonding, provides the lowest-cost alternative.

References

1. Mukul Luthra, “Process Challenges And Solutions for Embedding Chip On Board Into Mainstream SMT,” SMTA International Proceedings, September 2003.
2. American Society for Testing & Materials, F72-06, Standard Specification for Gold Wire for Semiconductor Lead Bonding.
3. International Directory of Solder Reflow Thermal Processing Equipment, Chip Scale Review, April/May 2007.
4. Kuldip Johal et al, “Impacts of Bulk Phosphorous Content of Electroless Nickel Layers to Solder Joint Integrity and Their Use As Gold-Aluminum Wire Bond Surfaces,” SMTA Journal, April-June 2004.
5. Ron Iscoff, “Are Wire Bonders Running Out of Steam? How This Essential IC Assembly Tool is Keeping Pace,” Chip Scale Review, March 2007.
6. Shigeo Hashimoto, et al, “Electroless Gold Plating for Printed Circuit Boards,” IPC Apex Proceedings, February 2007.
7. Kuldip Johal and Gerry Brewer, “Are You in Control of Your Electroless Nickel / Immersion Gold Process,” SMTA International Proceedings, September 2001.
8. R.J. Klein Wassink, Soldering in Electronics, ISBN 090115024-X, 1984.

Ed.: This article was first published at SMTA International in October 2007, and is used with permission.

Mukul Luthra is business director and founder of Waterfall Technologies (waterfalltech.com); mukul@waterfalltech.com.

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