The perfect electronic product device should be as small and light as possible, while containing the maximum amount of electronic functionality and operating at the highest possible speed. To meet these needs, the electronic packaging industry has been driven to develop more and more advanced packaging methods, both by increasing the density of integrated circuits on a single PCB and also by combining multiple functionalities into single, dense packages.

Increased package and interconnection density have driven the evolution of assembly methods from throughhole technology (THT) to surface mount technology(SMT) and have led an increased use of wire bonding to attach devices to PCB substrates. Decreased interconnect pitch and the use of chip scale packaging(CSP) have enabled the increases in device density, while multichip modules (MCM) / system in package(SiP) approaches have allowed integration of functionalities that would be difficult to produce on a single wafer substrate.

When the semiconductor industry has concentrated for many years on increasing the performance of devices by reduction in critical dimensions, until recently, there has been less consideration of the fact that devices in an electronic system must communicate with each other through the packages that contain them. Large I/O requirements and signal transmission quality have emerged as key considerations for the semiconductor packaging industry, as have the assembly process requirements and the final finish of PCB substrates used to achieve reliable interconnections both within IC packages and for the second level packaging of devices onto PCB substrates.

This article describes the key factors affecting interconnection reliability, especially focusing on the performance of finishes for gold wirebonding applications.

These limitations have provided an opening for electroless process options. These include electroless nickel immersion gold (ENIG), electroless nickel electroless gold (ENEG) and electroless nickel electroless palladium immersion gold (ENEPIG).

Of these three options, ENIG is not generally considered to have an acceptable process window for high reliability gold wire bonding (although it has been used for some lower end consumer applications) and ENEG suffers from many of the same process cost issues as electrolytic nickel gold, with additional challenges from operating more complex electroless gold processes.

While electroless nickel electroless palladium immersion gold (ENEPIG) first emerged in the late 1990's, its market acceptance was delayed by the very volatile price of palladium metal around 2000. However, in more recent years, the market demand has shown strong growth as users have begun to appreciate the potential of ENEPIG to address many of the new packaging reliability needs, while also meeting lead free / ROHS requirements.

Apart from the advantage of packaging reliability, the cost of ENEPIG has now become a positive consideration. With recent increases in the price of gold price to levels above US$800 per troy oz, the production cost of electronic device that required thick gold electroplating becomes extremely difficult to control.
Since the cost of palladium metal (US$300 per troy oz) has remained relatively low in comparison to gold, an opportunity for cost saving by replacement of gold with palladium is now available.

The table below shows a comparison between these four final finishes and ENEPIG. None of the four primary final finish options is perfect for lead-free assembly, due to a variety of different concerns including multiple reflow cycle capability, shelf life before assembly and wire bond capability. In contrast, ENEPIG shows substantial advantages combining excellent shelf life, solder joint reliability, gold wire bondability and usage as a contact surface.

The protection of the electroless nickel interface with the formation of a thin electroless palladium layer, prior to immersion gold, eliminates the potential for excessive gold attack on the electroless nickel surface.

Table 1 Comparison of Final Finish Performance

When we consider the final finish performance in a variety of different assembly methods, it can be seen that ENEPIG is suitable for a wide range of assembly requirements.