Mustafa Özkök

Direct EP and direct EPAG — Novel surface finishes for gold-, copper wire bonding and soldering applications

The direct deposition of palladium on copper is not a new surface finish process. Palladium was first introduced to the market as a surface finish on copper by Atotech more than 10 years ago. At that time, it was primarily used as a surface finish for soldering. In addition, it was employed as a contact surface in applications like touch key pads.

Forming solder filet on leadframe edges of a QFN with immersion tin

The QFN package is one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. The standard QFN Package soldered on a PCB is shown on fig. 1 a). The side edges of th. QFN are not covered by solder, which results in the fact that the package is soldered only planar in: dimensions from the QFN bottom side onto the PCB as shown on fig. 1 b). The QFN were the side edge is covered by immersion tin will keep the side edged of a QFN solderable. This allows the package to be soldered in 3 dimensions onto the PCB providing a stronger solder joint and; better reliability. The suggested paper describes a new process to cover the side edges of the package with immersion tin after the QFN package singulation step. The exposed lead frame edges are covered by immersion tin. No exposed copper is visible and delivers a three dimensional solder joint with better reliability and protection for the soldered package.

Palladium surface finishes for copper wire bonding (Part I: The selection of surface finishes)

During the past two years, fine pitch copper wire bonding has finally entered high volume production. It is estimated that nearly 15% of all wire bonders used in production are now equipped for copper wire bonding. Most of these are used exclusively for copper wire bonding. In terms of pitch, copper wire is only barely lagging behind the most advanced gold applications. The most commonly used copper wire is 20um in diameter and 18um copper wire is entering final qualification. Evaluations with even finer wire are underway. Although some technical challenges remain, many years of research have now resolved most of the problems associated with copper wire bonding and attention is beginning to shift from merely ensuring reliable manufacturing processes to optimizing processes for efficiency and throughput. The most advanced wire bonders now have pre-configured processes specifically designed for copper. In addition to throughput optimization, further cost reductions are being sought. Among these is the desire to eliminate the high-cost gold not just from the wire, but also from the substrate. On the substrate side the electronics packaging industry still works with electrolytic nickel / electrolytic (soft) gold (Ni/Au) for copper wire bond applications. This surface finish works with copper wire bonding but includes some disadvantages, such as: — Thick expensive Au layers of 0.1 to 0.4μm — Electrically connected pads (bussing for the plating) which require added space on the substrate. — Pd-coated copper wire often delivers better results on gold covered finishes, but is two to three times more expensive as pure copper wire. Furthermore electrolytic Ni/Au was not chosen as a result of in-depth investigations for the most effective surface finish. The selection was made because it was the surface finish with the highest distribution in the market for wire bond packages. This paper is offering the results of a two company joint work regarding an alternative copper wire bondable surface finish for substrates mainly with palladium as the final copper wire bondable layer. This offers further cost reduction possibilities. Furthermore, copper palladium intermetallics are regarded as very reliable.