Universal ASF II - Process Sequence
Universal ASF II outperforms ENIG as it shows finish shows consistently. Good pull strength results and a significant lower number of fractures in the intermetallic compound.

Universal ASF II: Nickel / Palladium / Gold for Advanced Packages

The trend in Advanced Packaging to smaller, more complex and more intelligent systems is still carrying on. In order to integrate the various functions in a densely packed space in an economic manner, System in Package Technology (SiP) is playing more and more a greater role.

On SiPs various assembly techniques are employed:

  • Soldering of Surface Mount Devices
  • Soldering of flip chips
  • Gold wire or aluminium wire bonding

ENIG has long been used as a surface finish within the PWB and IC substrate fabrication sectors. But the increased tendency to brittle fracture failures with decreasing BGA pad sizes was leading to more and more doubts regarding the reliability of ENIG for the advanced packaging systems.Another shortcoming is the incapability of conducting gold wire bonding on an electroless nickel/immersion gold surface finish.Electrolytic Nickel/Gold is also common for IC substrates, when gold wire bonding is needed. Disadvantages of this process are the bussing necessary to provide electrical connection to the circuit and the cost for gold as a relatively high thickness (0.5µm) of this precious metal is necessary. Soldering on a surface with high gold thickness is also critical as too much gold in the solder joint causes brittleness.

Atotech's Universal ASF II is a final finish, which consists of three metallic layers: nickel, pure palladium and gold. Nickel and palladium are plated autocatalytically. The palladium layer consists of pure palladium. The gold on top of the palladium is deposited by a displacement reaction and is virtually pure as well.

Universal ASF II performs better than ENEPIG with PdP in terms of Au-wire bond pull forces even with a lower gold thickness.

The performance of Universal ASF II in terms of solder joint integration is superior to ENIG as HSS and CBP tests results show. The occurrence of brittle fracture is banned from the scene.
When it comes to gold wire bonding it can be convincingly shown that Universal ASF II can compete with the commonly used final finish for this application, namely electrolytic nickel/gold.

Process Features:

  • Excellent cold ball pull test results
  • Excellent all shear test results
  • Excellent gold wire bonding results
  • Wide range of bonding parameter
  • Multiple reflow cycles do not degrade solder joint integrity or IMC build-up
  • Most cost effective surface finish for multi purpose assembly using Pd instead of thick gold layers