Due to the flexibility of chemical nickel palladium gold, it has gained more market share, especially as a gold wire bonding surface. If the residence time in the immersion gold is extended to meet the design of the gold layer thickness greater than 3.0 μin (0.075 μm) Requirement, bonding failures are occasionally observed. The failure is manifested by the floating of the metal wire bond. The failure analysis of the failed bond showed that there was a separation phenomenon at the Ni/Pd interface. The nickel surface is black, which is obviously corroded as a "black disk". Nickel corrosion in ENIG occurs during the immersion gold deposition step, usually due to damage to the nickel surface (unevenness), plus aggressive immersion gold solution (low gold concentration, low pH) and residence time in the gold solution Caused by extension. The longer the residence time is to achieve the thicker immersion gold layer required by the design. When the gold immersion step is not applicable in theory, how does the nickel covered by the electroless palladium layer corrode?
Immersion gold deposition is a displacement reaction in which one nickel metal atom is oxidized to nickel ions, releasing two electrons. These two electrons are absorbed by the two positively charged gold ions in the solution, and on the contrary, they are reduced to metal and deposited on the surface of the nickel substrate.
The driving force of the redox reaction can be derived from the electric potential. The potential sequence is a list of chemical elements (atoms, molecules, and ions), arranged according to their tendency to gain or lose electrons (reduced or oxidized respectively), expressed in volts, and measured with reference to the hydrogen electrode. The hydrogen electrode is regarded as a standard. The potential is arbitrarily assigned to 1.0 v.
The driving force of Ni/Au exchange is 3.4 times that of Pd/Au exchange. If immersion gold ions can contact the underlying nickel, this will be the way to minimize the resistance to gold reduction. Gold will be reduced at the expense of nickel, so that the nickel is oxidized or corroded, and gold is deposited on top of the palladium layer. Research and failure mechanism analysis show that under certain conditions (not up to the standard nickel deposition plus longer residence time in the aggressive gold immersion solution), the immersion gold can contact the underlying nickel.
IPC revised ENEPIG specification IPC-4556-A stipulates that the thickness of the immersion gold layer is 1.2 μin-2.8 μin (0.03 μm-0.07 μm). Although the data shows that the thickness of this kind of gold is sufficient for gold wire bonding, in order to open the gold wire bonding window, the thickness of gold (>3.0 μins/0.075 μm) is usually specified in the circuit board design.
There are three ways to slow down the nickel corrosion in ENEPIG deposition. The first method is to control the electroless nickel plating process to produce a uniform deposited layer with the smallest gap, increase the phosphorus content in the deposited layer by 1 or 2 percentage points, and do not try to deposit thicker gold in the immersion gold solution.
The second method is to deposit a thicker electroless palladium layer (6 μin-8 μins, 0.15 μm-0.20 μm), which can cover any defects in the nickel deposition layer and act as a barrier between the immersion gold and the underlying nickel. This is a relatively expensive intermediary layer because the price of an ounce of palladium now exceeds that of an ounce of gold.
The third method is to use reduction-assisted immersion gold (RAIG) instead of immersion gold (IG). RAIG is a mixed reaction liquid. The immersion reaction and the autocatalytic reaction start at the same time. When the substrate becomes inaccessible, the immersion reaction will decrease, and the autocatalytic reaction will dominate. RAIG is non-corrosive, does not produce substrate corrosion, and can deposit 4 μin-6 μin (0.10 μm-0.15 μm) thick gold layers in the same step.
The choice of the best mitigation method is determined by the manufacturer and its suppliers, and the process selection should be suitable for the production workshop and within the manufacturer's budget limit. It is worth mentioning here that RAIG solutions are becoming more and more popular and are becoming ENEPIG's choice, especially for specifying thicker gold in the design of circuit boards.