Bondable Gold

R&D utilizes soft gold plating chemistry at 99.99% purity with no brighteners or grain refiners added. Soft bondable gold is normally specified at 60 micro inch over 150 min Ni, Ni acting as the diffusion barrier for the base copper.

In the first image on your right: Grain bound diffusion. It has been shown that bondability issues may arise from impurity levels.

Ni and Cu metals, either deposited as base layers or codeposited as contaminates in the gold film, can quickly diffuse to the surface by grain boundry diffusion. These diffusing metals move rapidly through grain boundaries during high temperature processing, such as, die attach or plasma cleaning. See the second image to your right.

Metals in the upper right hand corner of the diagram move rapidly through the plated gold film at temperatures in the range of 100 degrees - 300 degrees Celsius in approximately one hour.

Ni and Cu are the most cited metals, which degrade bond-ability as well as pull strengths and reliability. Ni and Cu contamination may be accidentally introduced into the plating chemistry through small pieces of the metal such as Cu flashing falling into the bath and dissolving or by trace amounts being dragged in with the products.

Once these metals make their way to the surface, they quickly spread horizontally and diffuse across the surface. At the surface these two metals oxidize rapidly and cause serious bond-ability problems.

As you can see from the third image to your right, as the atomic % of Ni at the surface slightly increases the % of wire bond lifts dramatically increases, the same holds true for Cu.

It was previously stated that R&D uses no brighteners or grain refiners in their Au bath, while these additives permit for more rapid plating and brighter surface finish. It has been shown in numerous studies that these additives, particularly thallium, diffuse rapidly during the bonding process from the plated gold to the gold wire. The additive concentrates in the grain boundaries above the bond where it forms a low melting eutectic. This ultimately leads to breaks either during encapsulation or when thermal cycling parts.

An additional strategy R&D incorporates for providing a highly bondable and reliable surface finish is to minimize current density. This is due to the fact that impurity concentrations increase exponentially with current density.

Note: Thallium and lead content in gold deposits as a function of Current density etc.

As you can see in the fourth image on your right, maintaining a total impurity level of under 50 ppm is insufficient to assure bond-ability. It is also important to note than hydrogen gas forms at high current densities, which also degrades surface bond-ability. The Bond-ability of the gold film is found to be lowest in the region of 1.6 to 2.7 a/dm 2, this corresponds to the rapid evolution of hydrogen gas at the cathode.

As we have demonstrated, the problems involved in bondable gold plating are numerous and varied, not limited to bath impurity levels alone. It has been demonstrated that with any given Au bath bond-ability changes with impurity level, crystallographic structure, wave form, current density, plated impurity level, hardness, hydrogen content as well as bath temperature.

Therefore, there is no substitute for a full working knowledge of the bondable gold plating process and experience. To prove it, ask R&D for a free bold-able gold sample.