gold coating does not oxidise, its excellent wettability by molten solder does not degrade with time, and a plated finish maintains the flat surface of the copper lands. For these reasons a gold finish fulfills the requirements for boards designed for fine-line printing. The plated film has, however, to meet strict specifications to ensure that the subsequent reflow solder process will be satisfactory.
It used to be thought beneficial to have a comparatively thick (1.3–2µm) deposit of gold. However, with an immersion process, because nickel and gold ions are exchanged, a thick deposit can only be achieved when the gold is sufficiently porous to allow the underlying nickel to sustain the chemical reaction. But porous gold provides less protection for the nickel, which has an adverse impact on assembly. Current immersion gold processes plate 0.8–1.3µm of gold and are self-limiting, with reduced porosity compared with the earlier coatings.
Gold readily dissolves in molten solder and will be present in the reflowed solder joint. Intermetallic gold-tin compounds formed cause joints to be brittle, and the gold film must therefore be thin (<<1µm) to minimise the amount of intermetallics.
However, thin gold plating is porous, and will not protect the underlying copper against oxidation. Oxidised spots at the base of pores are a cause of dewetting, and copper can also diffuse to the gold surface during heat treatments, where it can oxidise and thus impair wettability. To prevent this, an electroless nickel underlayer is first deposited, to act as a barrier to copper diffusion: thickness specifications vary, but are usually in the 2–6µm range.
The ‘immersion gold’ plating process self-limits at around 0.05–0.1µm. Not only is this beneficial from the cost point of view, but this also reduces the possibility of gold embrittlement caused by the formation of a Au4Sn intermetallic phase. [Note that this process is not the same as that used to electroplate ‘gold finger’ edge connections]
