ENEPIG And Wire Bondable Pad Finishes

Some boards have to do two jobs at once. A board that carries a module, a bare die, or a hybrid circuit has pads that are soldered and pads that are wire bonded, and the same surface finish has to serve both. The solder joint needs a surface that wets and that does not contaminate the alloy with too much gold, while the bond needs a surface that forms a reliable intermetallic with the wire and that stays clean through storage.

This article explains why a standard electroless nickel immersion gold finish struggles with bonding, what the palladium layer in an ENEPIG finish changes, and how the finish is specified, controlled, and verified.

Why A Standard ENIG Finish Struggles

An electroless nickel immersion gold finish is a thin gold layer over a nickel layer that was deposited without current. The gold is applied by an immersion reaction that consumes the nickel beneath it, and if the reaction runs too aggressively the nickel surface becomes porous and the phosphorus content at the interface rises. That condition is known as black pad, and it shows up as a dark, brittle layer that does not bond well either to solder or to a wire.

The gold thickness is the second problem. A thin gold layer is not fully continuous, and the nickel beneath it can oxidise through the gaps, which is a poor starting point for a bond. A thick gold layer would solve the continuity, but it introduces the opposite problem in soldering: gold dissolves into molten solder and a high gold content in the joint produces a brittle intermetallic. The finish therefore has to satisfy two requirements that pull in different directions.

What The Palladium Layer Changes

Electroless nickel electroless palladium immersion gold adds a palladium layer between the nickel and the gold. The palladium is deposited by its own reaction rather than by consuming the nickel, so the attack on the nickel surface that causes black pad does not occur. The palladium also acts as a diffusion barrier, which allows the gold to be thinner while remaining continuous, and it protects the nickel from oxidation.

The result is a surface that is suitable for gold wire bonding and for aluminium wire bonding as well, since the thin gold over palladium limits the amount of gold that can react with the aluminium wire. It is also a good solderable surface, because the gold is thin enough to dissolve without embrittling the joint, and it presents a very flat pad, which is what a fine pitch solder joint and a bond pad both need. The cost is an additional plating step and tighter process control.

The Other Options

Where bonding is the only requirement, an electrolytic soft gold over nickel gives the thickest and most robust bond surface, at the cost of a plating bus that has to be routed to every pad and then removed. Where soldering is the only requirement, an immersion silver, an immersion tin, or an organic finish is cheaper and perfectly adequate. The mixed requirement is what makes the choice difficult, and it is the reason ENEPIG is specified.

An immersion tin finish cannot be wire bonded, and an organic finish is removed by the bonding process, so neither can serve a mixed board. An immersion silver finish can be bonded with some processes but is sensitive to tarnish and to the environment, and it is often avoided where a bond has to be reliable over a long storage period.

Plating Sequence And Control

The sequence is a nickel layer of three to six micrometres with a controlled phosphorus content, typically in the mid range, followed by a palladium layer of a few hundredths to a tenth of a micrometre, and then a gold layer of similar thickness. Each bath has its own control parameters, and the palladium layer is the one that has to be continuous: a gap in it exposes the nickel to the gold bath and reintroduces the attack that the palladium was added to prevent.

Thickness is measured by X-ray fluorescence on the pad surface and, where the holes matter, on a coupon that includes a plated through hole, because the thickness inside a hole is lower than on the surface. The palladium and the gold are thin enough that the measurement has to be made with a suitable instrument and a defined spot size, and the results are recorded per lot rather than per panel.

Reliability Of The Bond

The bond between a gold wire and an aluminium pad, or between an aluminium wire and a gold surface, forms intermetallic compounds. At elevated temperature the growth of those compounds and the associated voiding, which is a Kirkendall effect, weaken the bond and eventually raise its resistance. The rate depends on the temperature and on the amount of gold available to react, which is why the gold layer is kept thin and why an elevated temperature storage test is part of the qualification.

The solder side has its own limit. Gold that dissolves into a solder joint raises the proportion of gold in the alloy, and above a few percent the joint becomes brittle. A thin gold layer disappears into the joint without reaching that level, while a thick layer does not, which is the second reason the gold is specified thin. Both limits point in the same direction, and the specification is a compromise that has to be met consistently rather than approximately.

Verification And Storage

The finish is verified in three ways. The thickness is measured by fluorescence, the bond strength is measured by a pull test on a bonded wire and by a shear test on a ball bond, and the interface is examined in a microsection after a thermal ageing step. Those tests are performed on a qualification vehicle and repeated when the plating chemistry or the supplier changes.

Storage is the last variable. A bond pad is a small area of exposed metal, and it is sensitive to contamination from handling, from packaging, and from the environment. Boards are stored sealed with a desiccant, handled with gloves at the bond area, and used within the period the finish allows, because a bond that is made onto an oxidised or contaminated pad fails at a fraction of the expected strength. The plating processes involved are described under electroplating additives, the pad geometry under pad design standards, and the fabrication sequence under PCB design and fabrication.

FAQ

Why is palladium used instead of a thicker gold layer? Because the palladium is deposited without attacking the nickel, which prevents black pad, and because it lets a thin gold layer remain continuous. Thicker gold would create a brittle solder joint.

Can an ENIG board be wire bonded? It can, but the process window is narrow, because the thin gold is not always continuous and the nickel beneath it may have been attacked. For a reliable bond, ENEPIG is the usual choice.

How thick should the gold be? Thin, typically a few hundredths of a micrometre, so that it dissolves into the solder without embrittling the joint and so that the gold available to react with an aluminium wire is limited.