Wire Bondable Soft Gold PCB: Finish Selection

Why a Bondable Finish Is Different

A solderable finish has to survive reflow and form a joint with molten solder. A wire bondable finish has to do something harder: it has to accept an ultrasonic or thermosonic weld to a wire a few tens of micrometres across, and hold that weld reliably for the life of the product without any solder involved.

The finish that achieves this is a thin layer of pure gold over a nickel barrier, and the requirement is not simply the presence of gold but its purity, its thickness, its surface roughness and its freedom from contamination. The same board may also need solderable areas for the other components, and reconciling the two requirements is what makes a wire bondable board a specialist product.

Soft Gold Against Hard Gold

The two finishes are often confused because both are gold.

Soft gold is electroplated or immersion deposited pure gold, usually with a purity above 99.9 percent, over a nickel layer. It is soft, ductile and welds to a bonding wire, and it is used on bond pads and on surfaces that must be bonded. It is not intended to survive repeated wiping or mating, because being soft it wears quickly.

Hard gold is a gold alloy containing cobalt or nickel to increase hardness, typically to a Knoop hardness several times that of soft gold. It is used on edge fingers, on card connectors and on switch contacts, where the requirement is wear resistance over thousands of mating cycles. It bonds poorly or not at all, and it must never be specified on a bonding pad.

Where a board needs both, the two finishes are applied selectively: hard gold on the connector fingers and soft gold on the bond pads, using a selective plating process or a mask. Specifying one where the other is needed produces a product that either fails immediately on the bonder or wears out in the field. Our notes on PCB manufacturing describe how the selective finishes are applied.

The Layer Stack

Three layers do three jobs, and each has a specification.

• Copper. The conductor. Its surface is prepared before plating, and a roughness that is too high produces a rough gold surface that bonds inconsistently.
• Nickel. A barrier, typically two to six micrometres, that stops the copper from diffusing into the gold. Without it the gold becomes contaminated with copper at the surface and the bond fails. The nickel also provides a hard underlayer that supports the bond.
• Gold. The bonding surface. Its thickness is chosen from the bonding process: aluminium wire bonding generally uses a thinner gold, of the order of 0.05 to 0.1 micrometres in a conventional electroless nickel immersion gold process, while gold ball bonding or thermosonic bonding uses a thicker deposit, typically 0.3 to 1.0 micrometres or more, because the gold has to survive the bonding process itself.

Thickness alone is not enough. The bondability depends on the grain structure, the purity and the surface. A plated gold that is thicker but contaminated will bond less reliably than a thinner, cleaner deposit.

Bonding Processes

Aluminium wedge bonding. An aluminium or aluminium alloy wire is pressed onto the gold and vibrated ultrasonically. The weld forms at room temperature. It is the most common process in power electronics and in module assembly, and it works well on electroless nickel immersion gold with a thin gold layer, because the bond is effectively formed by the aluminium displacing the thin gold and reaching the nickel beneath.

Gold ball bonding. A gold wire is melted into a ball and bonded thermosonically, with heat and ultrasonic energy. It requires a thicker gold surface, because the process is sensitive to the underlying metal and to any contamination, and it is used where the pitch is fine and the wire count is high.

Thermosonic and thermocompression. Variants of the same idea, used where the joint has to be very reliable or where the pitch is extremely fine.

The process choice therefore sets the finish requirement, and the finish specification should name the bonding process rather than only the gold thickness. A fabricator cannot choose the correct deposit without knowing which of these is being used.

Surface Control and Contamination

A bond pad is a surface measurement, and the surface is affected by everything that touches the board.

Mask and plating chemistry. Residual organics from the mask, the plating bath or the resist removal show up as a carbon layer on the gold and prevent a reliable weld. The cleaning sequence after plating matters as much as the plating itself.

Handling. Bare fingers deposit oils. A bondable board is handled with gloves and packed with an interleaving that does not shed.

Storage. Gold does not oxidise, which is why it is used, but the surface still attracts organic contamination from the air and from packaging. A shelf life is defined, and the boards are stored in a controlled environment and in a sealed bag with a desiccant.

Plasma cleaning. Where bondability is critical, the board is plasma cleaned immediately before bonding to remove the organic layer. That step is part of the assembly process rather than the fabrication process, and it is scheduled into the flow.

The verification is a bond pull test or a bond shear test on a sample, performed with the same wire and the same machine settings as production. A finish that passes a visual inspection and fails the pull test is the classic failure of a bondable board, and the pull test is the only reliable check. Our notes on quality management describe the qualification records, and our notes on PCBA testing cover the test methods.

Solderable Areas on the Same Board

Most bondable boards also carry components that are soldered. Two approaches are used. A selective finish applies electroless nickel immersion gold to the whole board and then adds a thicker soft gold only on the bond pads, which keeps the solderable areas usable. Or the board uses organic solderability preservative or another finish for the soldered areas and selective soft gold for the bond pads, which requires two masking and plating operations.

Where the whole board is covered with soft gold, the solderable areas still work, because gold dissolves into solder, but the gold thickness must be limited: more than about one percent gold by weight in the joint makes it brittle. On a thin electroless gold this is a non-issue; on a selectively plated thick soft gold it is a real risk, and the soldered areas are kept away from the thick deposit.

Design Rules

• Keep the bond pads a uniform size and pitch. The bonding machine is programmed per pad, and uniformity reduces the setup and the risk.
• Do not place a via in a bond pad. The filled and plated via produces a dished or uneven surface that the bond cannot tolerate.
• Leave room around the pad. The capillary needs clearance for the wire and the ball, and a pad surrounded by tall components cannot be bonded.
• Keep the mask away from the pad edge. A mask dam that overlaps the gold surface contaminates it and reduces the bondable area.
• Plan the cleaning and the storage. The board is a controlled surface from the moment it leaves the plating line, and the handling instructions are part of the specification. Our notes on PCB design and layout cover the layout rules, and our notes on PCB assembly cover what happens at the bonder.

What It Costs

A bondable finish adds a plating step and a set of controls, so the board is more expensive than one with an ordinary finish. Electroless nickel immersion gold with a controlled thickness is a moderate premium. A selectively plated soft gold of significant thickness over nickel is a larger one, and a board with both hard gold fingers and thick soft gold bond pads is more expensive again, because it needs two selective plating operations and two sets of masks.

The premium is small compared with the cost of a failed bond, which is usually discovered at the assembly stage when a die is already attached to the board. That is why the qualification is done on a real board with a real wire pull test before the design is released.

FAQ

Can hard gold be used on a bond pad? No. Its hardness and its cobalt or nickel content make it unreliable or impossible to bond, and it must never be specified for a bonding surface.

How thick should soft gold be? It depends on the bonding process: a thin deposit for aluminium wedge bonding and a thicker one, of the order of 0.3 to 1.0 micrometres or more, for gold ball bonding.

Why is nickel plated under the gold? To stop the copper diffusing into the gold, which would contaminate the surface and prevent a reliable weld.

How is bondability verified? With a bond pull or shear test on sample boards, using the production wire and machine settings, before the design is released.

Conclusion

A wire bondable soft gold finish is a surface specification rather than a plate thickness. Pure gold over a nickel barrier, deposited with the right grain and cleanliness for the bonding process, handled and stored as a controlled surface, and verified with a pull test. Keep hard gold for the connectors, keep the bond pads uniform and via free, and tell the fabricator which bonding process is being used so that it can deposit the right thing.