The aerospace customer called us in June. They needed a metal dome that would work in a satellite ground station, where the temperature swings from -55C in winter to +85C in summer, and the humidity can hit 100% during monsoon season. Their previous supplier used a standard nickel-plated dome, and after 18 months in the field, the contact resistance had drifted from 20 milliohms to over 200 milliohms. The buttons were becoming intermittent.
We tested the failed domes under a microscope. The nickel plating had corroded at the contact point, forming a green patina. The contact resistance was no longer stable. We told the customer they needed a gold-plated contact, not nickel. They agreed, but they wanted us to plate the gold in-house, not send the domes out to a third-party plater. A third-party plater would add 7 days to the lead time, and they could not guarantee the gold thickness to ±0.01um.
We installed our own dedicated gold plating line in August. The first production run shipped last week.
Why gold, and why in-house
Gold is the best contact material for high-reliability applications because it does not oxidize. Nickel oxidizes over time, especially in humid environments. Silver tarnishes. Copper grows oxide layers. Gold stays clean, which means the contact resistance stays low and stable, for years.
The problem with gold is thickness. Too thin (less than 0.03um), and the gold wears through after a few thousand actuations. Too thick (more than 0.5um), and the dome becomes soft and the snap feel changes. The sweet spot for most applications is 0.05um to 0.1um, which is called "gold flash."
We used to send our domes out for gold plating. The problem was consistency. One batch would come back at 0.04um, the next at 0.08um. The customer could not rely on the contact resistance from batch to batch. With our own line, we control the current, the bath temperature, and the plating time, to ±0.01um tolerance.
Gold-plated contact points on stainless steel metal domes for high-reliability applications.
What the new plating line does
The new line is a vertical barrel plating system, which is the gentlest way to plate small parts. Barrel plating tumbles the domes in a rotating barrel with the plating solution, which ensures every dome gets a uniform coating. Rack plating would work too, but it is slower and more expensive for high volumes.
The line has six stages:
Stage 1: Cleaning - ultrasonic cleaning in alkaline solution, to remove any oil or stamping residue from the dome.
Stage 2: Activation - dilute acid dip, to remove any oxide that formed during cleaning.
Stage 3: Nickel underplate - 1-2um of nickel, to provide a barrier between the stainless steel and the gold. Without the nickel, the iron from the stainless steel would diffuse into the gold, causing corrosion.
Stage 4: Gold flash - 0.05um to 0.3um of gold, depending on the customer spec. We use a hard gold alloy (cobalt-hardened), which is more wear-resistant than pure soft gold.
Stage 5: Rinse - three rinses in deionized water, to remove any residual plating solution.
Stage 6: Drying - hot air blow-off, then 30 minutes in a vacuum dryer at 60C.
Four-legged stainless steel domes with gold-plated contact points ready for assembly.
How we verify the plating
Every batch goes through two tests before it ships.
X-ray fluorescence (XRF) thickness measurement - we measure the gold thickness at five points on each of 10 domes from the batch. The thickness must be within ±0.01um of the target. If any point is out of spec, the whole batch goes back through the plating line.
Contact resistance measurement - we actuate each dome 10 times, measuring the resistance on the 10th actuation. The resistance must be below 30 milliohms. If it is higher, the plating is either too thin or there is contamination on the contact.
We also run a salt spray test every quarter, on a sample of domes from the production line. The sample goes through 500 hours of neutral salt spray, and we re-measure the contact resistance after. The resistance must not drift by more than 10 milliohms.
What this means for our customers
If you need gold-plated metal domes for an aerospace, medical, or industrial application, we can now produce them in-house, with a 7-day lead time, at ±0.01um thickness tolerance. We can also plate dome sheet arrays after the domes are mounted on the film, which eliminates the need for the customer to plate the contacts separately.
We also support silicone buttons with gold-plated domes, and membrane switch panels with gold-plated contact pads. If you have a high-reliability application that needs stable contact resistance for years, talk to our engineering team. We can recommend the right gold thickness for your environment and your cycle count.
- John Smith, Senior Application Engineer, Shanyo
