A customer sent us a sample back in January. They had opened a box of metal domes that had been sitting in their warehouse for 8 months. The domes were supposed to be silver-colored, but when they opened the box, the domes had turned a dull brown, and some of them had green spots. The contact resistance had gone from 20 milliohms to over 500 milliohms. The buttons were intermittent.
The customer blamed us. They said our domes were defective. We asked them what the storage conditions were. They said the warehouse was in Shanghai, and the temperature was between 5C and 35C. The humidity was not controlled - it ranged from 30% in winter to 85% in summer. The domes were in the original plastic bags, but the bags were not sealed - the operator had just folded the top over.
The domes were not defective. They had oxidized in storage. Stainless steel is called "stainless" because it resists rust, but it is not rust-proof. In high humidity, with no barrier between the steel and the air, the chromium oxide layer on the surface breaks down, and the iron underneath starts to corrode.
Here is what we tell every customer who stores domes for more than 3 months.
Why stainless steel domes oxidize
Stainless steel has a thin layer of chromium oxide on the surface, which protects the iron from rusting. This layer forms automatically when the steel is exposed to oxygen. It is very thin - about 2-3 nanometers - but it is enough to prevent rust in normal conditions.
The problem is that this layer can break down in high humidity, especially when there is salt in the air. In a coastal city like Shanghai, the air has salt particles from the ocean. The salt absorbs moisture, and the moisture sits on the dome surface, dissolving the chromium oxide. Once the oxide layer is gone, the iron starts to corrode, forming iron oxide (rust) and iron chloride (the green spots).
Dome sheets that oxidized after 8 months in uncontrolled warehouse conditions.
What we do to prevent it
We take three steps to prevent oxidation during storage.
1. Vacuum-sealed packaging. Every dome sheet goes into a metalized PET bag, which is vacuum-sealed. The metalized layer blocks oxygen and moisture, so the domes stay dry and isolated from the air. We also put a desiccant pack in the bag, which absorbs any moisture that gets trapped during sealing.
2. Nitrogen purge. Before we seal the bag, we flush it with nitrogen gas. Nitrogen is an inert gas, so it does not react with the steel. The nitrogen displaces the oxygen and moisture inside the bag, so there is nothing left to cause corrosion. This is the same process used for semiconductor packaging.
3. Humidity indicator card. We put a humidity indicator card inside every bag. The card has dots that change color from blue to pink when the humidity inside the bag goes above 30%. If the customer opens the bag and sees pink dots, they know the seal has been broken, and they should use the domes within 30 days.
Four-legged stainless steel domes in vacuum-sealed metalized bags with desiccant.
What the customer should do
Even with our packaging, the customer has a responsibility to store the domes correctly.
Keep the bags sealed. Do not open a bag until you are ready to use the domes. Once the bag is open, the nitrogen is gone, and the domes are exposed to the air. We recommend using up an open bag within 30 days, and keeping the rest sealed.
Store in a dry environment. The warehouse temperature should be between 10C and 30C, and the humidity should be below 60%. If the warehouse is in a coastal area, or if the humidity is above 70%, the domes will start to oxidize even in the sealed bags, because the bag is not 100% impermeable. We recommend using a dehumidifier in the storage area.
Use FIFO. First in, first out. Do not let the domes sit in the warehouse for more than 12 months. Even with the best packaging, the seal will slowly degrade, and moisture will get in over time. We recommend using up the oldest stock first, and ordering only what you can use in 6 months.
What if the domes already oxidized?
If you find oxidized domes in your warehouse, do not try to clean them. The oxide layer is not just on the surface - it has penetrated into the steel, and cleaning it will damage the dome geometry, changing the actuation force. The domes are scrap, and you should order new ones.
If the oxidation is mild - just a slight discoloration, no green spots - you can test a sample of the domes to see if they still work. Measure the contact resistance. If it is below 50 milliohms, the domes are probably still usable. If it is above 100 milliohms, they are not.
If you need domes that will sit in storage for a long time, we recommend ordering gold-plated domes. Gold does not oxidize, so the domes will stay good for years, even in high humidity. The gold plating costs about 30% more than the standard nickel plating, but it is worth it if you need long-term storage.
If you have questions about storage conditions for your dome sheet arrays, silicone buttons, or FPC assemblies, talk to our engineering team. We can recommend the right packaging and storage conditions for your specific application.
- John Smith, Senior Application Engineer, Shanyo
