A customer emailed us last month with a strange failure. They had pulled 50,000 loose metal domes from a warehouse shelf in Malaysia, stock that had been sitting there for 18 months. When they assembled the domes onto a PCB, 12% of them had no tactile click at all. The domes were flat. Not bent, not corroded, just permanently flattened, as if they had been pressed 100,000 times already.
We traced the problem to storage conditions. The warehouse had no climate control. The temperature ranged from 28C to 38C, and the humidity hit 90% during monsoon season. The domes were in a sealed plastic bag, but the bag had been opened once, 12 months earlier, and not resealed properly. The combination of heat, humidity, and time had caused the SUS301 stainless steel to relax. A metal dome works because the steel is spring-tempered, holding a curved shape that snaps through when you press it. After 18 months at 90% humidity and 35C average, the spring temper relaxed, and the dome stayed flat.
This is the most common storage failure we see. It is not corrosion, and it is not physical damage. It is spring relaxation, and it happens slowly, invisibly, and only when the stock sits in the wrong environment.
What we recommend
We ship all our dome sheet arrays in anti-static vacuum bags with a desiccant packet. The vacuum seal keeps humidity out, and the desiccant absorbs whatever moisture was trapped inside the bag. As long as the bag is unopened, the domes are good for 24 months from the date of manufacture.
Once the bag is opened, the clock starts running faster. We recommend using the opened stock within 6 months, and storing it at 23C ±5C, 40-60% RH, away from direct sunlight. If you cannot meet those conditions, reseal the bag with a heat sealer and add a fresh desiccant packet.
Vacuum-packed dome sheet arrays stored in a climate-controlled warehouse.
What happens when storage goes wrong
We have seen three storage failure modes over the years:
Spring relaxation - the dome loses its snap. This is what happened to the Malaysian customer. It happens when the dome sits at high temperature (above 30C) for months, especially with high humidity. The SUS301 stainless steel loses its spring temper, and the dome stays flat after the first press. You can test this with a force gauge: if the actuation force is 30% below spec, the dome has relaxed.
Contact corrosion - the gold or silver plating on the contact point tarnishes. This shows up as a dark spot on the top of the dome. The tarnish increases contact resistance from 20 milliohms to over 100 milliohms, which causes intermittent signals. We test this with a four-point probe after storage.
PSA adhesive failure - the pressure-sensitive adhesive on a dome sheet loses its tack. The dome sheet peels off the PCB during reflow. This happens when the adhesive is exposed to high humidity for months. The PSA absorbs moisture, and the bond strength drops by 50%.
How to check old stock before assembly
If you are about to assemble a batch of domes that have been in storage for more than 12 months, do these three checks before you run the line.
First, visual inspection. Pull 100 domes at random and look at them under 10x magnification. Look for discoloration on the contact point, rust spots, or flat domes that should be curved. If more than 2% show visible issues, quarantine the whole lot.
Second, force test. Use a force gauge to measure actuation force on 20 domes. If the average force is within ±15% of the spec, the domes are good. If it is more than 20% off, the spring has relaxed. Do not assemble them.
Third, contact resistance test. Press each dome 100 times on a test fixture and measure the contact resistance. If it stays below 50 milliohms consistently, the plating is intact. If it jumps around, the contact point is tarnished.
Four-legged stainless steel domes, inspected and tested before assembly.
What about silicone buttons and membrane switches?
The same rules apply to silicone buttons and membrane switches, but with longer shelf lives. Silicone rubber is more forgiving than spring steel. A silicone button stored at 25C will last 36 months without compression set. The adhesive on a membrane switch is the weak point, and it should be used within 12 months of opening the bag.
FPC assemblies have the same storage concerns. The polyimide film is stable, but the copper traces can oxidize if exposed to humidity for too long. We ship FPCs in dry-pack bags, and we recommend baking them at 60C for 4 hours before reflow if the bag has been open for more than 3 months.
If you are ordering custom metal domes or dome sheets, tell us your storage conditions and your monthly usage rate. We will recommend the right pack size so you do not have a half-opened bag sitting on the shelf for a year. Most of our customers order 2-4 weeks of stock at a time, which is the sweet spot between too-frequent reordering and expired stock.
- John Smith, Senior Application Engineer, Shanyo
