A customer sent us a Gerber file last year for a 12-button keypad. The buttons were arranged in a 3x4 grid, with 12mm domes, and the center-to-center spacing was 14mm. That is 2mm gap between the edges of the domes. We told them this would not work, and asked them to increase the spacing to at least 18mm center-to-center. They pushed back, saying they needed the smaller pitch to fit the keypad in a handheld enclosure. We sent them a sample: 12mm domes on a 14mm pitch, assembled on a test PCB.
When they pressed the domes, two adjacent buttons triggered at the same time. The reason: when a 12mm dome snaps, it deflects outward by about 0.5mm on each side. With only 2mm between dome edges, the deflection from one dome was touching the adjacent dome, causing a parasitic trigger. The customer redesigned the PCB to 18mm pitch, and the false triggers went away.
This is the most common layout mistake we see in dome sheet array design. The pitch is too tight, and adjacent buttons crosstalk. Here are the layout rules we follow on every design.
Minimum center-to-center spacing
The spacing between domes depends on the dome size and the dome type. Here are our recommended minimums:
6mm round dome - minimum 9mm center-to-center (3mm gap between edges)
8mm round dome - minimum 12mm center-to-center (4mm gap)
12mm four-legged dome - minimum 18mm center-to-center (6mm gap)
20mm cross dome - minimum 28mm center-to-center (8mm gap)
These are minimums. If you have a thin silicone boot (less than 1mm), you can go tighter. If you have a thick boot (2mm or more), you need to add 2-3mm extra, because the boot spreads the press force across a wider area.
Typical dome sheet array layout with recommended spacing between buttons.
Spacing from domes to other features
The dome is not the only thing on the PCB. You need to leave room for vias, traces, standoffs, and the enclosure wall. Here are the minimum clearances we specify:
From dome edge to via - 1.5mm minimum. A via too close to the dome edge can cause the dome foot to sit on the via pad, which changes the contact resistance and the snap feel. If you must place a via close to the dome, use a blind via with a flat cap, not a through-hole via.
From dome edge to trace - 0.5mm minimum. A trace running under the dome foot can cause the dome to sit unevenly, leading to a mushy feel. We recommend routing traces around the dome pad, not through it.
From dome edge to enclosure wall - 2mm minimum. The enclosure wall needs to align with the silicone button boot. If the dome is too close to the wall, the boot will not seat properly, and the button will feel stiff.
Array size and sheet shape
A metal dome sheet array is cut from a polyimide film, typically 0.125mm thick. The film has a PSA adhesive on the back, which sticks to the PCB. The sheet can be any shape, but there are practical limits.
The maximum sheet size we can produce is 200mm x 200mm. Beyond that, the film tends to curl during shipping, and the adhesive loses its tack. The minimum sheet size is 10mm x 10mm, for a single small dome.
Four-legged stainless steel domes arranged in a precision die-cut array.
We also recommend leaving a 2mm border around the edge of the array. This border is the "frame" that holds the domes in place during assembly. If the border is too narrow (less than 1mm), the film can tear when you peel the release liner off.
How we help with layout
If you are designing a new keypad, send us your Gerber file or your mechanical drawing. We will review the layout for free, checking the dome spacing, the pad size, the solder mask opening, and the adhesive border. We will tell you if anything needs to change before you send the PCB to fabrication.
We can also provide a recommended layout drawing, showing the exact dome positions, the pad sizes, and the clearances. This drawing becomes part of your design package, and it is referenced during our first-article inspection.
If you are building a membrane switch panel or an FPC button assembly, the same spacing rules apply. The only difference is that on an FPC, the traces are on the flexible substrate itself, so you need to be even more careful about trace routing under the dome.
We have seen customers save thousands of dollars in rework by getting the layout right the first time. A 2mm spacing error on a 10,000-piece production run means 10,000 scrap PCBs. A 30-minute layout review before fabrication is cheaper than a week of rework on the assembly line.
If you have a new keypad design and want a second set of eyes on it, send us your file. We will turn it around in 24 hours, and the review is free.
- John Smith, Senior Application Engineer, Shanyo
