A customer sent us a video last year. They had assembled 10,000 metal domes onto a new PCB, and about 8% of the buttons felt "mushy" - no tactile click, just a flat press. The domes themselves were fine. We tested them on a known-good test fixture, and every dome snapped through with the correct 160g force. The problem was not the dome. It was the PCB pad.
We asked for three failed PCBs and three good ones. On inspection under a 20x microscope, the failed boards had a solder mask misalignment problem: the green solder mask was offset by 0.15mm, covering part of the contact pad where the dome feet sit. The dome was not sitting on the bare copper pad, it was sitting on the solder mask, which is 0.02mm thick and slippery. The dome could not snap through because it was sliding on the mask instead of gripping the pad.
This is the most common pad defect we see. But it is not the only one. Here are the five PCB pad problems that turn good domes into field failures.
1. Solder mask misalignment
Solder mask misalignment happens when the PCB fab prints the solder mask layer out of register with the copper pad. The mask should cover the area around the pad, not the pad itself. When it shifts by more than 0.1mm, it covers part of the pad, and the dome loses grip.
The fix is a tighter fabrication tolerance. We specify a ±0.05mm solder mask registration tolerance on all our dome sheet array PCB drawings. We also specify a "solder mask defined" (SMD) pad opening that is 0.2mm larger than the copper pad on all sides, so even if the mask shifts 0.1mm, the pad is still fully exposed.
PCB pad inspection under magnification, checking solder mask alignment.
2. Uneven pad plating
Some PCB fabs plate the contact pad with ENIG (electroless nickel immersion gold), but the plating thickness varies across the pad. The center of the pad might have 0.05um of gold, while the edge has 0.1um. When the dome sits on the pad, the uneven gold layer changes the contact resistance, and the dome feels inconsistent from button to button.
We specify a gold thickness of 0.05um ±0.01um on all contact pads, and we require the PCB fab to provide a plating thickness report for each lot. We also test contact resistance on every incoming PCB batch, measuring 50 pads per lot. If the resistance varies more than 10 milliohms across the batch, we reject the lot.
3. Pad burrs and edge roughness
When the PCB is etched, the copper pad can develop a burr along the edge. The burr is a thin sliver of copper that sticks up 0.01-0.02mm above the pad surface. When the dome sits on the pad, one of its feet rests on the burr instead of the flat pad. The dome tilts slightly, and the snap-through force is uneven.
We check for pad burrs with a profilometer on the first article of each new PCB design. The specification is: pad edge roughness less than 0.005mm. If the burr is taller than that, the PCB fab has to re-etch or re-plate the pads.
4. Pad contamination
Flux residue, oil from the assembly line, or finger grease on the pad changes the contact resistance. A clean pad measures about 18 milliohms. A pad with flux residue can measure 50-100 milliohms, which causes intermittent signals or no signal at all.
The fix is a cleaning step after reflow. We specify an aqueous clean using a neutral flux remover, followed by a DI water rinse and a dry bake at 80C for 30 minutes. We also recommend wearing finger cots when handling the PCB, because skin oil on the pad is the hardest contamination to remove.
Four-legged stainless steel domes properly seated on clean PCB pads.
5. Wrong pad size
Every dome size has a recommended pad size. A 6mm round dome needs a 5.5mm pad. A 12mm four-legged dome needs a 10mm x 10mm pad. If the pad is too small, the dome feet hang off the edge and the snap-through is weak. If the pad is too large, the dome has too much contact area and the snap-through feels mushy.
We provide a recommended pad layout drawing with every dome order. The drawing specifies the pad diameter, the pad spacing, the solder mask opening, and the plating finish. If you are designing a new FPC or rigid PCB for silicone button applications, send us your design file and we will review the pad layout for free before you send it to fabrication.
The bottom line: a metal dome is a precision part, but it only works correctly on a precision pad. We have seen customers save money by choosing the cheapest PCB fab, only to lose more money in assembly rework and field returns. Specifying the right pad tolerances upfront is cheaper than fixing 8% failure rate on the assembly line.
- John Smith, Senior Application Engineer, Shanyo
