Keypad assemblies designed correctly and made with suitable materials can be highly reliable. That’s not the case if poor design and manufacturing choices are made.
The top factors that impact keypad longevity are:
- Contamination entering the switch cavity
- Contamination inside the switch cavity
- Exceeding switch contact point electrical ratings.
We’ll look into these in greater detail and explain how to avoid problems that can shorten keypad life. But first, let’s gain a foundational understanding of keypad assemblies.
Keypads: The Basics
There are two types of low-profile keypad assemblies.
One is the conventional flexible printed membrane keypad. The switch circuitry of this type of keypad is silver conductive polymer inks screened onto 1.25 mm (5-mil) polyester sheets. A thin adhesive or polyester layer with a hole at each switch location separates opposing polyester circuit layers. When a finger puts pressure on the switch, it deforms the top circuit layer, completing the connection to the bottom circuit layer. A metal snap dome or embossed dome formed in a polyester sheet (sometimes called poly domes) indicates the proper pressure point.
The second type of keypad assembly incorporates a printed circuit board (PCB) with metal snap domes. All the keypad’s circuitry is on the printed circuit board. The snap domes indicate where to apply pressure. They also serve to complete a circuit path to trigger the switch.
Both types of keypads are typically finished with a graphic overlay or other mechanism, such as a plastic or molded rubber key top, to more clearly indicate where to trigger the switch.
Now that we have a foundational understanding of how switches work let’s examine how they can be corrupted and the best ways to extend their lives.
Contamination Entering the Switch Cavity
The first factor that adversely impacts keypad reliability is contamination entering the switch cavity. Contamination can take the form of non-conductive dirt, such as lint, dust, and other particles. It can also be liquid containing suspended or dissolved dirt.
Keypads, no matter which type they are, all contain air inside the switch cavity. When a finger presses on the switch, the air inside the cavity must reduce in volume, making it possible to complete the circuit. That’s why it’s necessary to provide a way for the switch cavity air to escape. Air escape is typically accomplished by cutting narrow air channels in the spacer layer between the silver-printed polyester circuit layers running from the switch cavities and its edges. Vents in PCB keypads are usually simple holes in the printed circuit board under the snap dome.
The issue is that vents not only let out air but also allow contamination into the switch cavity. Pressing the switch pushes out air. When the finger is removed, a partial vacuum is formed in the switch cavity, which sucks in outside air along with dirt particles or liquid. Contamination will prevent the electrical switch from closing correctly because it’s likely to be non-conductive, resulting in a faulty switch.
External contamination can be prevented from entering a switch by designing it correctly and working with an experienced circuit board manufacturer like Tramonto Circuits.
Contamination Inside the Switch Cavity
The second factor that negatively impacts keypad reliability is contamination in the switch cavity. It takes the form of oxide growth on the switch contact surfaces. Circuit performance of common metal oxides except silver can be affected by oxide growth, making them non-conductive, resulting in faulty switches.
Silver-printed polyester membrane switches are, by their nature, immune to oxide growth-related failures. That’s because silver is the only common metal that has a fully conductive oxide. Leveraging silver in circuits is a proven way to ensure good performance over time.
Using gold plating over nickel barrier plating can prevent non-conductive oxide growth on PCBs. An electro-less gold plating process provides two to four micro-inches of thickness. This thickness is necessary because the metal snap dome feet touch the gold surface, which will wear down thinner plate.
Almost all metal switch domes today are manufactured from stainless steel alloy. Many believe stainless steel doesn’t rust. However, that isn’t wholly true. Visible rust is prevented by the formation of chromium oxide (a form of rust) on the surface of stainless steel. A small amount of chromium is found in all stainless steel alloys. The chromium oxide is invisible, sealing the surface and making it impossible for iron particles to form iron oxide. Put simply, chromium oxide makes stainless steel stainless.
As with most metal oxides, chromium oxide is non-conductive, making it unsuitable for switch surfaces. Stainless steel domes are typically plated with nickel, and nickel oxide does a relatively good job of preventing chromium oxide growth. While this is true, gold plating also prevents chromium oxide growth and electrically performs better than nickel. The issue is that standard gold plating processes can cause significant mechanical damage to highly stressed, punched stainless steel snap domes.
Standard gold plating is hydrogen-based. It makes stainless steel brittle, which can negatively impact the life of metal snap domes, sometimes by as much as 90 percent. A knowledgeable circuit manufacturer like Tramonto Circuits can leverage the correct form of gold plating chemistry to prevent this problem.
Clearly, internal contamination of keypad circuits is a complex and common issue. You owe it to yourself to work with an experienced PCB designer and manufacturer to ensure your switches are durable and long-lasting and don’t fail in the devices you use them in.
Exceeding Switch Contact Point Electrical Ratings
A third issue that negatively affects keypad dependability is exceeding the electrical ratings for switch contact surfaces. Low-profile keypads are intended for logic-level circuits. Any excess open circuit voltage or closed circuit current — or reactive components in the switched load, such as inductors or capacitors, will rapidly deteriorate switch surfaces.
Once switch surfaces are damaged in this way, switch performance issues such as intermittent operation, long contact bounce on make or break, and high resistance will occur. Once it happens, it’s irreversible. Of all the issues covered in this piece, exceeding electrical ratings is the most significant.
When a switch controls high voltage, high current, or reactive loads, the loads must be isolated from the keypad’s switch surfaces by solid-state relays or electronic buffering. An experienced circuit designer and manufacturer like Tramonto Circuits can help prevent these issues and possible related device failures.
Keypad Circuit Issues: The Final Word
In short, a device or application is only as good as its keypad. The keypad must last as long as the device’s expected life.
The keypad — and the circuits controlling it — deserve as much design and manufacturing attention as the rest of the devices or applications they control. You owe it to yourself to partner with a top keypad circuit developer like Tramonto Circuits to get the durable circuits you need delivered on time and within budget.