Common Field Failures for Polymer Thick Film Membrane Switches

Membrane switches offer a reliable economical, cosmetically appealing answer for electronic switching applications. They provide a normally open, momentarily closed switch system. Used in a wide variety of applications, they can meet stringent requirements for shock and vibration; water, UV and chemical resistance and longevity. With proper design and choice of materials, membrane switches can provide the performance and durability necessary for the most demanding applications.

membrane keypad

The following discussion covers common failure points that can be mitigated, avoided or eliminated with proper design and materials selection.

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Silver Migration: Silver migration problems will be greatest in applications where moisture, heat and humidity are in prevalent. Silver migration is the ionic movement of silver between two adjacent traces where a voltage potential exists. Over time, silver migration can cause an electric short. Silver migration is commonly enabled by moisture. There are several methods to mitigate silver migration, including adhesive sealing, full perimeter gasketing and other design barriers. The mechanical configuration, bond to the device enclosure and connecting hardware can all contribute to sealing failures.

Dielectric Cross-over's, Jumpers or Bridges: When OEM requirements call for dense circuit layouts to meet the need for a high number of switches in a limited X-Y dimension, most membrane switch manufacturers use printed cross-overs, jumpers or bridges (all referring to a similar method). In its simplest manifestation, a silver trace is printed first, followed by a printed dielectric material, and finished with another printed silver trace over the dielectric. The dielectric insulating layer is the weak point in this construction. Because of the inherent properties of the material and application methods, a common failure is micro shorts that occur between the two overlapping traces as the silver leaches through the porous dielectric. While there are manufacturing and test methods to help mitigate this issue, the best way to avoid this failure is to avoid cross-over or jumpers altogether. Some membrane switch manufacturers can provide a double sided circuit where a layer of polyester provides insulation and completely eliminates the risk.

Graphic Overlay: The graphic overlay gets a surprising amount of stress in areas where switch contacts are made. Improper selection of the overlay materials can result in cracking and delaminating graphic layers. This cracking can occur in just a few hundred actuations. While initially manifesting itself as a "cosmetic issue" it can progress to the point of functionally failing non-tactile switch closure.

Collapsed Switches: A non-tactile membrane switch connection is made when a finger or probe pushes a shorting element against a nested finger circuit layer to momentarily close the circuit. The contact surfaces are often only separated by a few thousandths of an inch. Variations in temperature, air pressure or even multiple actuations can cause air to escape from the contact chambers which create a pressure differential leading to collapsed switches. A proper membrane switch design will mitigate this issue. Sealing, tactile domes and venting are a few methods commonly used to prevent collapsed switches.

Plastic domes can collapse due to extreme operating temperatures (high and low) and should be limited to applications in controlled environments. The actuation force is also influenced by temperature, with high temps producing very low actuation force and operator feedback, and low temperature actuation forces producing high actuation force and a "crunch" sound and feeling to the operator. Design parameters for plastic domes are critical to their performance and durability. There is no recovery for a collapsed plastic dome once its geometry is compromised.

Common causes of collapsed metal domes include, but are not limited to the following. If a metal dome is supported by a plastic injection molded enclosure surface, there can be variances in flatness or even pockets or cups that do not support one or more legs of the metal dome. Another manifestation of an irregular base surface is if the metal dome is placed into the membrane switch assembly with one or more legs supported on top of the adhesive spacer layer. Each of these cases cause premature dome failures. Furthermore, care should be taken to choose a metal dome manufacturer with a history of consistently acceptable production processes and choice of materials.

Cracked traces: It is very common for the dielectric substrate to bend, fold or be twisted as part of the assembly process. Polymer thick film traces printed on the substrate are subject to cracking when repeatedly bent or sharply creased on the outside radius. Proper design with good material selection and trace lay out can eliminate most causes of cracked traces. Manufacturers of double sided polymer circuitry have the best solution by placing circuit tail traces on the back or inside radius of the circuit tail and eliminating cross-overs, bridges, jumpers, etc. which can be fraught with cracked traces from printed stack ups over dielectric and conductive circuit traces.

Poor Tactile Life: Tactile feedback switches, if properly designed and manufactured can last millions of cycles. But improper design, material selection or poor manufacturing methods can create switches that lose their tactile response in a short period of time. Contact pad damage, corrosion, poor ventilation, material incompatibility and poor fabrication methods are some of the common causes for poor tactile life.

Creating a robust membrane switch requires an experienced manufacturer who has expertise in materials, reliability, design and robust manufacturing methods. Each and every new custom design should be thoroughly tested before launching into production. Make sure that the membrane switch manufacturer you select has the experience and technology to handle your requirements.

Common Field Failures for Polymer Thick Film Membrane Switches

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