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Thick Film backgroundThick Film technology for the production of electronic circuits has its origins from the fine china industry dating back to the late 1800s. The first conductive inks used to make electronic circuitry were modeled after gold inks used to decorate china. The first silver conductive inks for electronic applications were developed in the late 1930s. The development of resistor inks in the early 1960s enabled production of screen-printed microcircuit networks. Hybrid circuits manufactured on ceramic substrates continue to be produced today in high volume.
The concept of screen printing an electronic circuit onto a substrate using conductor, resistor and dielectric inks led into polymer thick film technology. Unlike the standard hybrid circuit compositions which must be fired at temperatures up to 1,000 degrees C, polymer thick film materials process at much lower temperatures, typically below 130 degrees C. Membrane touch switches use polymer thick film conductors and dielectrics screen printed onto polyester substrates.
The use of membrane touch switches in the appliance industry has grown significantly. Today, most all microwave ovens use membrane switches for the cooking controls. Membrane switches are also common in dishwashers and ranges and have found expanded uses in refrigerators, washers, dryers and small appliances.
Inside PTF conductorsPolymer thick film conductors are formulated with a few critical components, and are screen-printed onto a polyester or other plastic substrate. After curing, the PTF network of circuit traces acts as wire to carry electric signals to power appliance controls. PTF conductor technology is popular because it is highly conductive, low cost, low profile and can be easily used in large-scale manufacturing.
The three basic components of PTF materials include a metal powder, an organic polymer and a solvent. The function of the metal powder is to provide adequate conductivity. The size and shape of the particles play a critical role in determining the conductivity of the screen-printed conductor traces. Due to its relatively low cost and high conductivity, silver is the most common choice as the metal powder. The organic polymer determines many properties of the conductor such as adhesion, abrasion resistance, compatibility with other materials and circuit reliability. The solvent is used to solubilize the polymer and determines how long an ink can remain on a screen before drying out.
While the high conductivity and relatively low cost of silver conductors are strong advantages, one problem that has historically plagued silver conductors is that of silver migration, which impacts reliability.
Silver migration issueReliability continues to be a key issue in the membrane touch switch (MTS) industry and, in particular, the appliance industry. Switches are exposed to relatively harsh environments, in some cases both high temperatures and high humidity. The reliability of membrane switches has been improved over the years with advancements in materials, switch design and construction. Although switches can fail for a variety of reasons, the high heat/humidity conditions encountered in appliance applications, exposes a key weakness of silver conductors known commonly as silver migration.
In the presence of moisture, and under an electrical bias, silver will migrate, or form a dendrite, from one silver conductor trace to another, thus causing a short to occur.
Silver migration can cause rapid failure of switch circuits or it can be a very slow process. Direct exposure of silver traces to water from condensation, for example, will cause a silver migration failure in a matter of minutes. A membrane switch used in a high humidity environment with an electrical voltage applied may develop a short related to silver migration over a period of years. These slowly developing defects can be very frustrating to appliance and switch manufacturers since there is no way to test each switch to prevent failure.
"DuPont challenged its scientists to overcome this issue, and as a result, a new silver-migration-resistant conductor technology has been introduced, which dramatically enhances membrane switch reliability," says Brad Shickling, product manger for DuPont Microcircuit Materials.
New conductor technologyThe two new screen-printable, silver-based polymer PTF conductive compositions - APP11 and APP21 - utilize a unique polymer chemistry, which dramatically improves resistance to silver migration. The polymer has a higher Tg (second-order glass transition temperature) than traditional polymers for this use, and the cross-sectional densities as determined by scanning electron microscopy indicate much greater densities in the microstructure of APP11 and APP21. Shickling notes, "In addition to the improved migration resistance, other properties important for membrane switch manufacturing are excellent, including good printability, high abrasion resistance and good flexibility/actuation."
APP11, APP21 and more traditional type PTF materials were tested at an independent laboratory, LabTech and Advanced Technologies Testing Laboratory, using an accelerated aging test by exposing test parts to 85?C/85 percent relative humidity with a 10 V bias. Under these conditions, the new compositions showed three times to five times improvement in the average time to failure.