Section: INTERCONNECTS & PACKAGING

Flexible circuits have a wide range of applications, from low-cost consumer products, biomedical, automotive and hand held LAN devices to highly reliable medical, military and aerospace products.

The materials used in flexible printed circuits will determine their performance. For most consumer products (all the way up to many medical, military and aerospace products), polyester or polycarbonates are being used, along with some highly advanced screen-printable conductive and dielectric materials. Through-hole flexible circuits eliminate added thickness and the cost of additional layers of substrate and/or adhesives that may be used to bond them together.

Questions had been raised about the processability and reliability of through-hole circuitry on thin- or thick-film flexible circuitry. But recent advances in polymer-based conductive materials and mating dielectric technologies (both thermal and ultra violet), and the ability to apply substantial process control to their application have elevated the science of screen-printing on these types of materials.

Of course, advances in substrate technology over the past five years have assisted in pulling the entire process together more easily.

The objective of through-hole screen-printing on flexible circuitry is to provide the ability to manufacture a more-compact and cost-effective flexible circuit, while still offering the end user a high-quality, highly reliable product. Following is an example that illustrates the point.

Aztec Industries has been a leading manufacturer of membrane switches, flexible circuits, and graphic overlays since 1966. The company, which also provides a wide range of services, has manufactured momentary open/closed thick-film circuits since 1977. These devices, commonly called membrane switches, are widely used and accepted as a cost-effective, environmentally safe, user-friendly alternative to conventional hard-key, hard-wired switch devices.

In practice, the circuit includes a stack build of a top flexible circuit, which includes a lightning-rod electrostatic discharge (ESD) shield on both top and botton of top circuit. A bottom circuit, which includes a through-hole printing that makes contact with a mechanical-rivet connection to anelectrolumiescent backlight, is on the bottom of this layer.

This process is achieved on thick-film 0.005-inch clear polyester by a number of proprietary preparation steps and registration cut of small through-hole. In the example case, a 0.062-inch through-hole was cut in the top circuit and 0.125-inch in the bottom circuit.

Conductive material is then screen-printed in a process-controlled environment. In this circuit example (Fig. 1) we screen-print two conductive prints on both top and bottom circuits, after special 0.062-inch through-holes are cleanly die cut. Die cut is achieved with the use of a laser-cut steel die that has a tolerance of + - 0.005 inch.

Screening conductive inks requires that special processes and manufacturing controls be used in the application of the first conductive layer down. A conductive layer is applied to the surface of 0.005-inch polyethylene terephtalllate, or PET, a polyester, at a thickness of up to 0.0008-inch mil. The conductive material is screen-printed onto the through-hole area and pushed by mechanical screen-print means, toward the back side of the bottom circuit. In this case, the bottom circuit will be mechanically crimped using a small rivet, eventually used to connect to the electroluminescentlight system.

Flip and mate

After the "face" of the bottom circuit is screen-printed, the bottom circuit is then flipped over and mating screen printing is done on the now face (far side) of bottom circuit. Conductive material flows into the through-hole and makes contact with the through-hole deposit, made from face side of the bottom circuit. This completes the electronic connection from top to bottom (Fig. 2). Conductive material screen-printed here must be very flexible. In this process case, material can be bent creased at a 180 degrees angle many times over without fracturing, making it perfect for this type of application.

The top portion's through-hole conductive pad side of the circuit is screen-printed after the 0.125-inch-diameter through-hole is die-cut into the clearly polyester. With careful mechanical registration, the top circuit pads along with the ESD lightning rod, are screen printed going around the outside of the unit and through the 0.125-inch-diameter through hole. The circuit is then flipped, and the ESD lightning rod on the non-pad side of the top circuit is screen-printed along with the through-hole pad, completing the conductive connection.

Averting penetration

The ESD lightning rod is now in place around the entire top and bottom perimeter of the top circuit area, preventing an electrostatic discharge from penetrating the switch areas or the device itself from front or side. This particular ESD top-circuit shield will be grounded to the supplied customer case, thus completing ground of any unwanted charge.

If through-hole printing had not been used in this device, an additional layer of conductive material would have been needed to shield both the top and bottom sides of the top circuits. It also would have required an adhesive layer to bond an additional conductive layer to the top circuit, adding both material and labor costs, as well as possible unwanted thickness or weight, or both.

In the case of the bottom-circuit through-hole, it enables the user to connect and power a value-added lightning layer directly through the circuit without tying up an additional circuit layer or connect to a light source, and without adding unneeded cost, thickness, and/or weight.

Through-hole flexible circuits are available on a variety of substrates, with thicknesses ranging from 0.004 inch to 0.007 inch. Operation voltages are from 0 to 30 Vdc, with 1 W of operating power. Normal circuit resistance is less than 100 mega. Capacity is 27 pf maximum between traces.

DIAGRAM: COMPLETING THE CONNECTION

SOURCE: AZTEC INDUSTRIES

DIAGRAM: SCREEN PRINTING GETS BOOST FROM NEW MATERIALS

SOURCE: AZTEC INDUSTRIES

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By TIMOTHY J. MACK, DIRECTOR OF ENGINEERING AZTEC INDUSTRIES, MESA, ARIZ.