Repairing HSC and Mylar flexible cable technology

Repairing or reworking connections in telecommunications devices such as pagers, two-way radios and cellphones requires an understanding of the physical properties of the connective materials.

Two types of flexible cable are used for various purposes in telecommunications devices. The most prevalent use is connecting a liquid crystal display (LCD) to a printed circuit board (PCB).

Mylar technology can be recognized by the presence of solder joints, an amber or dark green color and the presence of a TAB chip bonded to the cable. A heat-sealed connector (HSC) can be recognized by the absence of solder; soft, pliable cable; and black, green and yellow coloring. (See Figure 1 below.)

All Mylar cable technology can be repaired, reworked and replaced with a soldering iron, wire solder and solder paste, or a bonding unit. HSC technology is divided into three groups, each having a different criterion for repair, and it can only be replaced with a process bonding unit. For factory-bonded results, HSC material requires the application of controlled pressure and specific temperature for a precise time.

HSC technology Heat-sealed connectors are not soldered materials but, rather, a flexible substrate with conductive traces attached. The joint created by bonding (electrically and mechanically attaching) the connecting end of the HSC-type flex cable is the heat-sealed connector. The conducting traces, or “wires,” are filled with conductive particulate material. This technology was developed to provide a quick and easy, but still reliable, method to connect an LCD to a printed circuit board. Telecommunications devices such as pagers, cellphones and two-way radios use this technology extensively. It is not necessary to understand the precise mechanics of the technology to replace or repair devices that incorporate HSC material. However, a technician will be more comfortable knowing why a repair or replacement is successful instead of only knowing how to perform the replacement.

Information about conductive epoxy and polymer technology, and specifications, is not readily available to the community of engineers working in the PCB environment, but understanding the physical properties and limitations is sufficient for successful repairs.

A good analogy is to think of the flex (ribbon) connector as a flat wire harness with each wire replaced by a bead of conductive epoxy. Visualizing this bead of conductive epoxy as being similar to a bead of caulking compound filled with metal particles will help illustrate the reasons for various time, pressure and temperature profiles that are required for different types of HSC material.

Temperature, time and pressure cause the conductive material to become plastic and make a mechanical, as well as an electrical, connection to the circuit card pad area. Different types of material require different profiles. Each type of HSC material has different metal particles in various amounts and sizes. Some materials require higher temperatures than others to change from a rigid to a plastic state. The pressure requirement relates to forcing the particles to make contiguous contact to ensure conductivity.

Three main types of HSC materials are in current use for telecommunications. Each type has its own mechanical and electrical characteristics that are unique and provide a range of price-performance features that can be applied to various products. The pitches, the spacing and size of the trace are different as well, as shown in Figure 2 below. Understanding the properties of the three types of HSC materials provides guidance for the repair of various products, as well as an appreciation of their use, removal and replacement limitations.

Monosotropic – Used for fine pitches, monosotropic material can be recognized by its diminutive traces. If you can hardly see it, it is monosotropic. This material can be made to a pitch as fine as 0.22mm. The material contains gold and nickel particles and has a low contact resistance. Its yellow color denotes the titanium dioxide used in the manufacturing process when coating the connector with Thermoset adhesive. Monosotropic materials are used in Motorola Memo Express alphanumeric pagers, for example.

Anisotropic – The lowest-cost material to yield reliable bonded joints, anisotropic material is filled with gold-plated nickel particles, and it is used in most pagers. Anisotropic material is also being used to manufacture replacement parts for planar materials used in pager technology. Anisotropic material can be produced in pitches as fine as 0.29mm, but for such a fine pitch, monosotropic material is usually used. The material’s color is green-and-white or black-and-white. Anisotropic materials are used in Bravo Express pagers.

Planar – The original pager connective material, planar is limited to pitches of 0.3mm or larger. It is more expensive than the other two types and contains no metal particles. Planar material is yellow with black traces that are usually easy to see. Planar materials are found in Bravo Alpha pagers.

Bonding (establishing a mechanical and electrical connection) of these materials requires temperature, pressure and time As shown in Figure 3 above, each of the three materials has different bonding requirements. There is an operating time, temperature and pressure envelope for each type of material.

When replacing an HSC connection, note the properties of the material used. When bonding a monosotropic material, for example, if an anisotropic pressure of 70psi is applied, then the bonded joint will look fine, but it will not have a proper mechanical connection to the pad. The joint may even work for a short time_then fail when the device is dropped or undergoes an ambient temperature change. If an anisotropic material is bonded at a monosotropic temperature, the insulating material between the conductive traces could melt and short. If a planar material is bonded at the high end of the monosotropic pressure envelope, then the bonding head could cut the material.

If the HSC cable is peeled away from the circuit card, then the residual conductive material on the pad areas of the PCB will reveal the way a properly bonded joint appears after bonding, as shown in Figure 4 at the left.

Mylar technology Flexible cable, using Mylar technology, is essentially a flexible PCB. The conductive traces are copper and, in most cases, are plated with solder. The techniques of working with flexible cable are the same as those used for working with PCBs, but the material will not tolerate the temperatures of 6008F-7508F used to repair PCBs. The best working temperature is beween 3758F and 4008F. A high thermal mass, digitally controlled soldering iron or bonding unit will ensure damage-free rework of devices using Mylar.

Flexible circuit replacement Mylar – Removal of the old flex cable is usually performed with a digitally controlled soldering iron set at less than 7008F. As the joints reflow, apply a peeling pressure to disconnect the solder joints. After cooling, the receiving pad areas on the PCB card should be tinned with a 63/37 soldering paste or wire as shown in Figure 5 above. Some Mylar flex cable requires solder tinning prior to installation. Two types of pad areas on Mylar flex cable are supported, meaning that Mylar material is between each finger (such as in the Advisor pager), and unsupported, meaning there is no material between the fingers. Before attempting to tin unsupported fingers, tape a piece of Kapton anti-static tape over the fingers. The tape will prevent the fingers from bending or distorting out of shape.

HSC technology – To remove a damaged or defective HSC cable, secure the LCD and gently peel the cable from the PCB pad areas. A residue of conductive material will be on the PCB fingers, as shown in Figure 6 below. The residue must be removed completely prior to installing a new HSC cable. Residual material can cause an electrical short in the device after the installation.

Scheu is president of A.P.E. South, Key Largo, FL. A.P.E. (Automated Production Equipment) manufactures equipment for rework and repair of intergrated chips and printed circuit boards.