HSC – Mylar Flexble Cable Technology
There are two types of flexible cable used in telecommunications devices. The most prevalent use is for connecting a LCD (liquid crystal display) to a PCB (printed circuit board).
Mylar technology can be recognized by the presence of solder joints, an amber dark green color and the presence of a TAB chip bonded to the cable. HSC (heat sealed connector) can be recognized by the absence of solder, sot pliable cable and black, green, yellow coloring.
All Mylar technology can be repaired, reworked and replaced with a soldering iron, wire solder, solder paste or a bonding unit. HSC technology is divided into three groups each having different criteria for repair and can only be replaced with a process controlled bonding unit. For factory bonded results HSC material requires the application of controlled pressure and specific temperature for an exact time.
HSC (heat sealed connectors) are not a soldered material, rather a flexible substrate with conductive traces attached. The joint created by bonding (electrically and mechanically attaching) the connecting end of HSC type flex cable is the HSC (HEAT SEALED CONNECTOR). The conducting traces – wires, so to speak – are filled with conductive material (fig 2)
This technology was developed to provide a quick and easy – yet reliable – interconnect method for connecting LCD (liquid crystal displays) to printed circuit boards. Telecommunications devices such as pagers, cell phones, two way radios, fax machines and business telephones use this technology extensively. It is not necessary to understand the precise mechanics of this technology to replace or repair devices which have used HSC material; however, a technician will be more comfortable knowing why a repair or replacement is successful instead of only knowing how to do the replacement.
Conductive epoxy and polymer technology, information and specifications are not readily available to the community of technicians and engineers working in the PCB environment. It is not necessary to understand the nuances of this technology to successfully deal with it in the repair of products using it, an understanding is sufficient to successful repair. To that end, it is more effective to use analogous examples to explain how to deal with equipment using the technology.
A good analogy, is to think of the flex (ribbon) connector as a flat wire harness with each wire replaced by a bead if conductive epoxy. Visualizing this bead of conductive epoxy as being much the same as a bead of caulking compound filled with metal particles will help in seeing the reasons for various time, pressure and temperature profiles which are required for different types of HSC material (fig 3)
The object of the temperature, time and pressure profile is to get the conductive material to become plastic and make a mechanical, as well as an electrical connection to the circuit pad area. Different types of material require different profiles. Some HSC material has different metal particles and different amounts and sizes of metal particles. Some require higher temperatures than others to change from the solid to the plastic state, The pressure requirement is easily understood if it is thought of as having to squeeze the particles together to get them to contact each other so they will be electrically conductive (fig 4)
Presently, in telecommunications, there are three main types of HSC materials in use. Each type has its own mechanical and electrical characteristics which are unique and give a range of price – performance features which can be applied to various products depending on cost vs. performance considerations. The electrical and mechanical differences as well as pitches – the spacing and size of the trace (fig 5) – are different as well.
Understanding the properties of the various HSC materials will allow an intelligent approach to repair of various products, as well as an appreciation of the restraints of their use and removal and replacement limitations.
MONOSTROPIC – Used for fine pitches. It can most readily be recognized by how small the traces are. As a rule of the thumb, if you can hardly see it – it is MONOSTROPIC. This material can be made to a pitch as fine as .22mm. The material contains gold and nickel particles and has a very low contact resistance. It has a yellow color from the titanium dioxide used in the manufacturing process of coating the connector with Thermoset adhesive. i.e. MEMO EXPRESS
ANISOTROPIC – Is the lowest cost material to yield very reliable bonded joints. This material is filled with gold plated nickel particles and is found on most pagers. It is also replacing planar material in pager applications. Many of the pagers manufactured with PLANAR technology have ANISOTROPIC replacement parts. This material can also be produced in pitches as fine as .29 mm, but at these fine pitches look for the MONOSOTROPIC type. Look for green and white as well as black and white to recognize this type. i.e. BRAVO EXPRESS
PLANAR – The original pager material. It is limited to .3mm or larger pitches and is more expensive than other types and contains no metal particles. It is yellow with black traces which are usually visually easy to see. i.e. BRAVO ALPHA
Bonding (establishing a mechanical and electrical connection) of these materials requires temperature, pressure and time. The (fig 6) shows materials and their respective parameters.
Note that there is an operating time, temperature and pressure envelope for each type of material. When replacing an HSC connection, it is important to note the properties for each type of material.
If, for instance, when bonding a monosopotric material, and anosopitric pressure of #70 lb. is applied – the bonded joint will look fine; but, it will not have a proper mechanical connection to the pad. It might even work for a short time – the fail is the device is dropped or sees an environmental temperature change. Is an anisotopic material is bonded at a monosopotric temperature, the insolating material between the conductive traces could melt and short.
If a planar material is bonded at the high end of the monosotropic pressure envelope – the bonding head could cut the material.
If the HSC cable is pealed from the circuit card, the residual conductive material on the pad areas of the PCB will reveal the way a properly bonded joint appears after bonding.
Flexible cable, using MYLAR technology, is essentially a flexible printed circuit board. The conductive traces are copper which, in most cases are plated with solder. The techniques of working with this cable is the same as PCB’s; but, the material will not tolerate the usual 600 F – 750 F used on PCB’s. The best working temperature is between 375 F to 400 F. A high thermal mass, digitally controlled soldering iron or bonding unit will ensure damage free rework of devices using MYLAR.
Preperation for Flexible Circuit Replacement:
Removal of the old flex cable is usually performed with a digital temperature controlled soldering iron set at less than 700 F. as the solder joints reflow, apply a pealing pressure to disconnect the solder joints. After cooling, the receiving pad areas on the PCB card should be tinned with a 63/37 paste or wire.