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Flexible Flat Cables or Flex are most often found in LCD displays, Laptops , cameras , cellular phones , and other high density handheld electronic devices. These cables allows extreme space savings and provides the needed flexible connection for devices with movable parts such as Laptop displays, clamshell

How to repair damaged Flexible Flat Cable from LCD displays?

Electronics Reworking

Wilbert Lao

/ slider cellular phones and other gadgets. Almost all equipments with LCD displays utilize flexible flat cables to connect the display to the main circuit board.

An advancement of Flexible flat cables are the Flexible circuits, these flexible circuits provide increased space savings as components are directly attached to the flexi-ble board.

The wide usage of flex cables and flex circuits in components utilizing LCD dis-plays has led to service stations pilling up with LCD modules with broken flex but working LCD display panels. The ability of these service stations to repair these LCD modules can mean additional income to its business and provide sav-ings for its customers.

Here we will detail the procedures and equipments needed to rework the flex in an LCD module.

© 2008 TYQ Int.

© 2008 TYQ Int. Sample Flex Circuit

© 2008 www.aoyue.com used with permission LCD Flex Reworking machine From Aoyue International

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Obtain an LCD module with a defective flex. Most LCD modules have a simple plastic casing that can easily be detached. After detaching the case a few thin films of plastics and metal-lic sheets can be found in-between the case and LCD panel. These are backlight diffusers and reflectors, set them aside as we will only need the LCD panel with the flex attached.

Parts of an LCD panel with attached flex

LCD main display panel viewable area

Main Control chip for the LCD panel

Contact area between Flex and LCD panel

Flexible cable or Flex circuit

Next we would need to detach the damaged flex, A Hot Bar soldering system set at around 230 to 260°C would be enough to break down the

adhesive bonding of the flex and LCD panel. Lightly heat the con-tact area between the Flex and LCD panel using the hot bar tool. The flex can then be easily peeled off.

© 2008 TYQ Int.

© 2008 TYQ Int.

© 2008 TYQ Int.

© 2008 TYQ Int.

LCD panel and its Flex

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Pressure Heat

Examining the contacts under the magnified lenses of the Aoyue 8011 Hot bar soldering system, We can see the leftover bonding ad-hesive on the contacts. The contact can be cleaned using a solution that softens it for easy removal. A panel which is properly cleaned should have bright and shiny contacts with no visible dark spots.

Dirty Contacts Cleaned Contacts

© 2008 TYQ Int.

© 2008 TYQ Int.

© 2008 TYQ Int.

Bonding of flex requires heat and pressure. A Hot bar soldering system provides both the required heat source and applied pressure to bond the flex and LCD contacts.

The bonding adhesive is an anisot-ropic conductive film adhesive. This adhesive film contains tiny conduc-tive spheres surrounded by insulating and bonding materials. The applied heat and pressure will break away the insulating materials thus allowing electrical conductivity between the contacts.

Hot Bar Bonding how it works?

Flex Cable or Flex Circuit

Anisotropic conductive film adhesive

PCB or LCD Panel

© 2008 TYQ Int.

© 2008 TYQ Int.

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© 2008 TYQ Int.

As heat and pressure is applied the conductive spheres are pushed in place and their insulat-ing coating breaks away allowing the sphere to conduct electricity. The adhesive then cures locking the sphere in this conductive state.

Attaching the bonding adhesive to the flex re-quires pressure and heat. The Aoyue 8001 Hot bar preheating system with temperature set at 150°C would provide the necessary heat and pressure to transfer the adhesive to the flex.

Attaching Anisotropic Conductive Adhesive to the new flex

The sphere’s insulation breaks away allowing conduction between contacts of the flex and LCD panel. Spheres still with insulated coating intact stays in the gaps in between the contacts.

© 2008 TYQ Int.

Heat and Pressure Exerted on flex and board

© 2008 TYQ Int.

© 2008 TYQ Int.

© 2008 www.aoyue.com used with permission Hot bar pre-heating machine From Aoyue International

Place the flex on a flat surface then carefully apply the adhesive. The Bonding adhesive is normally kept at low temperatures of –10 to 5°C . Let the adhesive return to ambient temperature before usage. After apply-ing the adhesive use the Hot bar pre-heating system to allow it to firmly at-tach to the flex. The protective film of the conductive adhesive can then be peeled off. This will leave only a thin clear film of conductive adhesive.

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Place the cleaned LCD panel on the LCD plat-form. Then attach the flex cable to the micro adjust platform. The micro adjust platform has 3 axis adjustments available, Slowly adjust these three knobs until the contact of the flex is perfectly lined up with the contacts of the LCD panel.

Normally these LCD flex has a certain guide marking located at the end of the last vertical strip. In this case our guide is the lone horizon-tal bar seen in this color LCD monitor. The dual camera and color LCD display monitor in the Aoyue 8011 hot bar system will allow simulta-neous monitoring and adjustment of the flex to match the LCD panel’s contacts.

Aligning flex contacts with LCD panel

© 2008 TYQ Int.

© 2008 TYQ Int.

© 2008 www.aoyue.com used with permission

Hot bar Soldering System From Aoyue International

While adjusting the flex to match the LCD panel’s contacts keep the flex completely flat on top of the LCD panel. A piece of thin plastic can be used to press down on the flex momentarily. With the flex flat on the LCD panel look at the LCD monitors to confirm the correct alignment. Alignment of the hori-zontal bars and vertical stripes on each end will guarantee perfect alignment for the entire flex and LCD panel.

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With both flex and LCD panel aligned, we proceed to soldering. In the Aoyue 8011 system we have the option of using the manual or automatic mode. The manual mode as the name suggest allows us to manually control the movement of the platform and hot bar tool. The automatic mode requires us to input the desired temperature and soldering duration while it automatically moves the platform and soldering process.

Heat and pressure soldering

© 2008 TYQ Int.

Platform slides in waiting for the Hot bar tool to move down

and begin soldering

Hot bar tool applying heat and pressure to the LCD panel and flex

© 2008 TYQ Int.

© 2008 TYQ Int.

The Platform in the initial position

Hot bar reworking of a broken flex is sim-ple and quick. It can bring those piles of LCD modules back to life and Repair cus-tomer’s flex problems in a few minutes . Hot bar reworking allows higher produc-tivity, savings and increases customer sat-isfaction. All we need are the right equip-ment and a couple of old LCD modules to practice on.

Easy Hot Bar Reworking

Automatic mode soldering

First we set the temperature to 260 °C and the timer to 15 seconds. Then we simultaneously press the start automate buttons to begin the automate cycle.

The system now automatically moves the plat-form in, then lowers the hot bar tool. When the hot bar tool touches the Flex countdown timer commences.

After timer expires the hot bar tool moves back up and the platform slides out again. Within 20 seconds the system is ready for the next items to be soldered.