FAQ: Hot Bar Soldering for TAB or COF Style LCD Modules

March 8, 2016 4:23 pm

Question

Some of your LCDs have a flexible connection, but I can’t find any connectors that fit them. What is the best way to connect these modules to a PCB?

Closed Flex Solder Tail - crystalfontz.com

Closed Flex Solder Tail

Open Flex Solder Tail - www.crystalfontz.com

Open Flex Solder Tail

Answer

There are three kinds of modules that have this kind of construction:

  • TAB (Tape Automated Bonding)
  • COF (Chip On Flex)
  • and some COG (Chip On Glass) modules

The benefit of this kind of construction is low cost — there is no connector involved at all, just the flex soldered directly down to the PCB. The drawback is that the machine used in the LCD manufacturing process to connect them is expensive, and they are near impossible to re-work.

The connector is a TAB mated to a COF (Chip On Flex) style flex tail or a COG (Chip On Glass) LCD module construction.

This style of connector is designed to be soldered directly to corresponding pads on your PCB by using a hot-bar soldering machine.

Hot Bar Soldering

High volume contract manufacturers will be familiar with this type of construction and its assembly methods.

There are bondmaster machines made by APE that are designed for prototype, rework or repair work. We have had good experience with the APE Bondmaster, and their price is very reasonable ($US4K at last check).

Here are some other possible solutions:

The process is as follows:

  1. The pads on the PCB are tinned.
  2. The tail is aligned to the PCB using the alignment holes or visually.
  3. The tail is held in place relative to the PCB with kapton tape.
  4. The bondmaster head is lowered, applying pressure between the tail and the PCB.
  5. The bondmaster is “cycled”, which means it heats up to the point of melting the solder and then cools down.
  6. The bondmaster head is raised.

Hand-Soldering the Tail to the PCB

It is possible to hand solder the tail to the PCB. Great care must be taken since the conductors of the tail are completely exposed in the area where they are soldered.

Below is a good example of hand soldering of a picture of a closed style tail.

Hand-Soldered - crystalfontz.com

Hand-Soldered Connector

Kapton tape should be used to secure the tail to the PCB, to insulate it and to reduce the chance of it flexing and breaking.

Hand-Soldered With Kapton Tape - crystalfontz.com

Hand-Soldered Connection Secured With Kapton Tape

Another approach uses ACF (Anisotropic Conductive Film) combined with heat and pressure to make the electrical connection between the flexible tail and the PCB. This two-step process still uses heat, but the temperature involved is much less than soldering.

Product Development

For development work, we have found that the SchmartBoards company makes a number of breakout boards in a number of pitches that work with our displays.

Questions?

For any questions about TAB, COB, COF, COG, hot bar soldering, or what LCD module is best for your product, please contact our knowledgeable and friendly support staff via email, phone, or chat.

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