ACF Bonding Principle

Rework of ACF Bonded Devices

Anisotropic Conductive Film (ACF) is a smart adhesive assembly technology with numerous benefits, such as

• fast and clean application
• long durability of applied adhesive film
• no potlife issues when assembly is on hold
• very flexible as stockbuilding is possible, ACF application as stand-alone solution for several assembly lines
• the transparent ACF allows visual inspection of fiducials and substrate structures over a long time
• no air inclusions
• no fluctuating dosing quantities
• fast pre-bonding, a separate post-curing is possible
• unlike most adhesive bonding technologies, ACF-bonded devices are reworkable

This bonding technology combines excellent quality and high reliability and therefore is a long-time premium choice for a lot of consumer products with ACF-bonded components integrated in great numbers.

Of course, bond failures or failures of substrate or chip still can happen. And in a good deal of failure cases, reworking the defect is - from an economic perspective -  absolutely reasonable because the affected product features a long value chain and is built of pricey components and materials. Possible fields of application could be repair services under warranty or a separate rework in manufacturing outside a production line (flexibility).

Employing a FINEPLACER^® allows you to rework ACF-bonded components and even reassemble exploited materials. Using materials exploited from defect products could be a way when

• there is no access to virgin materials
• virgin material is too expensive to keep it economically reasonable
• virgin material is only obtainable in great quantities
• virgin material is no longer available (EOP)

A good example how much reworking ACF-mounted components can make sense is the repair of failed CSP driver chips at pricey TFT or LCD screen and display panels.

In the past it was only possible to assemble virgin material (a chip or chip-on-flex) using a thermocompression bonding process. High accuracy sub-micron bonder FINEPLACER^® Lambda provides you with a reliable solution to rework ACF-bonded devices. Now it is possible to use reworked material for all chip-on-glass or chip-on-flex applications such as bonding CSP drivers on TFT panels.

Chip on Glass (CoG):
The chip is ACF-mounted directly to the glass panel

Chip on Flex (CoF):
The chip is ACF-mounted to a flex interposer.

Perspective View Layer Structure

1. Flex cable connector between panel and driver PCB

2. Panel bottom layer

3. Driver chip (CSP)

4. ACF (Anisotropic Conductive Film)

5. Panel top layer

Top View Layer Structure

1. Panel Bottom Layer with strip line (input signal)

2. Driver Chip (CSP)

3. ACF (Anisotropic Conductive Film)

4. Globe top between bottom and top layer

5. TFT Panel Bottom Layer with strip line (output signal), look through top layer

6. Panel Bottom Layer

In order to prepare a TFT panel for bonding of  a chip or a chip-on-flex it is necessary to detect the failed chips with a pre-test of the TFT screen. Which materials can be used for rework purposes will be visible optically when running the defect panel. Corrupt connections or chips evoke distinguishable errors like line, area or pixel failures.

There is easy access to the worksite since all screwing, clamping and plug connections of TFT screens can be opened and re-established easily.

If one or several chips-on-glass have failed, they need to be removed and the glass panel needs to be cleaned. Afterwards, fresh ACF can be applied to the glass substrate. The combination of FINEPLACER^® system, tooling and defined process parameters allow a safe and reproducible separation of the materials.

The FINEPLACER^® solution allows exploiting fully functional materials from defect and irreparable TFT panels to reassemble them to another TFT panel . This way it is possible to get two functional TFT panels out of three defect panels, even if there is no access to virgin material!

Again, a pre-test in a first step detects which chip or chip-on-flex is still functional and can be used again. A functional chip-on-glass can be removed, it is required to clean the chip bumps before the chip can be used again.

Special material-friendly cleaning technologies help reliably to remove adhesives from the panel and the bump gaps down to a pitch of 60 µm. The up to > 300 bumps on the chip will be preserved from damage and can be used for new contacting.

A functional chip-on-flex can be reworked by removing the flex from the substrate. Afterwards the flex contacts may be cleaned (conductors typically structured on polyimide, connections to the panel and to the driver PCB).

Now the reworked chip for COG or the chip-on-flex can be mounted to another TFT panel.

The picture overlaying of the fiducials of substrate and chip can be carried out easily in combination with a suitable ACF. By activating programmed camera positions, quick adjustment and elimination of the angular error is possible.

FINETECH provides you with a convenient rework solution for TFT panels even when there is no virgin materials available for assembly. Use the FINEPLACER^® Lambda system with specialized tooling and processes developed for removal, cleaning and assembly.

Approved Dimensions:

• sizes of TFT panel up to 32" (l x w = 700 mm x 400 mm) approved
• different chip sizes (e.g. l x w x h = 20 mm x 1.5 mm x 0.35 mm) approved
• chips with more than 300 I/O bumps approved
• pitch up to 60 µm (line / gap = 25 µm / 35 µm) approved

FINEPLACER® Lambda (motorized configuration)

The manual high accuracy sub-micron flip chip bonder FINEPLACER^® Lambda provides a placement accuracy of up to ± 0.5 µm.

Preferred applications for the versatile FINEPLACER^® Lambda system are assembly of opto-electronic components (e.g. VCSEL, photo diodes, power and single lasers or laser bars), advanced device packaging (MEMS, sensors, micro-optics, embedded components, surface mount photonics), flip chip bonding and precise die attach.

More information on FINEPLACER^® Lambda.