Peter J. Opdahl
ACF (Anisotropic Conductive Film) is a thermoset epoxy system that includes conductive particles uniformly distributed in a non-conductive adhesive film. ACF is well suited for flip chip assembly in very fine pitch chip-on-board (COB), chip-on-flex (COF), or chip-on-glass (COG) applications. It is used commercially for millions of units of COG modules, with pad areas as small as 800 square microns with 15-micron spaces.
In ACF assembly, heat and pressure are applied to trap the conductive particles between the substrate pads and the IC bumps, forming permanent electrical connections as the epoxy film cures. Tutorial 5 gives details and diagrams of the ACF assembly process.
An attraction of ACF assembly, in addition to the fine pitch and high volume capabilities, is that it eliminates the costly and time-consuming underfill step required by most flip chip processes. The cured non-conductive adhesive film acts as an underfill, strengthening and protecting the assembly. However, because the adhesive is a thermosetting epoxy, its strength and permanence has always been a limitation to reworking the completed assembly.
That limitation is now being overcome by a new class of rework solvents that are completely different from the MEK and Methyl Chloride – Formic Acid solutions used in the past. These new solvents are non-corrosive. They may safely be used in ultrasonic baths or in heated tanks to accelerate the ACF removal process.
The new rework process is safe, simple, and effective:
1. Apply the AFC rework solvent “ACF-X,” available from ITO America.
Application may be either with a dispenser or with a swab if no tank system is being used.
If a swab is used, the solvent should be puddled around the die so that the solvent can begin to penetrate and attack the epoxy from all directions. ACF-X is similar to a light cooking oil in appearance and viscosity, so it is easy to control the flow of the material.
If a tank system is used, the package should be placed in a carrier and lowered into the bath. If heat is used, the maximum amount of heat should be 60C. If an ultrasonic bath is being used, relatively low ultrasonic energy is recommended.
2. Allow the rework solvent to penetrate and break up the ACF.
The amount of time required for this depends upon the ACF used, the level of cure of the ACF, and the size of the die. Experiments with a 2.5mm x 11mm die assembled to a glass substrate showed that ACF-X rework solvent puddled over the die for 8 hours at room temperature allowed the die to be removed with fingertip pressure.
“Energized” processes using heat or ultrasonic vibration typically show a three to four times reduction in the required cycle times. ACF-X has an extremely low rate of evaporation and in a tank system may be filtered and used several times without losing its effectiveness.
3. Remove the die.
ACF-X works differently from standard solvents. It does not dissolve the epoxy, but rather destroys the epoxy’s ability to bond to the substrate and die. As a result, the epoxy lifts off and then tends to fragment in the solution. Once the die has been removed from the substrate, both may be washed with an aqueous solution, with acetone, or with another distillate. ACF-X is entirely water-soluble.
This new class of solvents removes the rework limitations of ACF, without compromising the strength and protective features of the high-volume, fine-pitch flip chip assemblies.