Adhesive materials can be applied between two join partners, such as die and substrates in various ways: dispensing, stencil printing, pin transfer or as a film working as an intermediate connection.
Thermal Conductive Adhesives
Thermal conductive adhesives can be used to mount chips on boards (COB), die on substrates and substrates on heat sinks. Electrical interconnections can be made by wire bonding, if needed.
According to the dimension of the adherent surface area, the needed amount of glue can be screen printed, stamped by pins or punches, dipped with a die or dispensed by fine cannulas. The adhesives can be cured directly after assembly with a heated tool, or chuck by snap curing within a few seconds, or it can be cured in a batch oven later.
Die Attach Film
Whereas most common NCA and TCA come as pastes, a special type is unfilled adhesive in film layer and can be used to mount chips on boards (COB), panels or wafers. Electrical interconnections can be made by wire bonding in the working steps after.
The DAF material can be pre applied to the substrate over larger surfaces or it can be applied directly to the bottom side of the die by using stamping or even to apply the film layer to the components on wafer level before dicing.
The thickness of these films is rather low and the uniformity quite high, which makes it suitable for applications with demands for small bond line thickness (BLT) and precise volume control. Directly applying force and heat makes DAF a quick thermal curing solution.
Isotropic Conductive Adhesives - ICA
If the electrical contact is to be made by the glue itself, adhesive materials filled with conductive particles can be dispensed as isotropic conductive paste (ICP) as a solder replacement.
These adhesive materials can be handled quite similarly to thermally conductive adhesives as described above. If surface mount related electrical connections are required, without the introduction of the heat needed for a soldering process, you may substitute solder by an appropriately filled adhesive and cure it in a much lower temperature process. ICA is used for backside contacted LEDs, standard SMD elements and even flip chip mounted devices with a wider pitch.
Anisotropic Conductive Adhesives - ACA
Anisotropic conductive adhesive (ACA) is filled with conductive particles but is not electrically conductive itself. As this adhesive is pressed between two surfaces, the particles will touch the surfaces and each other and a conductive layer will be established in one direction. These adhesives are available as paste (ACP) or film materials (ACF).
For mechanical fixture, thermal connection and connecting multiple electrical contacts with only one adhesive material, ACA can be used and simplifies the flip chip process. The main advantage of these materials is the low bonding temperature. The connection is cured very quickly by introducing force and heat from above and additional heat from below. One drawback is the high bonding forces required to crush the conductive particles within the material and to create the conductivity. Accurate alignment combined with reproducible bonding forces guarantees a steady interconnection with undamaged joined partners.
Attaching LCD drivers to LCD panels is one example for using ACF with chip on glass bonding (COG). In contrast to most other bonding methods, ACF bonding is reworkable. The defective component can be removed and the glass panel can be cleaned. Then new ACF preforms are placed either by an ACF module or manually by using a pair of tweezers.
When using anisotropic conductive film (ACF) materials, several process steps are necessary:
- Pre bonding (force and temperature) of the ACF to the substrate after removal of the safety layers of the film
- Alignment of chip to substrate
- Placement of the chip onto the film
- Final bonding by force and temperature
In RFID applications anisotropic conductive paste (ACP) is used to fix die on antenna films. The die have only a few pads, which are embossed by bumps or coined bumps. The conductive particles are pressed between the contacts and snap cured within a few seconds by a heated tool and may be supported by a heated chuck.
A suitable dispensing module is required for integrating ACP processes in your flip chip bonding system.
Stud Bump Bonding
Another possibility for electrically connecting two contacts is to use unfilled, nonconductive adhesives (NCA) and stud bumps. Conductive material (e.g. gold) is bumped on the pads of one chip. The adhesive is applied between the chip and substrate and then both join partners are pressed together. While pressing, the adhesive is displaced by the gold bumps and creates the mechanical support and fixation.
This technique of connecting electrical contacts needs well prepared flip chip die with gold bumps. These stud bumps can be added by a wire bonder (ball-bonder) working in stud bumping mode. The die are then mounted by a flip chip bonder and the gold bumps are pushed against the target pads.
After placing the chip into the adhesive, it has to be held by the tool in this particular position and cured by temperature or activated by UV light.
For optical application, it may be important to use glue with appropriate optical characteristics, such as index of refraction.
If adhesive material will be positioned in the principal axis of an optical system and so will affect the optical characteristics, a suitable adhesive should be used. After dispensing and attaching, these adhesives can immediately be cured by UV spot lights. Applications using optical adhesives could be mounting of glass lids, lens fixation or fiber chip coupling.
The Finetech Solution
High Force ACF Bonder
As the versatile FINEPLACER® pico ma system continues to evolve, a new optional configuration has been developed for the latest Anisotropic Conductive Film (ACF) applications.
- Bonding forces up to 700 N
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FINEPLACER® Bonding Systems
Due to the modular design approach, FINEPLACER® bonding systems can be configured for virtually any application challenge.
The main distinguishing features between the machines are the
- degree of automation
- optical resolution and
- placement accuracy
Browse our product range or get in contact with your sales contact to figure out the best equipment solution for your specific application requirements.