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Flip Chip Bonding Technologies |
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| Flip Chip describes an advanced method of electrical interconnection. A Flip Chip device, equipped with conductive “bumps", is assembled in a “face down” orientation to a package carrier (substrate) by using a Flip Chip Bonder (principle sketch).
Flip Chip bonding technologies:
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Using eutectic solder as bump materials is a well known technology since IBM introduced their C4 Flip Chip technology years ago. Solder is applied to the chip by ball placements or solder paste is printed to the pads. By using a reflow process the solder is melted and formed to balls. The process steps are described in the schematic. Samples of solder materials are 63Sn 37Pb or high lead eutectic 90Sn 10Pb.
Recommended FINETECH modules by side the base machine:
It is possible to use FINETECH equipment for rework of eutectic solder interconnects before applying underfill. The failed chip can be replaced by a new one after chip removal, residual solder removal, fluxing and soldering of a new chip.
However, if the underfill is cured, the repair gets much more complicated. Using special tooling, the chip can be removed after applying heat and mechanical force. When followed by careful manual cleaning of the substrate, chip replacement is possible.
Using reworkable underfill simplifies the operation. Depending on the underfill composition, solvents may be available to remove the underfill easily. |
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Especially in the field of opto-electronically applications the proven solder materials is AuSn (80Au 20Sn) which is used in a flux less process.
Layers or bumps of AuSn material located on chip and substrate are soldered together in a temperature and force process. Typical solder temperatures are in the range of 320 C. The bonding forces have to be controlled very precisely during the process.
Application examples:
- Soldering of laser bars to heat sinks
- Soldering of single laser or VCSEL to sub mounts
- Assembly of high brightness LED
- Assembly of electro-optical modules
Recommended FINETECH modules by side the base system:
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After accurate alignment and placement of the chip to the substrate the chip and substrate are heated up to the melting temperature of the interconnect material. If gold is used the melting temperature will be about 320°C. Simultaneously force is applied to the chip and controlled during the whole bonding process. Typically force range is between 50-80cN per bump.
Recommended FINETECH modules by side the base machine
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After accurate alignment and placement of the chip to the substrate the chip is activated by ultrasonic power into a one direction oscillation (about 50 – 60 KHz). Supported by force and heat the chip bumps are scabbing on the surface of the substrate pads and creating a connection between bumps and pads. As interconnect material mainly gold is used as well for the bump as for the substrate pad surface. By using gold stud bumps the bond is often mechanical supported by underfill. Standard gold bumps as well as gold stud bumps are currently used in the field.
Recommended FINETECH modules by side machine base configuration:
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| Adhesive material can be applied between the chip and substrate in various ways: dispensing, stencil printing, pin transfer or as film works as interconnect between the two partners.
Using the anisotropic conductive adhesive the electrical contact is limited in only one direction (z). These adhesives are available as paste or film materials. The main advantage of these materials is the low bonding temperature. Opposite are the high bonding forces which are needed to crash the conductive particles within the material and to create the conductivity. In case of using conductive paste often stud bumps are used. Beside the support of electrical contact the adhesive material takes care for mechanical support and chip fixation. After placing the chip into the material the adhesive has to be cured by temperature or activated by UV light. |
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Anisotropic Conductive Adhesive |
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Non Conductive Adhesive |
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In case of using anisotropic conductive film (ACF) materials several process steps are necessary
- Pre bonding (force and temperature) of the ACF to the substrate after removing of the safety layers of the film
- Alignment of chip to substrate
- Placement of the chip into the film
- Final bonding by force and temperature
Recommended base systems are FINEPLACER® Lambda and Pico. As additional process modules recommended are:
Application examples:
- Bonding of LCD driver chips to glass substrates (recommended base system FINEPLACER® Lambda, placement accuracies less than 3 microns requested)
- Bonding of flex materials to substrates
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