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Underfilled Flip-Chip
Conductive "bumps" are soldered directly on the die surface. The bumped die is then turned over -- faced down -- with the bumps connecting directly the carrier. |
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Molded Flip-Chip
After the die is soldered, in the case of "underfilled flip-chip", an underfill resin is added to fill the gap between the die and the substrate. |
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| Flip-Chip Assembly Flow |
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In addition to density and performance considerations, flipping a die and having it connected through its bumps on a substrate is undoubtedly a great simplification of the packaging process. By the elimination of the wire bonding step, the Flip-Chip concept is linked to an assembly cycle time reduction as well as equipment cost and footprint savings. This is particularly true in the case of high pin count products.
In addition to density and performance considerations, flipping a die and having it connected through its bumps on a substrate is undoubtedly a great simplification of the packaging process. By the elimination of the wire bonding step, the Flip-Chip concept is linked to an assembly cycle time reduction as well as equipment cost and footprint savings. This is particularly true in the case of high pin count products. |
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Fluxing & Flip-Chip Attach
The wafer is bumped and sawn. A fluxing step ensures then a good wetting of the bumps on the substrate pads. Moreover, the flux tackiness allows the die to keep its right place on the interposer before the reflow operation. The fluxing step can either be managed through the dipping of the die bumps into a paste or the dispensing of liquid flux over the die position on the substrate. The bumped die is then picked up from the wafer frame, flipped and assembled onto the package substrate.
The package obtained is introduced in a reflow oven to ensure a good connection both electrical and mechanical between the package and the die.
Package Cleaning
The compatibility between the solid residues and the encapsulation compound makes the package cleaning operation optional. A "no-clean" flux does not require any wet washing. Nevertheless, a plasma cleaning can be performed to ensure a perfect adherence of the epoxy resin to the die and board surfaces including the "hidden" zone in the gap between the die and the board. This gap is moreover partially closed by the array of solder bumps attaching the die to the package.
Underfilling Encapsulation
The underfill encapsulation is still the most commonly used and reliable Flip-Chip process, especially for large dice packages.
Molding Encapsulation
The transfer molding brings an increase in encapsulation throughput over the capillary flow of liquid compound. In addition the die is protected by the structure and the package is fully compatible with the standard wirebonded BGAs. This new concept is not yet widespread but it appears promising. |
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