Flip chip technology offers a direct and efficient bond between chips and substrates, replacing traditional wire interconnections.
The flip chip process includes wafer bumping, alignment, reflow, and encapsulation for reliable and efficient electronic packaging.
Different wafer bumping processes, such as solder bumping, plated bumping, stud bumping, and adhesive bumping, provide flexibility for diverse application requirements.
Chip after wafer bumping step
The flip chip packaging process is an important method for electrically connecting a die to a package carrier. Unlike the traditional approach of using wire interconnections, flip-chip packaging enables a direct and efficient bond between the chip and the substrate or package. This versatile process is compatible with various substrates, including laminate PCB, polyimide, glass, ceramic, silicon, and plastic package lead frames.
The flip chip packaging process extends its utility beyond individual die packaging. It can be used to directly attach dies to PCB boards, regardless of whether surrounding components utilize flip chip technology, a configuration commonly referred to as flip chip on board.
By eliminating performance issues associated with inductance and capacitance related to bond wires, flip chips have further solidified their position as a reliable and efficient packaging solution in the electronics industry. See below for a walkthrough of the flip chip packaging process.
Flip Chip Packaging Process
Flip Chip Packaging Process Steps
Metallization of attachment pads on the chip's surface and deposition of electrically conductive bumps or solder balls onto the bond pads.
Cutting of chips, flipping, and precise alignment of the die with conductive bumps to ensure accurate placement.
Heating and melting of conductive bumps to allow uniform spreading of conductive material across bond pads. Enhances solder's wettability and reduces gap or standoff between die and substrate.
Filling gaps between the die and substrate with an encapsulant. Careful deposition along the edges of the die and flow across the gap. Enhances mechanical strength, addresses thermal expansion mismatch, and more.
Step 1: Wafer Bumping
During the manufacturing process, the attachment pads on the chip's surface are metalized to enhance their solder receptivity. To facilitate flip chip bumping (or simply wafer bumping), electrically conductive bumps or solder balls are deposited onto the bond pads of the die. This can be achieved through various processes, including solder bumping, stud bumping, and adhesive bumping, collectively known as wafer bumping. These bumps serve multiple functions, such as establishing electrical connections, facilitating thermal conduction, preventing electrical shorts, and providing mechanical support to the flip chip.
Step 2: Alignment
After the bumps are created, the chips are cut, and the die with conductive bumps is flipped and aligned to ensure precise placement. The accuracy of the alignment is crucial, with requirements in the range of a few microns for reliable functioning.
Step 3: Reflow
During the reflow stage, after aligning and flipping a die onto the substrate, the conductive bumps undergo a process of heating and melting to allow the conductive material to spread uniformly across the bond pads. This reflow process enhances the solder's wettability and diminishes the gap or standoff between the die and substrate. The reflow is commonly achieved through thermosonic bonding or a reflow solder process.
Step 4: Encapsulation
Encapsulation involves filling the gaps between the die and the substrate. The encapsulant is carefully deposited along the edges of the die and flows across the gap between the die and substrate, effectively filling the space between the bumps. Additional deposition is performed on the edges of the die to complete the encapsulation process. The underfill material serves multiple purposes, including enhancing mechanical strength and reliability. It plays a crucial role in addressing the thermal expansion mismatch between the die and substrate, reducing thermal fatigue on the conductive bumps and extending their lifespan. Moreover, it creates a small space between the chip's circuitry and the underlying mounting. In many instances, an electrically-insulating adhesive is utilized for underfilling. This adhesive provides a more robust mechanical connection, acts as a heat bridge, and prevents stress on the solder joints.
Once the underfill material is cured, the chip is packaged and securely attached to the board, completing the flip chip packaging process.
Wafer Bumping Process Variations in Depth
The first step of the flip chip packaging process, known as wafer bumping, can be accomplished through a variety of different processes, each with its unique characteristics.
Solder bumping involves placing underbump metallization (UBM) over the bond pad, removing the oxide layer, and defining the solder-wetted area. This UBM deposition can be done through sputtering, plating, or similar methods, acting as an interface between the bond pads and the solder bumps.
Plated bumping, on the other hand, utilizes wet chemical cleaning processes to remove the oxide layer on the Al bond pad, followed by electroless nickel plating to form the foundation of the bump. This plating provides the desired thickness and coverage.
Another flip-chip bumping process is stud bumping, which involves melting the end of a wire to create a free-air ball or sphere, attaching it to the bond pad, and then breaking off the wire. The resulting stud on the bond pad is utilized to bond flip chips to a substrate using conductive adhesives or the thermosonic process. Stud bumping can be carried out using standard wire bonding equipment.
Finally, adhesive bumping employs conductive adhesive stenciled over an underbump metallization layer on the bond pad. This process forms conductive bumps that can be mounted on the substrate using conductive adhesives. Adhesive flip chip technologies encompass various adhesive types, including conductive adhesive polymers and anisotropic conductive adhesives, although non-conductive adhesives can be used as well. While adhesive technologies do not require cleaning, they are more challenging to rework and may have higher thermal and electrical resistance, making them less suitable for specific applications.
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