Leadframe Support Features for Flip-Chip Die Collapse Control
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Solution Overview
Problem
Flip-chip semiconductor die collapse during reflow processing due to inconsistent flux thickness and coplanarity, leading to uneven melting of connector bumps and inadequate stand-off height, which prevents proper contact with heat sinks.
Innovation Solution
Creating support features on the leadframe with a higher liquidus temperature than the connector bumps, configured to maintain the desired stand-off height by melting and forming into support features during the reflow process, and optionally coating these features with materials like epoxy or silver, nickel, and gold.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If flux is applied to connector bumps before reflow, then bonding between die and leadframe is achieved, but inconsistent flux thickness causes uneven melting and collapse of connector bumps
Solution Approach 1:
The patent introduces support features as intermediary structures between the connector bumps and the leadframe. These support features have higher liquidus temperature than the connector bumps, so they remain solid during reflow while the connector bumps melt. This mediates the bonding process by providing structural support that prevents collapse, while still allowing the connector bumps to bond to the leadframe through flux activation.
Solution Approach 2:
The patent changes the temperature parameter by selecting materials with different liquidus temperatures. The support features are made from materials whose liquidus temperature is higher than that of the connector bumps, creating a temperature differential that allows selective melting. This parameter change enables the connector bumps to melt and bond while the support features maintain structural integrity.
2Reliability
If connector bumps are allowed to melt during reflow for bonding, then die-to-leadframe connection is achieved, but stand-off height becomes inconsistent preventing proper heat sink contact
Solution Approach 1:
The support features act as intermediary structures that maintain the stand-off height between the die and the leadframe. While the connector bumps melt to create electrical connections, the support features with higher liquidus temperature remain solid and preserve the vertical spacing. This ensures consistent stand-off height for proper heat sink contact while still achieving reliable electrical bonding.
Solution Approach 2:
The support features provide beforehand cushioning by pre-establishing a structural framework that prevents excessive collapse of the connector bumps during reflow. The support features are positioned to cushion and limit the downward movement of the die, ensuring that even if some connector bumps collapse, the stand-off height remains within acceptable tolerances for heat sink contact.
3Manufacturing precision
If support features with higher liquidus temperature are added to the leadframe, then connector bump collapse is prevented, but manufacturing process complexity increases
Solution Approach 1:
The support features are not added throughout the entire leadframe but only at specific locations where connector bumps are positioned. This local quality approach means the higher liquidus temperature material is applied only where needed to support individual connector bumps, rather than making the entire leadframe complex. The support features are localized to the bonding areas, maintaining simplicity in non-critical regions.
Solution Approach 2:
The leadframe structure is segmented into functional zones: areas with support features for critical connector bumps and areas without for less critical connections. This segmentation allows the manufacturing process to handle different regions differently, applying the more complex support feature technology only where necessary to achieve stand-off height control, while keeping other areas simpler.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Minimizes the collapsing effect of connector bumps, ensuring uniform melting and maintaining the necessary stand-off height, thereby ensuring effective heat dissipation and device functionality.
Implementation Method 1
performing a reflow process on the leadframe, wherein the metallic solder material is melted to the top surface of the leadframe and formed into a plurality of support features
Implementation Method 2
support features that are configured to support a semiconductor die... maintaining the necessary stand-off height
Data Source
AI summary
A support feature on a leadframe to support a semiconductor die during placement of the die on the leadframe and minimize the collapsing effect of the connector bumps of the die after reflowing. In some embodiments, the support features are formed from material that is different from the leadframe, such as by a ball drop process or a plating process. In some embodiments, the support features are formed from the leadframe material, such as by etching. In some embodiments, the support features are covered with a coating material.