Stacked Semiconductor Package Alignment via Segmented Solder Reflow
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Solution Overview
Problem
In the semiconductor industry, there is a challenge in achieving high-reliability connections between stacked semiconductor packages, particularly in compact and high-performance devices, where misalignment during fabrication can lead to inefficient alignment and increased thermal stress due to differences in thermal expansion coefficients.
Innovation Solution
A method involving the use of dummy balls and solder balls on both packages, where the dummy balls are joined at a lower temperature to create a connection solder with increased surface tension, allowing for self-alignment, and subsequently, the solder balls are joined at a higher temperature to form a connection terminal, ensuring both physical and electrical connections while incorporating a dummy terminal to absorb thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If dummy balls and solder balls are joined simultaneously at the same temperature, then the fabrication process is simpler, but alignment precision deteriorates and thermal stress increases
Solution Approach 1:
The joining process is segmented into two distinct stages: first joining dummy balls at a lower first temperature, then joining solder balls at a higher second temperature. This segmentation allows each stage to serve a specific function - dummy ball alignment without premature solder reflow, followed by secure electrical connection, thereby resolving the contradiction between process simplicity and alignment precision.
Solution Approach 2:
The dummy balls are joined in advance at a lower temperature before the solder balls are joined. This preliminary action establishes precise alignment between upper and lower packages through the dummy ball connection, creating a stable foundation that prevents misalignment during subsequent solder ball joining at higher temperature.
2Loss of time
If dummy balls and solder balls are joined at the same temperature, then the process time is reduced, but thermal stress increases due to simultaneous heating
Solution Approach 1:
The thermal process is segmented into two temperature stages: a first lower temperature for dummy ball joining, and a second higher temperature for solder ball joining. This segmentation controls the thermal history of the materials, allowing the dummy ball material to solidify and establish alignment before the solder material is heated to its reflow temperature, thereby minimizing thermal stress while maintaining reasonable process time.
Solution Approach 2:
The process utilizes parameter changes by varying the temperature between two distinct joining stages. The first temperature is controlled to be below the reflow temperature of the solder ball material but sufficient to join the dummy balls. The second temperature is raised to the solder reflow temperature for electrical connection. This parameter control resolves the contradiction between process time and thermal stress.
3Strength
If higher temperature is used for joining both dummy and solder balls, then connection strength is improved, but misalignment risk increases
Solution Approach 1:
The dummy balls are joined at a lower temperature in advance to establish precise alignment between the upper and lower packages. This preliminary alignment action occurs before high temperature is applied, ensuring that when the solder balls are subsequently joined at higher temperature for strong connection, the packages are already properly aligned, thus avoiding misalignment risk.
Solution Approach 2:
The dummy balls serve as an intermediary element that facilitates alignment between the upper and lower packages. By joining the dummy balls first at lower temperature, they act as a mediator that establishes the correct positional relationship, which then guides the subsequent solder ball joining process at higher temperature, ensuring both alignment accuracy and connection strength.
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
This method enables high-reliability connections between semiconductor packages by aligning the packages through surface tension and forming a connection terminal that resists thermal stress, improving the integration density and accuracy of semiconductor devices.
Implementation Method 1
liquefying the dummy members such that surface tension is created at surfaces of the liquefied dummy members
Implementation Method 2
forming internal electrical connections of the PoP including by reflowing the solder
Data Source
AI summary
A method of fabricating a semiconductor package includes providing a lower semiconductor package including a lower package substrate, and a lower dummy ball and a lower solder ball on a top surface of the lower package substrate, providing an upper semiconductor package including an upper package substrate, and an upper dummy ball and an upper solder ball on a bottom surface of the upper package substrate, joining the upper dummy ball to the lower dummy ball at a first temperature to form a solder joint, and joining the upper solder ball to the lower solder ball at a second temperature to form a connection terminal.