Multi-Layer Encapsulant Prefill to Prevent Trace Voids
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Solution Overview
Problem
In semiconductor device manufacturing, the reflowing process of wafer-level underfills often results in voids between metal traces, leading to electrical failures due to trapped air/gases and uneven flow, causing shorting and leakage.
Innovation Solution
A prefill material with a lower viscosity than the wafer-level underfill is applied directly to the substrate and trace peripheral surfaces before attachment, ensuring complete contact and reducing void formation by controlling the flow and attachment process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If wafer-level underfill is reflowed to fill space between die and substrate, then encapsulation is improved, but voids form between traces causing electrical failure
Solution Approach 1:
The patent applies underfill material to the substrate surface before attaching the die, allowing the material to be in place and ready to receive the die and traces. This preliminary application ensures that when the die is attached and reflowed, the underfill flows uniformly without forming voids between traces, as the material is already positioned correctly.
Solution Approach 2:
The patent controls the viscosity of the underfill material by temperature control during the reflow process. By heating the underfill to a specific temperature range, the viscosity decreases allowing complete flow between traces without trapping air. After cooling, the viscosity increases to maintain the filled state without voids.
2Ease of operation
If underfill viscosity is reduced to improve flow, then encapsulation coverage is improved, but air/gases become trapped causing voids
Solution Approach 1:
The patent utilizes phase transition of the underfill material from solid to liquid state during reflow heating. The material transitions to a low-viscosity liquid phase that flows completely between traces, then transitions back to solid phase upon cooling, trapping no air or gases. This controlled phase transition ensures complete filling without void formation.
Solution Approach 2:
The patent employs a periodic heating and cooling cycle during the reflow process. The heating phase reduces viscosity to enable complete flow, while the cooling phase increases viscosity to lock the material in place. This periodic temperature variation ensures thorough filling followed by stabilization without trapped air.
3Manufacturing precision
If reflow temperature is increased to reduce viscosity, then flow uniformity is improved, but air/gases are trapped causing voids
Solution Approach 1:
The patent maintains continuous heating throughout the reflow process to ensure the underfill material remains at optimal temperature for uniform flow. The continuous thermal energy input prevents localized cooling that would cause uneven viscosity and air trapping, ensuring consistent flow characteristics throughout the entire encapsulation area.
Solution Approach 2:
The patent introduces a fluxing agent as an intermediary substance mixed with the underfill material. This fluxing agent reduces surface tension and promotes wetting of the traces, allowing air bubbles to escape during flow rather than being trapped. The intermediary substance facilitates uniform flow while preventing void formation.
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 approach enhances the reliability of semiconductor devices by minimizing voids and reducing failure rates through precise application and attachment of the prefill material, ensuring better electrical connectivity.
Implementation Method 1
A prefill material with a lower viscosity than the wafer-level underfill is applied directly to the substrate and trace peripheral surfaces before attachment, ensuring complete contact and reducing void formation by controlling the flow and attachment process
Implementation Method 2
The attachment process can include reforming or reflowing the wafer-level underfill 130 (e.g., such as by controlling the temperature thereof to change its viscosity level), which travels downward as illustrated in FIG. 1 to fill a space between the die 110 and the substrate 102
Data Source
AI summary
A semiconductor device includes a substrate including traces, wherein the traces protrude above a top surface of the substrate; a prefill material over the substrate and between the traces; a die attached over the substrate; and a wafer-level underfill between the prefill material and the die.