STIM Lid Attach Using Low-Temperature Solder for Void-Free IMLs
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional STIM lid attach processes in IC packages result in voids and non-homogeneous intermetallic layers due to flux use, leading to weakened mechanical properties and impaired thermal performance.
Innovation Solution
Employ a low-temperature solder (LTS) paste instead of flux, using a low-temperature reflow process to form intermetallic layers without melting the solder preform, ensuring a thin, continuous, and void-free solder joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional flux and reflow process are used to attach STIM lid, then solder preform is melted and IMLs are formed, but voids are created in the IMLs or STIM bulk due to flux volatiles and non-homogeneous contour match
Solution Approach 1:
The patent changes the temperature parameter of the reflow process to be below the melting point of the solder preform, transforming the process from a high-temperature melt-reflow to a low-temperature solid-state bonding process. This parameter change prevents flux volatile generation and eliminates void formation while still achieving IML formation through solid-state diffusion.
Solution Approach 2:
The patent extracts and removes the flux material from the bonding process entirely, replacing it with a fluxless low-temperature reflow process. This elimination of flux prevents the generation of volatile substances that cause voids in the intermetallic layers and solder bulk.
2Manufacturing precision
If fluxless formic acid reflow is used to avoid voids, then void formation is reduced, but process complexity increases and safety concerns arise
Solution Approach 1:
The patent employs conventional, well-established low-temperature reflow processes that are already widely used in the industry, rather than introducing complex new fluxless formic acid systems. This approach achieves void-free bonding using simple, inexpensive, and safe processes that require minimal special equipment or safety infrastructure.
3Manufacturing precision
If polymer TIM is used to avoid voids, then void formation is prevented, but thermal performance decreases and bond line thickness must be very thin
Solution Approach 1:
The patent creates a composite structure consisting of the solder preform material combined with formed intermetallic layers through low-temperature reflow. This composite approach maintains the superior thermal conductivity of solder materials while achieving void-free bonding through the controlled formation of thin, homogeneous IMLs at the interfaces.
4Reliability
If high temperature reflow is used to melt solder preform, then IMLs are formed, but energy consumption increases
Solution Approach 1:
The patent fundamentally changes the temperature parameter of the reflow process from high-temperature (above solder preform melting point) to low-temperature (below melting point). This parameter change enables IML formation through solid-state diffusion rather than melting, dramatically reducing energy consumption while eliminating flux volatile generation and 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
The LTS-STIM lid attach process enhances thermal performance by eliminating voids and achieving a homogeneous intermetallic layer, improving mechanical stability and reducing energy consumption.
Implementation Method 1
a low-temperature reflow process to melt the LTS paste
Implementation Method 2
the LTS paste at temperatures (e.g., below 150° C.), which is lower than both the melting point of the solder preform and traditional solder pastes. The low-temperature reflow process forms two separate solder joints that connect the die and the lid to the non-melted solder preform
Data Source
AI summary
An integrated circuit package may include a die and a lid over the die. An solder preform core is between the die and the lid. A first intermetallic layer (IML) is between the die and the solder preform and a second intermetallic layer (IML) is between the lid and the solder preform, wherein the first IML and the second IML comprise low-temperature solder (LTS) material.