Copper Pillar Solder Coating for Underfill Filler Separation
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Solution Overview
Problem
In high-density flip chip attach methods using copper pillar bumps, the filler in underfill materials separates during heat curing, leading to areas with no filler and subsequent crack development due to unabsorbed differences in coefficient of linear expansion between silicon chips and substrates.
Innovation Solution
Controlling the separation of filler in underfill materials by dispersing or agglomerating it homogeneously, and curing the underfill in that state, through methods such as coating copper pillars with solder, forming resist patterns of greater thickness, neutralizing the underfill, applying electric fields, or shortening curing periods to prevent filler separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If underfill material containing basic curing agent and inorganic filler is used to improve connection reliability, then reliability against thermal stress is improved, but filler separates during heat curing causing areas with no filler
Solution Approach 1:
The patent applies preliminary action by coating the copper pillar bumps with solder before injecting the underfill material. This solder coating creates a barrier that prevents the filler from separating during heat curing, ensuring uniform filler distribution is maintained from the beginning of the curing process. The solder layer is applied in advance to prevent the harmful separation effect that would otherwise occur during curing.
Solution Approach 2:
The patent uses solder as an intermediary substance between the copper pillar bumps and the underfill material. This intermediary solder layer prevents direct contact between the underfill components that would cause filler separation, while still allowing the underfill to perform its stress-absorbing function. The solder acts as a mediating layer that maintains filler distribution uniformity throughout the curing process.
2Stress or pressure
If filler is mixed in underfill material to absorb difference in coefficient of linear expansion, then stress absorption is improved, but filler agglomerates in resin during heat curing creating areas with no filler
Solution Approach 1:
The patent applies preliminary action by coating the copper pillar bumps with solder before injecting the underfill material. This solder coating creates a barrier that prevents the filler from separating during heat curing, ensuring uniform filler distribution is maintained from the beginning of the curing process. The solder layer is applied in advance to prevent the harmful separation effect that would otherwise occur during curing.
Solution Approach 2:
The patent uses solder as an intermediary substance between the copper pillar bumps and the underfill material. This intermediary solder layer prevents direct contact between the underfill components that would cause filler separation, while still allowing the underfill to perform its stress-absorbing function. The solder acts as a mediating layer that maintains filler distribution uniformity throughout the curing process.
3Productivity
If copper pillar bumps are used for high-density mounting, then mounting density and thermal conductivity are improved, but filler separation occurs during heat curing leading to crack development
Solution Approach 1:
The patent applies preliminary action by coating the copper pillar bumps with solder before injecting the underfill material. This solder coating creates a barrier that prevents the filler from separating during heat curing, ensuring uniform filler distribution is maintained from the beginning of the curing process. The solder layer is applied in advance to prevent the harmful separation effect that would otherwise occur during curing.
Solution Approach 2:
The patent uses solder as an intermediary substance between the copper pillar bumps and the underfill material. This intermediary solder layer prevents direct contact between the underfill components that would cause filler separation, while still allowing the underfill to perform its stress-absorbing function. The solder acts as a mediating layer that maintains filler distribution uniformity throughout the curing process.
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
Improves connection reliability by ensuring homogeneous filler distribution, reducing the likelihood of crack formation and enhancing the absorption of thermal and physical stresses.
Implementation Method 1
The coefficient of linear expansion of the silicon chip is 4 ppm/° C., and the coefficient of linear expansion of the substrate, e.g., a glass epoxy substrate, is 20 ppm/° C. The filler is mixed in the underfill material to absorb the difference in coefficient of linear expansion.
Implementation Method 2
at least coating the surfaces of the metal pillars with solder; and injecting underfill material containing filler into space between the electronic component and the substrate, and heating and curing the underfill material
Data Source
AI summary
In flip chip attach of electronic components, underfill is filled between the component and the substrate to alleviate, for example, thermal stress. In electronic component mounting using copper pillars conducted so far, filler contained in the underfill may cause separation in the process of heating and curing the resin. Disclosed is plating the surfaces of the copper pillars with solder. Mobilization of the filler charged in the underfill due to electric fields produced by local cells that are developed upon contact between dissimilar metals, is suppressed, and occurrence of crack at connection portions is obviated. Thus, connection reliability is increased.