Flip Chip Solder Bump Reliability via Wetting Layer Barrier
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Solution Overview
Problem
The degradation of the under bump metallurgy (UBM) layer in flip chip technology due to reaction with tin from eutectic solder during reflow operations leads to solder joint failure, as the nickel in the UBM layer is consumed, causing cracking and reliability issues.
Innovation Solution
A solder bump with a frusto-conical or T-shaped profile and a wetting layer, such as gold, cobalt, copper, or nickel alloys, is used to prevent eutectic solder from reacting with the UBM layer, acting as a diffusion barrier and oxidation preventer, thereby reducing the need for wafer reflow and protecting the UBM layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If eutectic solder is used during substrate reflow, then the solder joint can be formed, but the tin from the eutectic solder reacts with the nickel in the UBM layer, consuming the nickel and degrading the UBM layer
Solution Approach 1:
A wetting layer is introduced as an intermediary between the eutectic solder and the UBM layer. This wetting layer preferentially reacts with the tin from the eutectic solder, preventing the tin from reaching and reacting with the nickel in the UBM layer. The wetting layer acts as a protective mediator that sacrifices itself to preserve the underlying UBM layer structure.
Solution Approach 2:
The wetting layer is applied in advance to the solder bump surface before the substrate reflow process. This preliminary action ensures that when the eutectic solder is applied, the wetting layer is already in position to prevent tin diffusion to the UBM layer, proactively solving the nickel consumption problem before it occurs.
2Reliability
If wafer reflow is performed to remove oxides from the solder bump, then the solder bump can be attached to substrate solder, but the high temperature causes tin to react with the UBM layer
Solution Approach 1:
The wetting layer serves as an intermediary that allows oxide removal during wafer reflow while preventing tin from the solder bump from reacting with the UBM layer. The wetting layer is positioned between the solder bump and UBM layer, mediating the thermal and chemical processes during reflow.
3Productivity
If multiple reflow operations are performed, then the solder joint can be assembled, but the repeated heating causes progressive degradation of the UBM layer
Solution Approach 1:
The wetting layer is applied beforehand to cushion and protect the UBM layer from repeated thermal and chemical stress during multiple reflow operations. This prior cushioning prevents progressive nickel consumption that would otherwise occur with each subsequent reflow cycle, extending the UBM layer's service life.
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 solution effectively prevents the consumption of the UBM layer's tin content, reducing the likelihood of solder joint failure by inhibiting eutectic solder flow and oxidation, thus enhancing the reliability of the solder joint during reflow operations.
Implementation Method 1
A solder bump with a frusto-conical or T-shaped profile and a wetting layer, such as gold, cobalt, copper, or nickel alloys, is used to prevent eutectic solder from reacting with the UBM layer, acting as a diffusion barrier
Implementation Method 2
acting as a diffusion barrier and oxidation preventer, thereby reducing the need for wafer reflow and protecting the UBM layer
Implementation Method 3
the eutectic solder 20 flows or wicks around the solder bump 12
Data Source
AI summary
In one embodiment, the invention provides a method comprising fabricating a die bump on a die, the die bump being shaped and dimensioned to at least reduce the flow of solder material used, to attach the die bump to a package substrate, towards an under bump metallurgy (UBM) layer located below the die bump. Advantageously, the method may comprise performing a substrate reflow operation to attach the package substrate to the die bump, without performing a separate wafer reflow operation to reflow the die bump.