Thick Polymer Layer for Solder Ball Stress Buffering
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Solution Overview
Problem
Current wafer level chip scale packaging (WLCSP) technologies are limited by the absence of underfill, which restricts die size due to thermal mismatch issues between the die and the PCB, leading to solder joint cracks during thermal cycling, necessitating flip-chip packaging for larger dies.
Innovation Solution
A post-passivation structure with a polymer layer thicker than 30 μm is introduced, acting as a stress buffer and increasing the stand-off distance between the die and the package component, reducing stress on solder balls and integrated circuit devices, while maintaining the die size by using a polymer layer with a thickness between 30 μm and 200 μm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If no underfill is used between WLCSP and PCB, then ease of manufacture is improved and cost is reduced, but reliability deteriorates due to solder joint cracks from thermal mismatch
Solution Approach 1:
The patent introduces a polymer layer as an intermediary substance between the die and PCB, functioning similarly to underfill but without requiring the complex underfill application process. This polymer layer absorbs thermal stress and protects solder joints from cracking during thermal cycling, thereby improving reliability while maintaining manufacturing simplicity.
Solution Approach 2:
The patent changes the thickness parameter of the polymer layer to greater than 30 μm, which is significantly thicker than conventional polymer layers. This parameter change enhances the stress-buffering capability of the polymer layer, allowing it to effectively compensate for thermal mismatch between the die and PCB, thus improving solder joint reliability without requiring underfill.
2Reliability
If polymer layer thickness is increased to protect solder joints, then reliability is improved, but device complexity increases
Solution Approach 1:
The polymer layer serves multiple functions simultaneously: it acts as a stress buffer to protect solder joints from thermal mismatch, provides mechanical support for the die, and maintains the electrical insulation between conductive elements. By consolidating these functions into a single layer, the patent improves reliability without proportionally increasing device complexity.
3Area of moving object
If die size is increased for larger applications, then functionality is improved, but reliability deteriorates due to increased thermal mismatch stress
Solution Approach 1:
The patent applies beforehand cushioning by introducing a thick polymer layer (>30 μm) that预先 buffers the thermal stress before it reaches the solder joints. This cushioning effect is particularly important for larger dies where thermal mismatch stress is more significant, allowing larger die sizes to be used without compromising solder joint durability.
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 solution allows larger die sizes to pass thermal cycle tests, as demonstrated by dies of 7 mm×7 mm successfully passing 500 thermal cycles with polymer layer thicknesses greater than 30 μm, whereas thinner layers fail, indicating enhanced reliability and durability.
Implementation Method 1
A post-passivation structure with a polymer layer thicker than 30 μm is introduced, acting as a stress buffer and increasing the stand-off distance between the die and the package component
Data Source
AI summary
An integrated circuit structure includes a substrate and a metal pad over the substrate. A post-passivation interconnect (PPI) line is connected to the metal pad, wherein the PPI line includes at least a portion over the metal pad. A PPI pad is connected to the PPI line. A polymer layer is over the PPI line and the PPI pad, wherein the polymer layer has a thickness greater than about 30 μm. An under-bump metallurgy (UBM) extends into an opening in the polymer layer and electrically connected to the PPI pad.