Flip-Chip Underfill Fillet Truncation for Smaller Keep-Out Zones

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Solution Overview

Problem

The existing flip-chip packaging method results in a large 'keep-out-zone' (KOZ) due to excess underfill material forming a fillet around the IC die, which occupies valuable substrate space and limits the placement of additional components.

Innovation Solution

A method involving the application of a release film and subsequent laser cutting of the underfill fillet to truncate it, reducing the underfill area and allowing for the placement of additional components closer to the IC die, thereby reducing the KOZ.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If underfill material is applied to strengthen die attachment and distribute thermal stresses, then reliability of solder joints is improved, but area occupied by underfill fillet increases, reducing available substrate space for additional components

Engineering Contradiction:

Improvesolder joint reliabilityVSAvoidsubstrate area

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The underfill fillet is segmented into two parts: a retained portion that provides the necessary reliability function (adhesion and stress distribution) and a removed portion that would otherwise occupy valuable substrate space. This segmentation allows the fillet to perform its protective function while minimizing its footprint on the substrate.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The excess portion of the underfill fillet is extracted from the final package structure through laser cutting and removal processes. This extraction eliminates the harmful effect of the fillet occupying substrate space while preserving the essential function provided by the retained fillet portion.

Inventive Principle:

Principle #2Taking out (Extraction)

2Strength

If underfill material flows beyond die edges to form a fillet, then adhesion between die and substrate is strengthened, but keep-out-zone size increases, limiting component placement density

Engineering Contradiction:

Improvedie-to-substrate adhesionVSAvoidcomponent placement density

Core Design Contradiction:

StrengthVSProductivity

Solution Approach 1:

The underfill fillet is treated differently in different locations: the portion adjacent to the die is retained to provide local adhesion strength where it is most needed, while the portion extending further onto the substrate is removed. This local differentiation optimizes both adhesion strength and component placement density.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

Instead of completely preventing underfill material from flowing beyond the die edges, the patent allows partial flow to occur (enough to form a useful fillet for adhesion) and then removes the excessive portion. This partial action approach achieves the necessary adhesion while minimizing the keep-out-zone.

Inventive Principle:

Principle #16Partial or excessive action

3Area of stationary object

If release film is applied and underfill fillet is laser cut to reduce underfill area, then substrate area for additional components is increased, but manufacturing process complexity increases

Engineering Contradiction:

Improveavailable substrate areaVSAvoidmanufacturing process complexity

Core Design Contradiction:

Area of stationary objectVSDevice complexity

Solution Approach 1:

A release film is introduced as an intermediary element that facilitates the selective removal of the underfill fillet. The release film acts as a temporary carrier for the fillet during the laser cutting process, enabling precise removal of excess material while simplifying the overall manufacturing process through standardized steps.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The release film is applied in advance before the underfill material is introduced, and the laser cutting is performed at a predetermined stage in the manufacturing process. This preliminary action and timing optimization allow for efficient integration of the fillet reduction step into the existing manufacturing workflow.

Inventive Principle:

Principle #10Preliminary action

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 significantly reduces the underfill area by up to 90%, enabling more components to be placed on the substrate and allowing for smaller package dimensions while maintaining the integrity of the IC die attachment.

Implementation Method 1

the underfill fillet is truncated (e.g., laser cut) after the underfill material is hardened or cured

Methodology Applied

Scientific EffectLaser cutting: Laser Ablation

Data Source

PatentUS11837476B2Flip-chip package with reduced underfill area

Publication Date: 2023.12.05 SANDISK TECHNOLOGIES LLC

AI summary

A flip-chip package and a method for assembling a flip-chip package includes positioning the die on a substrate and introducing an underfill material into a space between the die and the substrate, where a portion of the underfill material extends beyond an edge of the die and forms a fillet that at least partially surrounds the die. The underfill material is cured, and a portion of the fillet is removed to reduce the area of the fillet.