Re-workable Underfill for WLCSP Solder Ball Stress Reduction

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Solution Overview

Problem

As larger dies are bonded to printed circuit boards (PCBs) in wafer-level chip scale packages (WLCSP), the stresses on solder balls increase, and existing methods to reduce these stresses do not allow for re-workability, making it impossible to replace defective dies once bonded, leading to potential PCB failure.

Innovation Solution

A re-workable underfill is selectively dispensed into corner and edge regions between the die and PCB, protecting solder balls from high stresses while avoiding contact with the die to enable easy removal and replacement of defective dies, with the underfill being easily removable using solvents.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If underfill is applied to protect solder balls from stress, then solder ball reliability is improved, but re-workability deteriorates because defective dies cannot be removed from PCB

Engineering Contradiction:

Improvesolder ball reliabilityVSAvoidre-workability

Core Design Contradiction:

ReliabilityVSEase of repair

Solution Approach 1:

The patent applies underfill selectively only in corner regions between the die and PCB, rather than filling the entire gap. This localized application provides stress protection to solder balls where it is most needed (at the corners experiencing highest stress) while leaving other areas open, thereby maintaining re-workability for defective die replacement.

Inventive Principle:

Principle #3Local quality

2Area of moving object

If larger dies are bonded to PCB, then device functionality is improved, but stress on solder balls increases

Engineering Contradiction:

Improvedie areaVSAvoidsolder ball stress

Core Design Contradiction:

Area of moving objectVSStress or pressure

Solution Approach 1:

The patent applies underfill in advance to corner regions before the die is fully bonded to the PCB. This preliminary protective measure counteracts the high stresses that will occur during and after bonding, particularly at the corner solder balls that experience the most stress during thermal cycling and mechanical loading.

Inventive Principle:

Principle #9Preliminary anti-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

The selective application of re-workable underfill reduces stress on solder balls by up to 52% and allows for the replacement of defective dies without damaging the PCB, thereby extending the lifetime of the WLCSP and maintaining the re-workability of the bonding process.

Implementation Method 1

the underfill being easily removable using solvents

Methodology Applied

Scientific EffectSolvation: Solvation

Data Source

PatentUS9240387B2Wafer-level chip scale package with re-workable underfill

Publication Date: 2016.01.19 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A package includes a printed circuit board (PCB), and a die bonded to the PCB through solder balls. A re-workable underfill is dispensed in a region between the PCB and the die.