Pre-Reflowed Flip Chip Solder Joints With Low Void Density

Solution Overview

Problem

Conventional flip chip package solder joints often have high void densities due to trapped volatile components during the reflow process, leading to reduced reliability and strength, as well as potential cracking under stress.

Innovation Solution

A two-step solder reflow process is employed, where pre-reflowed solder is applied to the package substrate before mounting the IC die, allowing for easier removal of volatile components and forming a solder joint with a void density of ≤5% of the cross-sectional area, using a lead-free SAC305 solder composition and varying solder bump sizes and shapes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional single-step reflow process is used to form solder joints, then the manufacturing process is simple and fast, but the solder joints have high void densities leading to reduced reliability and strength

Engineering Contradiction:

Improvesolder joint reliabilityVSAvoidreflow process complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The soldering process is divided into two separate steps: a pre-reflow step that forms initial solder joints with low void density, and a final reflow step that completes the bonding. This segmentation allows each step to be optimized independently, with the pre-reflow step focusing on void reduction and the final step focusing on complete joint formation, thereby resolving the contradiction between reliability and process complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A pre-reflow step is performed before the final reflow process to pre-form the solder joints. This preliminary action removes volatile components and reduces void formation in advance, so that when the final reflow occurs, the solder joints already have low void density. This preliminary intervention directly addresses the reliability issue without requiring complete process redesign.

Inventive Principle:

Principle #10Preliminary action

2Manufacturing precision

If solder is applied directly during the final reflow step, then the process is simpler, but volatile components become trapped creating high void densities

Engineering Contradiction:

Improvesolder joint void densityVSAvoidsoldering process simplicity

Core Design Contradiction:

Manufacturing precisionVSEase of manufacture

Solution Approach 1:

The soldering process is divided into two separate steps: a pre-reflow step that forms initial solder joints with low void density, and a final reflow step that completes the bonding. This segmentation allows each step to be optimized independently, with the pre-reflow step focusing on void reduction and the final step focusing on complete joint formation, thereby resolving the contradiction between reliability and process complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A pre-reflow step is performed before the final reflow process to pre-form the solder joints. This preliminary action removes volatile components and reduces void formation in advance, so that when the final reflow occurs, the solder joints already have low void density. This preliminary intervention directly addresses the reliability issue without requiring complete process redesign.

Inventive Principle:

Principle #10Preliminary action

3Strength

If thin bond line thickness is used in solder joints, then the assembly is more compact, but the mechanical strength and electrical properties are reduced

Engineering Contradiction:

Improvesolder joint strengthVSAvoidbond line thickness

Core Design Contradiction:

StrengthVSLength of moving object

Solution Approach 1:

The pre-reflow process changes the physical and chemical parameters of the solder, creating a optimized microstructure with reduced voids and improved metallurgical bonding. This parameter change allows the solder joint to achieve higher strength at greater thicknesses than conventional single-step reflow, effectively resolving the contradiction between strength and compactness by showing that thickness can be increased without sacrificing strength when void density is reduced.

Inventive Principle:

Principle #35Parameter changes

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 solder joint voids, enhances joint reliability, and allows for thicker bond line thickness, improving the mechanical and electrical properties of the solder joints.

Implementation Method 1

a first reflow process forms the pre-reflowed solder on the bondable features of the package substrate

Methodology Applied

Scientific EffectReflow:

Implementation Method 2

a second reflow process forms a solder joint between the bondable features of the package substrate and the metal posts of the IC die

Methodology Applied

Scientific EffectMelting: Melting

Data Source

PatentUS20250022841A1Semiconductor device assembly with pre-reflowed solder

Publication Date: 2025.01.16 TEXAS INSTRUMENTS INC

AI summary

A semiconductor device assembly includes a package substrate having a top side including a plurality of bondable features, at least one integrated circuit (IC) die including a substrate having at least a semiconductor surface including circuitry configured for realizing at least one function including nodes coupled to bond pads with metal posts on the bond pads. The metal posts are attached by a solder joint to the bondable features. The solder joint has a void density of less than or equal to (≤) 5% of a cross-sectional area of the solder joint.