Balanced Metal Density Substrate Design for Flip Chip

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Conventional Bump-On-Trace (BOT) structures in flip chip packages experience bridging and poor solder joints due to high density and thermal expansion, leading to reduced assembly yield, especially in peripheral areas where metal traces are thin and solder volume is small.

Innovation Solution

The design balances metal and solder resist densities on package substrates by controlling the metal density difference to less than 10% and solder resist density difference to less than 50%, reducing warpage and improving assembly yield through optimized layer configurations and materials.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If BOT structures are used with high density to reduce chip area and manufacturing cost, then productivity and cost efficiency are improved, but solder bridging and poor solder joints occur more frequently, reducing reliability

Engineering Contradiction:

Improveassembly yieldVSAvoidsolder joint quality

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent changes physical parameters including trace thickness, solder resist thickness, and reflow temperature profiles to optimize BOT structure performance. By adjusting these parameters, the patent achieves reliable solder joints even in high-density configurations where bridging and poor joints were previously common problems.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies different properties to different regions of the substrate. Specifically, it uses varying trace thicknesses, solder resist thicknesses, and material compositions in different areas to localize stress distribution and prevent both bridging and poor joint formation in high-density BOT structures.

Inventive Principle:

Principle #3Local quality

2Area of moving object

If metal traces are made thin to achieve high BOT structure density, then chip area is reduced and productivity is improved, but warpage increases and solder bridging occurs more frequently

Engineering Contradiction:

Improvechip areaVSAvoidsubstrate warpage

Core Design Contradiction:

Area of moving objectVSStability of the object's composition

Solution Approach 1:

The patent employs composite material structures including copper traces on polymer substrates, multi-layer constructions with alternating conductive and insulating layers, and combined metal-solder-resist systems. These composite structures provide both the thin-profile needed for high density and the mechanical stability to reduce warpage.

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The patent uses counterbalancing techniques where thicker trace sections or additional structural layers are strategically placed to compensate for warpage-prone areas. This creates a balanced stress distribution that counteracts the warping forces generated by thin, high-density trace configurations.

Inventive Principle:

Principle #8Anti-weight (Counterweight)

3Reliability

If solder resist thickness is increased to prevent bridging, then reliability is improved, but poor solder joints occur due to restricted solder access

Engineering Contradiction:

Improvebridging preventionVSAvoidsolder joint quality

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent applies varying solder resist thicknesses in different regions. Thicker solder resist is used in areas where bridging is a concern, while thinner or patterned solder resist is applied in areas requiring optimal solder access and joint formation. This localized variation resolves the contradiction between bridging prevention and joint quality.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent employs dynamic solder resist configurations that may include break patterns, varying thickness profiles, or selective removal in certain areas. This allows the solder resist to provide bridging prevention where needed while maintaining solder access and joint quality in critical bonding regions.

Inventive Principle:

Principle #15Dynamics

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 increases assembly yield by minimizing warpage and preventing solder bridging and poor joints, with assembly yields exceeding industry specifications when metal and solder resist density differences are carefully managed.

Implementation Method 1

the shift of the BOT structures caused by the thermal expansion of the metal traces is more significant than in the areas close to the centers of the respective packages

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentUS10128195B2Substrate design with balanced metal and solder resist density

Publication Date: 2018.11.13 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A package includes a package substrate, which includes a middle layer selected from the group consisting of a core and a middle metal layer, a top metal layer overlying the middle layer, and a bottom metal layer underlying the middle layer. All metal layers overlying the middle layer have a first total metal density that is equal to a sum of all densities of all metal layers over the middle layer. All metal layers underlying the middle layer have a second total metal density that is equal to a sum of all densities of all metal layers under the middle layer. An absolute value of a difference between the first total metal density and the second total metal density is lower than about 0.1.