Solderless Silver Die Attach on Direct Bonded Aluminum Substrates

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Solution Overview

Problem

DBA-based power devices face failures in severe temperature cycling due to thermal expansion issues and the lack of solderability of aluminum substrates, which existing technologies have not adequately addressed.

Innovation Solution

A solderless silver-to-silver die attach process is implemented using silver nanoparticle paste deposited onto a DBA substrate, sintered to form a sintered silver feature, and then ultrasonically welded to a semiconductor die, eliminating the need for soft solder and enhancing thermal stability.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If aluminum plates are direct bonded onto ceramic substrate to form DBA substrate, then resistance to cracking and lift-off failures under thermal power cycling is improved, but solderability of the substrate deteriorates

Engineering Contradiction:

Improveresistance to cracking and lift-off failuresVSAvoidsolderability

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

A thin layer of solderable metal (copper, nickel, silver, or gold) is deposited onto the top surface of the aluminum plate to serve as an intermediary layer. This mediator provides solderability while the aluminum substrate maintains its superior thermal expansion matching properties, resolving the contradiction between reliability under thermal cycling and ease of soldering.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The DBA substrate structure combines aluminum (for thermal expansion matching and mechanical reliability) with a solderable metal layer (for ease of manufacturing and soldering). This composite structure integrates the advantages of both materials to simultaneously achieve high reliability and manufacturability.

Inventive Principle:

Principle #40Composite materials

2Ease of manufacture

If soft solder is used to attach semiconductor die to DBA substrate, then ease of manufacture is improved, but thermal fatigue failures under severe temperature cycling increase

Engineering Contradiction:

Improveease of die attachVSAvoidresistance to thermal fatigue failures

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The die attach method changes from soft solder (low melting point, poor thermal fatigue resistance) to silver nanoparticle paste sintering (high melting point, excellent thermal fatigue resistance). This parameter change in the attach material's thermal and mechanical properties eliminates thermal fatigue failures while maintaining manufacturing feasibility through controlled sintering processes.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The mechanical bonding method is replaced by sintering silver nanoparticles to form a metallurgical bond. This substitution creates a stronger, more thermally stable connection that resists thermal fatigue, replacing the conventional soldering mechanism with a superior bonding approach.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If silver nanoparticle paste is sintered to form sintered silver feature on DBA substrate, then reliability under severe temperature cycling is improved, but device complexity increases

Engineering Contradiction:

Improvethermal stabilityVSAvoidmanufacturing process complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The manufacturing process is segmented into distinct, manageable stages: depositing silver nanoparticle paste, sintering to form conductive features, and attaching the semiconductor die. This segmentation allows each step to be optimized and controlled independently, reducing overall process complexity while achieving high reliability.

Inventive Principle:

Principle #1Segmentation

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 prevents thermal fatigue failures by providing a strong, solderless bond that maintains integrity under extreme temperature cycling, avoiding cracking and lift-off failures, and ensuring reliable operation from -55°C to +150°C.

Implementation Method 1

The silver nanoparticle paste is sintered to form a sintered silver feature (also called a sintered silver structure or a sintered silver layer)

Methodology Applied

Scientific EffectSintering: Sintering

Implementation Method 2

A DBA substrate is then physically attached (for example, is ultrasonically welded) to a lead of a leadframe

Methodology Applied

Scientific EffectUltrasonic welding: Ultrasonic Vibration

Implementation Method 3

the silver layer of the semiconductor die fuses to the sintered silver layer of the DBA substrate

Methodology Applied

Scientific EffectFusion bonding: Melting

Data Source

PatentUS9111782B2Solderless die attach to a direct bonded aluminum substrate

Publication Date: 2015.08.18 LITTELFUSE INC

AI summary

A DBA-based power device includes a DBA (Direct Bonded Aluminum) substrate. An amount of silver nanoparticle paste of a desired shape and size is deposited (for example by micro-jet deposition) onto a metal plate of the DBA. The paste is then sintered, thereby forming a sintered silver feature that is in electrical contact with an aluminum plate of the DBA. The DBA is bonded (for example, is ultrasonically welded) to a lead of a leadframe. Silver is deposited onto the wafer back side and the wafer is singulated into dice. In a solderless silver-to-silver die attach process, the silvered back side of a die is pressed down onto the sintered silver feature on the top side of the DBA. At an appropriate temperature and pressure, the silver of the die fuses to the sintered silver of the DBA. After wirebonding, encapsulation and lead trimming, the DBA-based power device is completed.