Solder-Free Gold Bump Contacts for Thermal Stability
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Solution Overview
Problem
Gold bumps attached with solder experience significant electrical resistance increase or failure during temperature cycles, especially when exposed to high temperatures, leading to unreliable contacts in semiconductor devices.
Innovation Solution
Gold bumps are attached directly to copper contact pads with a thin nickel layer, using ultrasonic energy to create a stable gold-to-copper interdiffusion contact without solder, resulting in a nickel-free gold, copper, and tin alloy layer that maintains low electrical resistance even at elevated temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder is used to attach gold bumps to substrates, then initial electrical contact is established, but electrical resistance increases rapidly or contact fails after temperature cycles
Solution Approach 1:
The patent removes solder from the bonding interface between gold bumps and substrate, eliminating the solder layer that causes resistance increase and contact failure during temperature cycles. The direct gold-to-substrate contact without solder intermediary resolves the reliability issue.
Solution Approach 2:
The invention uses a composite metallurgical structure combining gold bump material with substrate metallization layers, creating a metallurgically bonded joint that maintains stable electrical properties across temperature variations, replacing the solder-based composite that failed under thermal stress.
2Reliability
If gold bumps are attached without solder using ultrasonic energy and nickel layer support, then contact stability is improved, but manufacturing process complexity increases
Solution Approach 1:
The patent introduces a nickel layer as an intermediary between the gold bump and copper substrate, which diffuses slightly into adjoining regions to create a stable metallurgical bond. This nickel intermediary enables reliable solder-free bonding while maintaining process feasibility.
Solution Approach 2:
The invention employs ultrasonic energy (mechanical vibration) to facilitate the bonding of gold bumps to the substrate without solder, using vibration to create intimate contact and promote metallurgical bonding at the interface, thereby achieving stable contacts through a controlled physical process.
3Ease of manufacture
If high temperatures up to 280°C are applied during assembly, then solder reflow is achieved, but gold bump dissolution or contact opening occurs
Solution Approach 1:
The patent eliminates solder from the high-temperature processing pathway, removing the material that dissolves gold bumps at elevated temperatures. By achieving bonding without solder, the process avoids the temperature range (260-280°C) that causes gold dissolution and contact failure.
Solution Approach 2:
The invention changes the bonding temperature parameters from traditional solder reflow temperatures (260-280°C) to lower ultrasonic bonding temperatures, thereby achieving reliable contact formation without exposing the gold bumps to temperatures that cause dissolution or contact opening.
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 solution achieves minimal change in electrical resistance (5-9%) after multiple temperature cycles up to 260°C, significantly outperforming conventional methods which often result in 5-10 fold resistance increase or complete contact failure.
Implementation Method 1
ultrasonic energy with suitable amplitude, time, and temperature is applied to create gold-to-gold interdiffusion contact
Implementation Method 2
the nickel layer diffused only slightly into the adjoining regions of gold and copper
Data Source
AI summary
A semiconductor device has a chip (101) with gold studs (212) assembled on a tape substrate (102), which has solder balls (103) for attachment to external parts. The tape substrate (about 30 to 70 μm thick) has on its first surface first copper contact pads (221) covered with a continuous thin nickel layer (222) of about 0.04 to 0.12 μm thickness. Gold including stud (212) is contacting the nickel. On the second substrate surface are second copper contact pads (231) covered with an alloy layer (about 2 to 3 μm thick) including gold, copper/tin alloys, and copper/nickel/tin alloys; the alloys are metallurgically attached to the second copper pad and substantially free of unalloyed nickel. A reflow body (103) comprising tin is metallurgically attached to the alloy layer of each second pad.