Lead-Free Solder Joining with Segmented Thermal Profiles
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The challenge in microelectronics is to reduce thermal exposure during the joining of semiconductor devices to laminate substrates and avoid remelting of earlier joined components, which can lead to defects such as chip package interactions, solder extrusion, and solder spitting, especially with the introduction of lead-free solders that have a reduced delta in melting points and limited options for controlled plating.
Innovation Solution
The method involves forming underbump metallurgy on semiconductor devices with lead-free solder alloys comprising at least 85% bismuth and tin, and on substrates with tin-bismuth-silver alloys, to create high melting temperature solder bumps and pads that reflow and join at specific temperatures, ensuring a stable and strong bond while minimizing thermal exposure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If lead-free solder with reduced melting point delta is used, then thermal exposure can be reduced, but the ability to control plating and availability of options on substrate side is limited
Solution Approach 1:
The patent segments the soldering process into two distinct stages: first forming high melting temperature solder bumps (≥260°C) on the semiconductor device, then joining them to low melting temperature solder pads (≤230°C) on the substrate. This segmentation allows optimization of each stage independently, achieving both reduced thermal exposure and controlled plating.
Solution Approach 2:
The patent applies different solder compositions to different locations: high Bi content solder (≥85% Bi) for bumps requiring high melting temperature, and lower Bi content solder with Ag (56-58% Bi, 0.5-1.5% Ag) for pads requiring low melting temperature. This local differentiation resolves the contradiction between thermal exposure reduction and plating control.
2Strength
If high melting temperature solder is used for joining, then joint strength is improved, but earlier joined components may be remelted causing defects
Solution Approach 1:
The patent divides the joining process into two temperature stages: first creating high melting temperature solder bumps (≥260°C) that provide strong mechanical bonding, then joining to low melting temperature pads (≤230°C) that prevent remelting of previously attached components. This temporal and thermal segmentation resolves the contradiction between joint strength and defect prevention.
Solution Approach 2:
The high melting temperature solder bumps are formed and allowed to cool and solidify before the low melting temperature pad solder is applied and reflowed. This preliminary action ensures that when the final join occurs at lower temperature, the bumps remain solid and provide structural support without remelting, preventing defects while maintaining strength.
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 raises the liquidus temperature of solder bumps and pads, providing a strong and reliable joint with reduced thermal exposure, enhancing electromigration performance and simplifying rework processes by maintaining solder mass on the correct side during rework.
Implementation Method 1
reflowing the lead free solder at 240-260° C. so that the lead free solder reflows to form solder bumps
Implementation Method 2
react with the underbump metallurgy to raise the liquidus temperature of the solder bumps by incorporating the metal from the underbump metallurgy
Implementation Method 3
heating at 240 to 260° C. the lead free solder ball and the high melting temperature lead free solder pad to cause the lead free solder ball and the high melting temperature lead free solder pad to join
Data Source
AI summary
A method and structure for joining a semiconductor device and a laminate substrate or two laminate substrates where the joint is formed with lead free solders and lead free compositions. The various lead free solders and lead free compositions are chosen so that there is a sufficient difference in liquidus temperatures such that some components may be joined to, or removed from, the laminate substrate without disturbing other components on the laminate substrate.