Sn-Bi Core Material Structure for Oxidation-Stable Bump Electrodes
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Solution Overview
Problem
Existing solder bumps with Bi concentration gradients in the solder plating layer can lead to defects such as oxidation, increased oxide film thickness, and misalignment during substrate reflow, causing short circuits and visual inspection failures due to uneven Bi distribution and volume expansion issues.
Innovation Solution
A core material with a (Sn-Bi)-based solder alloy and a Sn layer on the outer side, where the Bi concentration is uniformly distributed between 90% to 108.6% and the Sn layer thickness is 0.1 μm to 12 μm, with a Sn layer thickness of 0.215% to 36% of the solder layer, preventing oxide film thickness increase and ensuring uniform Bi distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If Bi concentration is high on the inner circumferential side of the solder plating layer, then the solder begins to melt from the inner side first, but the pressure difference causes the Cu ball to burst open
Solution Approach 1:
The patent applies local quality by creating a specific Bi concentration distribution in the solder plating layer where the Bi content is higher on the outer circumferential side and lower on the inner circumferential side. This non-uniform local composition ensures that the outer Bi-rich region melts first, generating upward pressure that prevents the inner Bi-rich region from causing core bursting, thus resolving the contradiction between melting uniformity and core integrity.
Solution Approach 2:
The patent changes the concentration parameter of Bi in the solder plating layer from a uniform distribution to a gradient distribution with specific concentration ratios. By controlling the Bi concentration ratio between inner and outer regions to be within 0.95-1.05, the patent achieves controlled melting behavior that prevents core bursting while maintaining reliable solder joint formation.
2Reliability
If Bi is distributed with concentration gradient in the solder plating layer, then melting behavior is affected, but oxidation increases and oxide film thickness increases causing visual inspection failures
Solution Approach 1:
The patent changes the Bi concentration parameter in the solder plating layer to a controlled gradient distribution with concentration ratios between 0.95-1.05. This parameter optimization ensures sufficient Bi content for reliable melting and bonding while preventing excessive oxidation that would cause visual inspection failures, thus resolving the contradiction between joint reliability and oxidation resistance.
3Area of stationary object
If solder ball is used to prevent short circuit, then mounting area is reduced, but the solder ball may be crushed due to weight causing electrode contact and short circuit
Solution Approach 1:
The patent uses a composite structure consisting of a Cu ball core providing mechanical strength and weight support, combined with a solder plating layer providing melting and bonding functionality. This composite material design allows the solder bump to withstand package weight without crushing while maintaining the ability to melt and form reliable electrical connections, resolving the contradiction between mounting area reduction and electrode isolation reliability.
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 prevents defects like misalignment and short circuits by maintaining uniform Bi distribution and reducing oxide film thickness, ensuring reliable solder bump formation and visual inspection pass rates.
Implementation Method 1
the surface of the solder plating layer contains a relatively larger amount of Bi, oxidation tends to proceed easily, and an oxide film thickness tends to increase
Implementation Method 2
When the semiconductor package obtained with the three-dimensional high-density mounting is the BGA and the solder ball is placed on the electrode of the semiconductor chip and reflowed
Data Source
AI summary
A core material has a core 12; a solder layer 16 made of a (Sn—Bi)-based solder alloy provided on an outer side of the core 12; and a Sn layer 20 provided on an outer side of the solder layer 16. The core contains metal or a resin. When a concentration ratio of Bi contained in the solder layer 16 is a concentration ratio (%)=a measured value of Bi (% by mass)/a target Bi content (% by mass), or a concentration ratio (%)=an average value of measured values of Bi (% by mass)/a target Bi content (% by mass), the concentration ratio is 91.4% to 106.7%. The thickness of the Sn layer 20 is 0.215% or more and 36% or less of the thickness of the solder layer 16.