Conductive Paste via-Hole Fillability
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Solution Overview
Problem
Multilayer ceramic substrates face challenges in achieving dense packing of conductive pastes into small-diameter via-holes due to high sintering shrinkage and large aspect ratios, leading to poor fillability and increased resistance, which affects the reliability and precision of the substrates.
Innovation Solution
A conductive paste composition with 88-94% Ag powder and 0.1-3% Pd powder, along with an organic binder, is used, where Ag crystal particles grow to 25 µm or more during sintering, reducing porosity and ensuring good fillability into via-holes with diameters as small as 150 µm or less, while maintaining low resistivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If conventional conductive paste is used in small-diameter via-holes with high aspect ratios, then the via-holes can be formed with smaller diameters, but the fillability of conductive paste deteriorates leaving large space after sintering
Solution Approach 1:
The patent applies parameter changes by carefully controlling the particle size distribution of silver powder (combining fine particles ≤3μm for flowability with coarser particles for packing density) and adjusting the organic vehicle composition to achieve optimal paste viscosity. This enables the paste to adequately fill high aspect ratio via-holes while maintaining good electrical conductivity after sintering.
2Manufacturing precision
If shrinkage constraining method is used to suppress shrinkage in X-Y plane, then dimensional precision is improved, but via-holes and printing patterns before sintering should be smaller resulting in larger aspect ratios
Solution Approach 1:
The patent applies preliminary action by pre-forming via-holes and conductor patterns at appropriately scaled dimensions before sintering, taking into account the expected shrinkage behavior. The via-hole diameters and conductor pattern sizes are deliberately designed to be smaller in the green state to achieve the desired final dimensions after sintering, while the conductive paste composition is optimized to ensure adequate fillability despite the high aspect ratios.
3Productivity
If via-hole diameters are reduced to achieve finer via-conductors, then the number of elements can be increased, but the aspect ratios become larger making filling more difficult
Solution Approach 1:
The patent applies parameter changes by optimizing the conductive paste formulation with a specific particle size distribution of silver powder (combining fine particles ≤3μm for flowability into narrow via-holes with coarser particles for packing density). This enables adequate filling of high aspect ratio via-holes even with reduced via-hole diameters, allowing increased element density while maintaining manufacturing feasibility.
4Length of stationary object
If green sheets are made thicker to compensate for thickness reduction in shrinkage constraining method, then the targeted thickness after sintering can be achieved, but via-holes have larger aspect ratios resulting in more difficult filling
Solution Approach 1:
The patent applies preliminary action by pre-designing the via-hole dimensions and conductor paste application parameters before lamination and sintering. The via-holes are formed at dimensions that account for the thicker green sheet requirement in shrinkage-constraining methods, and the conductive paste composition is optimized to ensure adequate filling capability despite the resulting high aspect ratios.
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 provides multilayer ceramic substrates with high connection reliability and excellent dimensional precision by ensuring dense packing and low resistivity of via-conductors, even with small-diameter via-holes, thereby alleviating issues of shrinkage and void formation.
Implementation Method 1
Ag crystal particles grow to 25 µm or more during sintering, reducing porosity and ensuring good fillability into via-holes
Data Source
Figure 1(a)~2
Figure 3~4
AI summary
A conductive paste comprising 88-94% by mass of Ag powder having an average particle size of 3 µm or less and 0.1-3% by mass of Pd powder, the total amount of the Ag powder and the Pd powder being 88.1-95% by mass. A multilayer ceramic substrate obtained by laminating and sintering pluralities of ceramic green sheets, and having conductor patterns and via-conductors inside, the via-conductors being formed in via-holes having diameters of 150 µm or less after sintering, containing Ag crystal particles having a particle size of 25 µm or more, and having a porosity of 10% or less.