Polyimide Layer Thinning Between Metal Pillars to Prevent Voids
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Solution Overview
Problem
In semiconductor packages with closely spaced metal pillars, the application of a polyimide layer followed by a mold compound often results in void formation due to the limited space between pillars, especially when large-filler mold compounds are used, leading to inadequate mold compound flow and increased void formation.
Innovation Solution
A modified reticle with specific light transmittance areas is used during photolithography to thin the polyimide layer between closely spaced metal pillars, allowing for adequate mold compound flow and minimizing void formation, even with large-filler mold compounds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a polyimide layer is applied to cover metal pillars with small pitch, then the polyimide layer thickness increases, but mold compound flow is restricted and void formation increases
Solution Approach 1:
The patent applies different polyimide layer thicknesses to different regions: a first thickness over the metal pillars and a second, thinner thickness in the regions between pillars. This local variation allows the mold compound to flow adequately between closely spaced pillars while maintaining sufficient polyimide coverage on the pillars themselves, thereby preventing void formation without restricting mold flow.
2Productivity
If metal pillars are placed closely together to increase terminal density, then the package capacity increases, but mold compound flow is blocked and voids form
Solution Approach 1:
The patent creates a local quality variation in the polyimide layer thickness, making it thinner in the inter-pillar regions to facilitate mold compound flow while maintaining adequate thickness on the pillars for proper coverage and protection.
3Reliability
If a thick polyimide layer is used to ensure adequate coverage, then pillar protection is improved, but mold compound flow is restricted
Solution Approach 1:
The patent implements a polyimide layer with spatially varying thickness, where the first thickness provides adequate pillar coverage and protection, while the second, thinner thickness in the regions between pillars allows sufficient mold compound flow, thus resolving the contradiction between coverage and flowability.
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 technique effectively mitigates void formation by ensuring the mold compound has sufficient space to flow freely, resulting in a more reliable and void-free semiconductor package, especially in applications with small pitch between metal pillars.
Implementation Method 1
A modified reticle with specific light transmittance areas is used during photolithography to thin the polyimide layer between closely spaced metal pillars
Data Source
AI summary
In some examples, a semiconductor package comprises a semiconductor die including circuitry, a first metal pillar coupled to the circuitry and extending away from the semiconductor die, and a second metal pillar coupled to the circuitry and extending away from the semiconductor die. A distance between the first and second metal pillars does not exceed 100 microns. The package also comprises a polyimide layer covering portions of the first and second metal pillars and covering a region between the first and second metal pillars. The polyimide layer in the region between the first and second metal pillars has a thickness not exceeding 15 microns and lacks a boundary between separate applications of polyimide to the region. The package also includes a mold compound covering the polyimide layer in the region between the first and second metal pillars. The package further includes conductive terminals coupled to the first and second metal pillars and exposed to an exterior of the mold compound.