PCB Solder Resist Opening Structure for Undercut Reduction
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Solution Overview
Problem
Conventional circuit boards face issues with solder ball separation from pads due to insufficient light transmission and curing in solder resist openings, leading to undercut formation and reduced reliability, especially in high-density PCBs.
Innovation Solution
A circuit board design with a first protective layer divided into portions, where the second portion has a wider opening and a recessed side wall, reducing the horizontal distance of the undercut and minimizing the distance between circuit patterns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the opening region width of the solder resist is reduced to improve I/O performance and accommodate smaller bump pitch, then the electrical connection density is improved, but the solder ball joint reliability deteriorates due to insufficient light transmission and curing
Solution Approach 1:
The solder resist layer is divided into multiple regions (first region, second region, third region) with different opening widths. The first region has a wider opening for reliable solder ball connection, while the second and third regions have narrower openings for high-density I/O performance. This segmentation allows different functional requirements to be satisfied in different areas of the same circuit board.
Solution Approach 2:
Different regions of the solder resist are assigned different local properties (opening widths) according to their specific functional requirements. The first region uses a wider opening configuration optimized for solder ball joint reliability, while the second and third regions use narrower openings optimized for I/O performance and bump pitch reduction.
2Strength
If the solder resist layer thickness is increased to improve protection and insulation, then the protective function is improved, but the undercut width increases reducing circuit board reliability
Solution Approach 1:
The solder resist layer is divided into multiple sub-layers (first solder resist layer, second solder resist layer, third solder resist layer) with different thicknesses. The first layer has greater thickness for adequate protection, while the second and third layers have reduced thickness to minimize undercut formation during development, thus maintaining both protective function and reliability.
Solution Approach 2:
Different thicknesses are assigned to different regions of the solder resist layer based on local requirements. The first region maintains greater thickness for protection, while the second and third regions use reduced thickness to prevent excessive undercut and ensure circuit board reliability.
3Manufacturing precision
If the opening region width is reduced to accommodate finer circuit patterns, then the circuit density is improved, but the light transmission to lower regions becomes insufficient causing inadequate curing
Solution Approach 1:
The circuit board is divided into regions with different opening widths in the solder resist layer. The first region uses wider openings that allow sufficient light transmission for complete curing, while the second and third regions use narrower openings to achieve high circuit pattern density, with each region optimized for its specific function.
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 design enhances electrical and physical reliability by reducing undercut distances, preventing short circuits and improving bonding force, allowing for finer circuit patterns and integration of multiple chips on a single board.
Implementation Method 1
there is a problem in that light is not sufficiently transmitted to a lower region of an exposure region of the solder resist layer, and accordingly, the lower region of the exposure region is not sufficiently cured
Data Source
AI summary
A circuit board according to an embodiment includes a first insulating layer; a first circuit pattern layer disposed on the first insulating layer; and a first protective layer disposed on the first insulating layer and the first circuit pattern layer, wherein the first circuit pattern layer includes a first pad, wherein the first protective layer is divided into a first portion disposed on the first insulating layer in a thickness direction and a second portion disposed on the first portion, wherein the second portion of the first protective layer includes an opening with a width greater than a width of the first pad, and wherein a side wall of the second portion forming the opening is provided with a recess portion recessed in an inward direction.