PCB Cavity Fabrication via Laser Drilling and Copper Barrier
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Solution Overview
Problem
Conventional methods for manufacturing cavities on printed circuit boards (PCBs) face challenges in achieving flat surfaces for chip mounting and making direct electrical contacts, leading to increased package size and cost due to reliance on wire bonding, and difficulties in flip-chip bonding and die stack integration.
Innovation Solution
A method involving forming bump pads on the substrate surface, covering with a second insulating layer, creating a copper barrier for protection, and laser-drilling the exposed area while removing the copper barrier post-drilling, followed by sand blasting to expose the bump pads, enabling direct flip-chip bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If laser drill is applied to fabricate cavity, then cavity can be formed, but bottom surface becomes non-flat and damaged
Solution Approach 1:
A copper layer is prepared on the PREPREG before laser drilling to protect the surface during the drilling process. This preliminary protective action prevents surface damage and maintains flatness while allowing cavity formation.
Solution Approach 2:
The copper layer acts as an intermediary protective barrier between the laser drill and the PREPREG surface. It absorbs the harmful laser energy and protects the underlying substrate while allowing the cavity to be formed underneath.
2Reliability
If copper layer is used to protect PREPREG surface, then surface damage is avoided, but direct electrical contacts cannot be made
Solution Approach 1:
The copper layer is selectively removed in the cavity region to create bump pads, while maintaining the protective copper layer in other areas. This segmentation allows simultaneous surface protection and direct electrical contact capability.
Solution Approach 2:
The copper barrier is selectively removed only in the cavity region where bump pads are needed, creating local variations in copper presence. This allows the copper layer to provide protection in most areas while enabling electrical contacts where needed.
3Adaptability or versatility
If bump pads are made by selective etch on copper barrier, then flip-chip bonding is enabled, but bump pads are easily peeled off
Solution Approach 1:
The bump pad pattern is first created on the copper layer, then the copper is selectively removed to create the final bump pads. This copying process ensures that the bump pads inherit the strong adhesion properties of the underlying structure while maintaining the desired pattern.
Solution Approach 2:
The copper layer is prepared and patterned before selective removal to create bump pads. This preliminary preparation ensures proper adhesion and structural integrity before the copper is removed in the cavity region.
4Adaptability or versatility
If wire bonding technology is used, then electrical contacts can be made, but package size and cost increase
Solution Approach 1:
The invention transitions from 2D wire bonding to 3D flip-chip bonding by creating bump pads on the bottom surface of the cavity. This dimensional change allows direct vertical electrical contacts, reducing the horizontal space needed for wiring and reducing package size.
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 method allows for direct electrical contacts and enhanced integration density while reducing manufacturing costs by facilitating flip-chip bonding and die stack mounting within the cavity.
Implementation Method 1
a cavity is formed by laser-drilling only the surface-exposed area of the third insulating layer
Implementation Method 2
The copper barrier is now removed by chemical etch once the laser drill is over
Implementation Method 3
The second insulating layer with bottom surface exposed will be removed via sand blast process, thereby exposing the bump pads
Data Source
AI summary
A step of forming bump pads on the surface of the substrate corresponding to the cavity region, and covering the whole surface with a second insulating layer, forming a copper barrier on the surface of a second insulating layer corresponding to the cavity region for protection of the second insulating layer, forming a third insulating layer, and forming a copper layer for electrical circuit. A mask is formed on the copper later of the external circuit so that only the cavity region is exposed. The cavity is formed by laser-drilling only the surface-exposed area of the third insulating layer. The bottom copper layer protects the second insulating layer and bump pads underneath from laser damages. The copper barrier is removed by chemical etch after the laser drill. The second insulating layer with the bottom surface exposed will be removed via sand blast process, exposing the bump pads fabricated.