Plated Through Hole Polymer Coating for PCB Reliability
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Solution Overview
Problem
Forming reliable plated through holes (PTHs) in high aspect ratio printed circuit boards (PCBs) is challenging due to sidewall roughness, which can lead to conductive anodic filament formation, insulation resistance failures, and laminate adhesion issues, exacerbated by increasing device density and decreasing wall-to-wall distances between PTHs.
Innovation Solution
A method involving selective etching of insulating layers to form copper lands and recesses, followed by applying a polymerization precursor mixture inhibited by copper to create a polymer coating in the recesses, which is then polymerized and washed with an organic solvent to reduce hole wall roughness and enhance PTH reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If aggressive etching is used to remove polymer coating, then electrical connection is improved, but hole roughness increases causing defects
Solution Approach 1:
The patent applies a polymer coating selectively in the recesses between copper lands, creating local quality differentiation. The coating is applied only where needed (in recesses) rather than uniformly across the entire hole surface, allowing aggressive etching to proceed while protecting against excessive roughness in critical areas.
Solution Approach 2:
The patent uses a composite structure combining copper lands with polymer-coated recesses. This composite approach allows the beneficial electrical connection properties of copper while using the polymer material to fill and smooth recesses, reducing overall hole roughness and preventing defects.
2Productivity
If drill speed and feed rate are increased to improve productivity, then manufacturing efficiency is improved, but hole roughness increases leading to PTH failures
Solution Approach 1:
The patent applies polymer coating to the recesses before the plating step. This preliminary action prepares the surface in advance, ensuring that even if high drilling speeds cause roughness, the subsequent polymer coating and controlled etching will mitigate the roughness before plating occurs, preventing PTH failures.
3Quantity of substance
If PCB thickness is increased to support higher device density, then device capacity is improved, but aspect ratio increases making PTH formation more difficult
Solution Approach 1:
The patent changes the surface topology parameters by creating controlled recesses between copper lands through selective etching. This parameter change in surface geometry allows the PTH process to accommodate higher aspect ratios by providing a more favorable surface for plating, enabling thicker PCBs to be manufactured with acceptable PTH quality.
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 polymer coating minimizes hole wall roughness, reduces the likelihood of conductive anodic filament formation, and improves laminate integrity and connectivity by absorbing mechanical and thermal stresses, resulting in more reliable PTHs with reduced roughness and improved adhesion.
Implementation Method 1
exposing the substrate to a polymerization precursor mixture comprising an initiator and a monomer or oligomer whose polymerization is inhibited by copper to form a precursor layer in the recesses of the through holes; polymerizing the precursor layer to form a polymer coating in the recesses of the through holes
Implementation Method 2
washing the through holes with an organic solvent
Data Source
AI summary
In some embodiments, methods include drilling one or a plurality of PTHs with any industrial grade drill to fabricate holes with positive etch back, flooding the PTHs with a dilute solution of an acrylate monomer/oligomer containing an appropriate level of peroxide initiator, polymerizing the acrylate, and then rising the PTHs with the solvent used in the formulation of the acrylate material. In one embodiment, the printed circuit board may include a substrate comprising a plurality of metal layers separated by a plurality of insulating layers; a plurality of plated through holes formed in the substrate, each plated through hole comprising: recesses formed at each insulating layer, copper lands between the recesses, a polymer coating in each recess, and a metal layer lining the plated through hole.