A Simulation of the Galvanic Copper Deposition in PCB Manufacturing
Rayming PCB & Assembly
Printed Circuit Board Manufacturing and PCB Assembly Services
Copper plating is a key process in the fabrication of printed circuit boards (PCBs). It deposits copper onto the dielectric substrate to form the traces and pads that interconnect electronic components.
This article provides an in-depth simulation of the galvanic copper plating process used in PCB production. It covers:
Understanding the intricacies of this copper patterning technique through modeling and simulation helps improve quality during manufacturing.
Overview of Galvanic Copper Plating
We first introduce the basics of electroplating copper for context.
Operating Principle
It works by passing electric current between two electrodes immersed in a copper salt solution (electrolyte). This causes copper to deposit onto the cathode PCB surface.
Process Steps
Role in PCB Fabrication
Electroplated copper is patterned to form:
High quality plating is imperative for reliability.
Governing Mechanism and Equations
The copper deposition process involves multiple chemical reactions at the cathode-electrolyte interface.
Electrochemical Reactions
The net redox reaction is:
Cu2+ + 2e− → Cu
Where cupric ions gain electrons from cathode to transform into copper atoms.
Complex additional reactions with additives also occur.
Transport Processes
Key Relationships
Applying Nernst-Planck equation gives ionic flux density:
Cu2+ + 2e− → Cu
Where:
Current density is proportional to flux density:
i = n F Σ Ni
Rate of deposition equals current density:
dM/dt = ρ i / (z F)
Where:
These govern local deposition rates across panel surface.
Characteristics of Plating Process
Key features of the copper plating process are:
Uniformity
Achieving highly uniform copper across entire panel area is challenging. But vital to prevent reliability issues. Involves managing anode placements, solution agitation, additive concentrations etc.
Conformal Coverage
Deposit needs to evenly coat surfaces irrespective of topology. Important for covering holes, vias, trenches with same thickness as planar regions.
Stickness Control
Deposit thickness dictated by current exposure over duration. Typical thickness from few microns for defined traces to 30μm+ for high current power/ground planes.
Miniaturization
Higher density PCBs demand plating finer features of line width/spacing under 8 mils accurately.
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Aspect Ratio Limitations
Increasing via depth/diameter aspect ratio stretches conformal coating capability driving move to newer methods.
Developing Plating Process Simulation
To gain further insight into the copper plating process, we develop a physics-based, transient multi-physics simulation model using COMSOL software.
Computational Approach
Our model couples charged species transport equations with evolving electrode reactions and liquid flow dynamics. Iteratively solved over temporal increments.
Key Simulation Details
Geometry - 2D/3D plating tank, anode-cathode assemblies Materials - Electrolyte mixture, copper Physics - Electrochemistry, Nernst-Planck, Navier-Stokes Mesh - Mapped mesh with boundary layers Solver - Time dependent coupled multi-physics nonlinear solver
Assumptions
Uniform anode corrosion rate, constant operating temperature, single additive agent, quasi-reversible electrode kinetics.
These simplify model complexity while still capturing most mechanisms of interest.
Simulated Copper Plating Behavior
We simulate the electroplating under different operating scenarios to demonstrate capabilities.
Baseline Conditions
A moderate set of input parameters reproduces well-characterized deposition behavior.
We see classical response - higher rate initially as concentration gradient induces diffusion flux but slowing over time as transport becomes limited.
Optimized Parameters
Adjusting anode placement, solution mixing rate, inlet additive concentrations can dramatically improve uniformity.
Copy thickness range decreases from 20% down to ~5% - huge enhancement.
Patterned Substrate
Switching to patterned cathode instead of blank surface leads to differential local growth dictated by surface currents.
Recesses plate slower than protruding areas.
Overall, excellent match to real expected performance establishes validity of model.
Using Simulation to Optimize
We leverage the simulation capabilities to determine operating guidelines that improve deposit uniformity which minimizes costly post-processing.
Anode-Cathode Spacing
Contour plots reveal preference for narrow ~50mm spacing between electrodes versus wide 200mm configurations prone to peripheral thickness variations.
Optimal Flow Regimes
High impingement jets introduce edge defects but excessive agitation accelerates solution degradation requiring careful tuning for smoothness.
Bath Chemistry Mixing
Introducing dynamic control of chemical replenishment and additive dosing ratios compensates for inherent process drift over long runs.
The model facilitates rapid what-if analysis to refine equipment configurations and process parameters.
Conclusion
A finite element based transient simulation of the complex galvanic copper plating process captures the interplay between electrochemistry, charge transport and hydrodynamics that governs deposition uniformity and thickness capabilities.
Calibrating against physical data at smaller scales can enhance model accuracy for production-scale PCB fabrication analysis. Similar approaches apply to other metallization processes likepattern plating, electroless coatings and alternatives like direct metallization.
Optimized configurations promise to reduce scrap and rework arising from plating non-uniformities - saving significant time and costs.
FAQs
What limits the thickness that can be plated?
Current density capabilities caps thickness but processes like pulse plating overcome limitations.
How to improve plate uniformity in recessed areas?
Optimizing fluid flow helps but often involves organic additives to locally enhance deposition rates.
What defects degrade plated copper quality?
Common defects are voids, pits, nodules and organic inclusions. Careful filtration and agitation reduces these.
What process paramters are measured for control?
Key measurements are cathode current, cell voltage differences, solution temperature and chemical concentrations for feedback loop control.
What alternatives to copper plating are emerging?
Direct metallization techniques like inkjet printing metallic nanoparticles and aerosol jet printing eliminate waste are show promise but need maturing.