ECDM Glass Microfluidics Prepress Guide

1. Principles of Electrochemical Discharge Machining (ECDM / SACE)

Electrochemical Discharge Machining (ECDM), also known as Spark-Assisted Chemical Engraving (SACE), is an advanced hybrid non-traditional machining technology specifically designed for non-conductive, chemically inert, hard, and brittle materials such as borosilicate glass (Pyrex, Borofloat), fused silica (quartz), sapphire, and technical ceramics. It is widely employed in the microfabrication of lab-on-a-chip microfluidic capillary channels, droplet generators, electrophoresis chips, and optical through-glass vias (TGVs).

The process operates in an aqueous alkaline electrolyte bath (typically 20–35 wt% KOH or NaOH). The miniature cathode tool (e.g., cylindrical tungsten carbide, micro-drill, or platinum-iridium electrode) is immersed near the workpiece opposite a large counter-electrode anode. When DC voltage exceeds a critical threshold ($V_c \approx 24 - 30\text{ V}$), the high current density causes vigorous hydrogen bubble nucleation, creating a continuous dielectric gas vapor sheath around the cathode. High-frequency electrical discharges ignite across this gas film, generating localized plasma thermal shocks ($> 1200^\circ\text{C}$) that rapidly thermally ablate and chemically leach the glass matrix.

2. Thermodynamic Formulations & Overcut Mechanics

Achieving micron-scale geometric accuracy in glass micro-channels requires modeling the hydrodynamic gas film and spark discharge overcut:

• Critical Film Breakdown Voltage ($V_c$):

  V_c = V_0 - k_c \cdot C_{KOH} - k_T \cdot (T_{bath} - T_{ref})

  where $C_{KOH}$ is electrolyte concentration in wt% and $T_{bath}$ is solution temperature.
• Dielectric Gas Film Thickness ($h_{film}$):

  h_{film} = h_0 + \beta \cdot (V - V_c)^{1.25} \cdot \left(\frac{\sigma_{electrolyte}}{\mu_{electrolyte}}\right)^{0.5}

  where $\sigma$ and $\mu$ represent the electrical conductivity and dynamic viscosity of the alkaline solution.
• Total Radial Overcut ($\Delta R$):

  \Delta R = h_{film} + S_{plasma} = h_{film} + \alpha \cdot \sqrt{E_{pulse}}

  Typical radial overcuts range from $12\text{ µm}$ to $45\text{ µm}$ beyond tool nominal radius.

3. Vector Prepress & CAM Micro-Toolpath Optimization

When converting CAD drawings and vector artwork into G-code for 3-axis CNC ECDM work cells:

1. Inward Path Compensation: Programmed centerline coordinates must offset the target channel geometry by $R_{comp} = R_{tool} + \Delta R$. If target channel width is $150\text{ µm}$ and tool diameter is $80\text{ µm}$ with an overcut of $25\text{ µm}$, the toolpath radius offset is exactly: R_{offset} = 40\text{ µm} + 25\text{ µm} = 65\text{ µm}
2. Constant Velocity Cornering: Because ECDM material removal rate is highly sensitive to tool residence time, abrupt decelerations at sharp $90^\circ$ corners cause localized thermal blowout and widening. Vector prepress requires all corners to be replaced with tangent arc fillets ($R \ge R_{tool}$) with lookahead feed control.
3. Z-Step Layering (Helical Milling): For deep microfluidic reservoirs ($> 100\text{ µm}$ deep), helical 3D ramp trajectories with small incremental depth steps ($\Delta z \le 2 - 5\text{ µm/rev}$) ensure continuous electrolyte exchange and bubble replenishment, avoiding thermal micro-cracking along channel sidewalls.
4. Entrance / Exit Tangent Leads: Micro-channel lead-ins must incorporate circular roll-in arcs outside the active fluidic region to allow the hydrodynamic hydrogen film to stabilize before cutting into the functional glass boundary.