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A wraparound conductor does not jump from front to back at a zero-thickness line. Current travels across the front approach, around a finite ceramic edge and onto the reverse face. Edge radius, fired thickness and usable width determine the local resistance and current density; attachment pressure can act on the same interface. The design must dimension the complete path and measure voltage around the edge before one total resistance hides the limiting segment.
Key design decisions
- Draw front, curved-edge and backside conductor segments separately.
- Use the actual edge arc and minimum conducting cross-section in the electrical screen.
- Place voltage taps so an edge drop can be separated from face approaches and external contacts.
- Review attachment load and current distribution together where a clip or terminal contacts the wrap.
1. Define the path around the physical edge
Show ceramic thickness, edge radius or bevel, wrap width, face overlap and termination contact. Identify whether the coating follows a semicircular radius, a chamfered path or an irregular broken edge. A nominal board thickness does not by itself specify conductor length. Preserve the local coordinate and face orientation in cross-sections.
Separate conductor geometry from the external joint. Solder, braze, adhesive or spring contact can change effective entry area and add resistance. The wrap may be electrically continuous yet mechanically vulnerable where a rigid terminal loads its edge. Put the contact footprint and force direction on the same detail.
2. Calculate path length from the edge profile
For a semicircular edge of radius r joining straight face runs L_f and L_b, use L_total=L_f+pi r+L_b. A chamfer uses its measured sloping length instead. The radius must refer to the conductor centerline or another declared surface; using outside diameter for one term and centerline area for another introduces a hidden bias.
If the wrap thickness changes around the curve, divide the path into segments rather than applying one average. Each segment resistance is rho L_i/(w_i t_i), and series terms add. Current density J_i=I/(w_i t_i) is highest at the smallest conducting cross-section, not necessarily at the longest segment.
3. Work a uniform-path example before adding detail
Assume two face runs of 0.30 mm each, edge radius 0.15 mm, width 0.50 mm, fired thickness 10 µm and effective resistivity 3×10^-7 ohm·m. Total length is 0.30+pi×0.15+0.30=1.071 mm. Cross-section is 0.0050 mm², giving modeled resistance about 64.3 mΩ.
At 0.80 A the uniform current-density screen is 160 A/mm² and dissipation is about 0.041 W. These hypothetical inputs are not a rating or manufacturing claim. If the curved segment thins to 7 µm, analyze that segment separately; one full-path average can conceal its higher density.
| Quantity | Example | Result |
|---|---|---|
| Path length | 0.30+pi×0.15+0.30 mm | 1.071 mm |
| Cross-section | 0.50×0.010 mm | 0.0050 mm² |
| Resistance | rho L/A | 64.3 mΩ |
| Current density at 0.80 A | I/A | 160 A/mm² |
4. Measure voltage on both faces and near the edge
Where artwork permits, add sense points before the front bend, after the backside bend and at the external contact. Measure with defined current, polarity, temperature and probe positions. Segment drops reveal whether the curve, one face overlap or the joint controls total resistance.
Compare cold resistance with the value reached during the intended current pulse or dwell. If the curved segment warms faster than the face runs, its voltage fraction should rise consistently with its material response; a sudden step instead suggests contact motion or a discontinuity. Thermal imaging needs declared emissivity and spatial resolution because a reflective termination can display a false cold region. A small temperature sensor can alter the same narrow path, so its attachment belongs in the uncertainty record.
Reverse current and reposition external contacts to expose thermoelectric or contact effects. Track voltage with time if self-heating matters. A two-wire reading across the complete assembly cannot distinguish a thin wrap from a poor clip contact. Preserve the pre-attachment baseline before applying solder or mechanical load.
5. Inspect width and thickness through the turn
Current may crowd at the inside of a bend or enter through only part of a termination footprint. Register cross-sections to the direction of current and measure thickness at several angular positions. A section perpendicular to the wrong direction can overstate usable area. Edge roughness and paste pullback also change the continuous path.
Use microscopy as geometry evidence, not automatic proof of electrical performance. Preparation may smear metal or round the fragile edge. Correlate each section with earlier voltage-drop data. If a narrow region is suspected, target it intentionally rather than choosing the easiest polished plane.
6. Keep load eccentricity in the same review
A spring or terminal pressing away from the ceramic mid-plane introduces a moment as well as contact force. Mark its line of action and support. Increasing force can lower contact resistance while raising stress at the wrap edge or pad interface. The electrically best force is not automatically the mechanically safest.
A voltage drop that changes immediately with force points toward contact area; a permanent change after unloading suggests slip, damage or conductor deformation. Cracking aligned with an eccentric contact indicates a mechanical path, while progressive resistance localized to the curved segment suggests thinning or metallurgical change. Confirm with controlled force and position rather than an uncontrolled manual press.
7. Dimension the complete wrap construction
The drawing should specify edge profile, wrap width, face overlap, protected regions and external contact envelope. Define the electrical requirement with current and temperature state, while leaving process-specific compensation to qualified manufacturing review. When minimum fired thickness is required, state how its sampling represents the curved surface.
Reopen the calculation when ceramic thickness, edge radius, paste system, wrap width, firing, attachment or current waveform changes. The model does not claim adhesion, current capacity, edge strength or operating life. Project validation must include the represented joint and mechanical restraint.
8. Send the edge profile and connection load
Provide cross-section and both-face artwork, ceramic edge geometry, wrap limits, external joint and allowed contact surfaces. Supply continuous and pulsed current, temperature boundary and acceptable voltage drop. Identify any requirement for soldering, bonding or spring contact.
For a problem, send segmented voltage data, force or attachment condition, cross-section orientation and observed peel, cracks or thinning. Include specimen identity and process state. This evidence supports a finite-radius calculation without inventing material properties or accepting a nominal front-to-back connection as a complete design.
Send the wraparound termination package
Define the curved conductor path, narrowest area and external contact so electrical and mechanical limits can be reviewed together.
- Ceramic thickness, edge radius or bevel and both-face conductor drawings
- Wrap width, face overlaps, thickness evidence and material system
- Current waveform, temperature state and allowed voltage drop
- Segmented sense locations and contact-resistance measurement boundary
- Terminal footprint, force direction, support and attachment process
- Section orientation, failure observations, acceptance owner and validation plan
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