SMT Copper Shim (Current Shunt) Calculator
A high-current trace can be reinforced by a copper shim soldered onto the PCB: from the shim size, parallel copper foil and solder-joint resistance it computes equivalent resistance, shim/copper current sharing, ampacity, voltage drop, losses and temperature rise — or back-solves the minimum shim thickness for a given current.
Key points
SMT Copper Shim (Current Shunt) Calculator: A high-current trace can be reinforced by a copper shim soldered onto the PCB: from the shim size, parallel copper foil and solder-joint resistance it computes equivalent resistance, shim/copper current sharing, ampacity, voltage drop, losses and temperature rise — or back-solves the minimum shim thickness for a given current. Formula: R = ρ(T)·L/(W·t) ｜ R_eq = R_shim ∥ R_pcb ｜ I = √(h·A_surf·ΔT/(R_eq + R_solder)). Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. The shim and PCB copper share current inversely with their resistance; the solder layer makes them a parallel circuit, and joint resistance strongly affects the total drop — only a large solder area approaches the theoretical value. Temperature rise uses a lumped heat balance: loss = h × cooling area × ΔT. The shim is soldered to the board, so it cools mainly through its top face, the two edges and the PCB copper.
Inputs
R = ρ(T)·L/(W·t) ｜ R_eq = R_shim ∥ R_pcb ｜ I = √(h·A_surf·ΔT/(R_eq + R_solder))Calculation model (updates with parameters)
| Loop resistance (incl. joints) | 51.52 µΩ | R_eq + 10 µΩ |
| Shim resistance R_shim | 51.15 µΩ | ρ(T)·L/(W·t), L = 15 mm |
| Skin depth δ | — (DC) | no skin effect at DC |
| Shim AC / DC resistance ratio | 1.00 × | shim thinner than the skin depth — DC calculation is fine |
| Effective conducting thickness | 0.60 mm | min(t, δ) |
| Parallel copper resistance R_pcb | 220.5 µΩ | 10 × 2oz × 2 |
| Equivalent resistance R_eq | 41.52 µΩ | R_shim ∥ R_pcb |
| Shim current share | 81.2% | shared inversely with resistance |
| Shim / copper current | 40.58 A / 9.416 A | |
| Shim current density J | 6.8 A/mm² | SMT shims commonly run 5–10 A/mm² on short paths |
| Shim ampacity (ΔT = 30 K) | 52.03 A | with the current shim plus parallel copper |
| Voltage drop ΔV | 2.576 mV | ΔV = I·(R_eq + R_solder) |
| Drop ratio | 0.05% | ΔV ÷ Vs (5.0 V) |
| Loss (shim / copper / joints) | 84.25 mW / 19.55 mW / 25 mW | |
| Estimated temperature rise ΔT | 27.5 K | P/(h·A_surf), h = 10 W/(m²·K) |
| Shim weight | 0.81 g | estimated at 8.96 g/cm³ for copper |
Engineering notes
- The shim and PCB copper share current inversely with their resistance; the solder layer makes them a parallel circuit, and joint resistance strongly affects the total drop — only a large solder area approaches the theoretical value.
- Temperature rise uses a lumped heat balance: loss = h × cooling area × ΔT. The shim is soldered to the board, so it cools mainly through its top face, the two edges and the PCB copper.
- Shims are usually T2 copper or tin-plated copper; resistivity is taken as 1.724e-8 Ω·m. Plating barely affects DC resistance and mainly improves solderability.
- For high current: make the shim at least as wide as the trace, open the solder mask to the shim width, and stitch several vias at each end (on multilayer boards) to bring current into the inner layers.
- Voids between the shim and pad after reflow raise the temperature rise; X-ray sample the power path, or switch to a crimped/bolted busbar when needed.
- An SMT shim only fixes the local bottleneck; connectors, fuses and shunts in the loop are often the real limit and must be checked together.
- Skin depth δ = √(ρ/(π·f·μ)): about 9.3mm in copper at 50Hz and 0.21mm at 100kHz. Under switching ripple the shim is often thicker than δ, so AC resistance grows according to the 1D slab approximation — use thin shims or stacked copper foil at high frequency.
FAQ
- What is SMT Copper Shim (Current Shunt) Calculator for?
- A high-current trace can be reinforced by a copper shim soldered onto the PCB: from the shim size, parallel copper foil and solder-joint resistance it computes equivalent resistance, shim/copper current sharing, ampacity, voltage drop, losses and temperature rise — or back-solves the minimum shim thickness for a given current.
- What is the formula for SMT Copper Shim (Current Shunt) Calculator?
- Core formula: R = ρ(T)·L/(W·t) ｜ R_eq = R_shim ∥ R_pcb ｜ I = √(h·A_surf·ΔT/(R_eq + R_solder)). The tool returns results instantly using this formula and the selected parameters.
- What engineering notes should I keep in mind when using SMT Copper Shim (Current Shunt) Calculator?
- The shim and PCB copper share current inversely with their resistance; the solder layer makes them a parallel circuit, and joint resistance strongly affects the total drop — only a large solder area approaches the theoretical value. Temperature rise uses a lumped heat balance: loss = h × cooling area × ΔT. The shim is soldered to the board, so it cools mainly through its top face, the two edges and the PCB copper. Shims are usually T2 copper or tin-plated copper; resistivity…
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