Tabbing wire is thin copper ribbon soldered from the top of one solar cell to the back of the next, joining them in series, with a thicker bus bar at the string ends to carry the current out. Use rosin flux and 60/40 solder with a temperature-controlled iron, work on a soft surface, and test every joint as you go, because the ribbon’s own resistance is negligible and a cracked cell or a cold joint is what actually costs power.
Part of Solar Panel Construction
Soldering copper ribbon between solar cells and bus bars at the end of a string.
The Joint Matters More Than The Ribbon
The carrier specifies thin copper ribbon 0.5–1 mm wide and 0.1 mm thick, rosin-core solder or conductive adhesive, and rosin flux dissolved in alcohol. A tab runs from the top (n-type) face of one cell down to the back (p-type) face of the next, so each cell adds about 0.5 V. Its warning is that silicon wafers crack easily: work on a soft padded surface and use a temperature-controlled iron, because too hot cracks the cell and too cool gives a poor joint.
Soldering practice is in Soldering Basics: 60/40 tin-lead solder melting at 183–190°C, heat the joint and not the solder, and look for a smooth, shiny, concave joint rather than a dull, granular one. A tinned iron tip transfers heat; an oxidized one does not.
One point the carrier does not address: solder wets metal and not bare silicon. A salvaged commercial cell already has metal pads on its faces for the ribbon. A wafer made from scratch would need metal contacts put on first, and the corpus gives no method for that, so treat the tabbing of a scratch-made wafer as unspecified.
What You Need
- Copper ribbon — 0.5–1 mm wide by 0.1 mm thick (carrier); salvaged ribbon from dead panels is best
- 60/40 rosin-core solder and rosin flux — Soldering Basics
- A temperature-controlled iron (or a copper-bit iron heated in a charcoal fire, as the soldering article describes, with care)
- A padded work surface
- A continuity meter — Continuity Test
- Heavier copper strip for the string-end bus bars — Bus Bars
Method: Tab A String
Step 1: Clean the pads and the ribbon with flux.
Step 2: Tin the ribbon ends briefly.
Step 3: Place and solder the first joint with light pressure, briefly. The joint should be smooth and shiny.
Step 4: Bend a small loop of slack between cells so heat and cold do not pull on the joints.
Step 5: Check continuity from cell to cell as each is added.
Step 6: Join the string ends to bus bars that lead to the junction box (Junction Boxes).
Worked Example: The Ribbon Is Not The Problem
The corpus gives copper’s resistivity as 1.72 × 10⁻⁸ Ω·m (Conductor Materials). A ribbon 0.75 mm wide (the middle of the carrier’s 0.5–1 mm) and 0.1 mm thick has an area of 0.75 × 0.1 = 0.075 mm² = 7.5 × 10⁻⁸ m². For a run of 5 cm (an assumed cell-to-cell length), R = 1.72 × 10⁻⁸ × 0.05 ÷ 7.5 × 10⁻⁸ = 0.0115 Ω. At the carrier’s 0.5 A the voltage lost is 0.5 × 0.0115 = 0.0057 V and the heat is 0.5² × 0.0115 = 0.0029 W. Across 35 joints in a 36-cell string that is 35 × 0.0057 = 0.2 V, about 1% of 18 V.
A single cold joint changes this. I²R gives a joint of 2 Ω carrying 20 A as 800 W; at this panel’s 0.5 A a 1 Ω cold joint costs 0.5 × 1 = 0.5 V, as much as one whole cell.
Common Mistakes
- Too much heat — cracks the wafer
- Too little heat — a grey, granular cold joint with high resistance
- No slack loop — the joint tears when the panel warms and cools
- Trusting appearance — measure the joint, as the continuity article says (under 0.01 Ω is good)
Safety
The iron burns and the string is live while the cells are lit
A soldering iron at joint temperature burns skin and starts fires, and flux fumes irritate the lungs, so work in moving air. A string of cells being tabbed in daylight is already producing voltage; a long string reaches a voltage that matters, so cover the cells with an opaque sheet while you work and treat the string as live regardless. Lead-bearing solder contaminates hands and benches, so wash before eating (Lead Exposure Prevention).
What’s Next
- Learn next: Series vs Parallel Configuration — arranging strings into a panel
- Combine with: Bypass Diodes — protecting a string from shade