Materials & making · Established
The joint that holds electronics together
Solder has to melt low enough not to damage the parts, wet both metals, and then survive years of thermal cycling as a structural and electrical connection.
Explore the supporting references at the foot of this article.
THE MATERIAL THAT DOES THE WORK
Chemical form
Modern electronic solder is mostly tin, alloyed with small amounts of silver and copper; the near-eutectic tin–silver–copper composition melts around 217 °C. The older tin–lead eutectic melts at 183 °C and is now restricted in consumer equipment. Antimony appears in some to strengthen the joint, and bismuth or indium in low-melting formulations.
- 01
Print the paste
Alloy powder in flux is stencilled onto the pads, with the deposit volume setting the eventual joint shape.
- 02
Place and reflow
Components are placed and the board follows a temperature profile that activates flux, melts the alloy and wets both surfaces.
- 03
Form the joint and live with it
A thin tin–copper intermetallic layer bonds the joint, and time above melting is bounded because too thick a layer is . Thereafter the joint absorbs the strain of mismatched expansion, and its structure decides how many thermal cycles it survives.
Why it works
A solder joint does three jobs at once: it conducts, it holds the component in place, and it accommodates the strain that arises because a package and a board expand by different amounts when they warm up. Tin is the base because it wets copper readily, forming a thin intermetallic layer that bonds the joint, and because its alloys melt in a usable window well below the temperatures that damage components.
How it is made & used
Solder paste — alloy powder suspended in flux — is printed onto the board through a stencil, components are placed, and the assembly passes through a reflow oven whose temperature profile activates the flux, melts the alloy and controls how fast it solidifies. The profile matters as much as the composition: cooling rate sets grain structure, and grain structure sets fatigue life.
Inside the application
Replacing tin–lead solder across the electronics industry was a materials problem disguised as a regulatory one. The lead-free alloys melt about 35 °C higher, which pushed component and board materials to their limits, changed the intermetallic chemistry at the joint and required most of the process window to be re-established.
Limits & alternatives
Tin-rich joints can grow whiskers — thin single that bridge adjacent conductors — and the mechanisms are still not fully predictable. Tin also undergoes a transformation to a brittle gray form at low temperatures, though alloying suppresses it in practice. Thermal cycling eventually cracks joints through the intermetallic layer, so reliability is specified in cycles rather than as a pass or fail.
Part of a larger system
This links a metal known since antiquity to the most volume-critical joint in modern manufacturing, and shows how an environmental restriction propagates through melting point, oven profile, board material and expected service life.
Sources & further reading
- LANL Periodic Table — Tin ↗Los Alamos National LaboratoryReference explanation · Source checked
- NIST — Tin ↗National Institute of Standards and TechnologyReference explanation · Source checked
- RSC Periodic Table — Tin ↗Royal Society of ChemistryReference explanation · Source checked
- LANL Periodic Table — Tin ↗Los Alamos National LaboratoryReference explanation · Source checked
- USGS — Tin ↗United States Geological SurveyReference explanation · Source checked
Sources and further reading. What the review labels mean