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This article explores soldering in electrical circuits, covering the properties of a good solder joint, the physics of soldering, the soldering process, and examples of good and bad solder joints. It also emphasizes the importance of proper techniques and safety precautions.
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Soldering in electrical circuits The National Electronics Museum October 26, 2016 Dan Zeitlin
Solder Joint Properties Soldering provides electrical conductivity and protection A Good Solder Joint • Makes electrical contact between conductors • Provides minor mechanical support • Not the main support • Encapsulates the joint • Prevents oxidation of the conductors (it’s gas tight)
Physics of Soldering Soldering creates a metallurgical bond between pieces • Heat is applied to metal* parts to be connected • Wires, terminals, board tracks • Solder is applied to the parts • Heated parts melt the solder, not the iron • Alloys form between the solder and metals • Alloys bond the metal parts to the solder • Joint must not be cooled quickly or be disturbed • Otherwise solder crystallizes, losing bond strength * Usually copper, tin, steel, or noble metal
Soldering Process It’s important to let the work pieces melt the solder • Attach work pieces to each other mechanically • Don’t rely on the solder for strength • Exceptions are light parts through or on circuit boards • Use flux core solder • Flux helps clean the interfaces • May leave residue – can be cleaned • Heat iron to appropriate temperature • 370ºC (700ºF) is common for electrical solders • Clean the iron’s tip • Lightly wet the tip with a little solder • Will fill small gaps and help heat transfer to the metals • Sometimes flux is added separately by brush or droplets
Soldering Process (continued) It’s important to let the work pieces melt the solder • Place iron tip so it heats all parts to be soldered • Make parts just hot enough to melt solder • Do not overheat by dwelling too long • Touch solder to opposite side of joint • Allow parts to melt the solder • Feed solder until the joint is filled or covered • Avoid applying excess solder • Remove iron smoothly and quickly • As soon as solder has flowed around it • A good joint will be shiny and well-wetted • Solder shape will be concave rather than convex (ball-like) • Sometimes flux is added separately by brush or droplets
Solder Joint Examples The good, the bad, and the ugly • Good Joint • Shiny • Volcano-like concave shape • Attached 100% to wires and pads Just right Almost Too much Just right too much solder not enough wetting OK Not great Cold joints • Bad Joint • Dull • Ball-like convex shape • Lumpy • Only Partial attachment
Soldering Summary Electronic parts are connected using solder Soldering Process • Solder bonds metal together • Usually copper wire and copper pads or terminals • Metal must be hot enough to melt solder to make it “stick” • Always heat the metals and let them melt the solder • Simply melting the solder alone makes a poor “cold joint” • Soldering temperatures are around 700 degrees F! • Be Careful • Wear safety glasses. Solder and flux may sputter • Only touch the soldering iron’s insulated handle • Never set the iron down anywhere but in its holder • Wires will get hot. Hold them with a tool or not at all.
