RACA Journal I July 2026 www. refrigerationandaircon. co. za 35
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• Thermal transfer and heat capacity: When the soldering iron tip touches a joint, it must not only melt the solder but also heat the copper pad and component lead. These act as heat sinks at room temperature. If the iron were exactly at 183 ° C, the moment it contacts the cooler workpiece, heat would flow out and the solder could solidify. Thus, the iron needs to be significantly hotter than the solder’ s melting point so that it can deliver enough heat into the joint quickly without the temperature dropping below liquidus. Being well above the melting point ensures a reserve of thermal energy, so the joint area reaches solder liquidus rapidly and stays molten long enough to form a quality bond.
• Flux activation and effectiveness: Solder flux is essential for removing oxides and allowing solder to bond to metal surfaces. Fluxes are formulated to activate within certain temperature ranges. If your soldering temperature is too low, the flux may not fully activate to clean the surfaces, or it may not evaporate solvents properly. Conversely, if the temperature is too high the flux can burn.
• Oxidation of solder and tip: Hot metals oxidise when exposed to air. The higher the temperature, the faster oxidation occurs. When solder is molten, oxide forms on its surface( the dull film you might see if solder stays liquid too long). Flux combats this by continually cleaning and scavenging oxides. The flux itself gets consumed in the process. Additionally, the soldering iron tip itself will oxidise faster at elevated temperatures, which is why irons left idling at 400 ° C will have blackened, non-wettable tips. From a maintenance perspective, lower tip temperatures extend tip life.
FACTORS INFLUENCING OPTIMAL SOLDERING TEMPERATURE Several factors influence what temperature( and time) will yield the best results:
• Solder alloy composition: Different solder alloys have different melting points, the general rule is to set the soldering tool about + 50 – 150 ° C above the alloy’ s melting point, depending on circumstances.
• A guideline for hand soldering is melting point + 100 ° C as an iron tip temperature. In practice, this means roughly 315 – 330 ° C for lead-free and a bit lower around 280 – 300 ° C for tin-lead solder.
• Time and technique: Temperature cannot be considered in isolation from time. A higher temperature allows a shorter contact time to achieve a good joint, whereas a lower temperature might require holding the iron on the joint longer.
For manual soldering with a soldering iron, a good rule of thumb is: set the iron to the lowest temperature that still allows you to form a proper joint in a couple of seconds.
In summary, for hand soldering, start around 315 ° C( leaded) or 330 ° C( lead-free) and adjust as needed. Use the lowest temperature that still allows soldering quickly. Many professionals rarely go above 350 ° C unless warranted.
CONSEQUENCES OF IMPROPER SOLDERING TEMPERATURE Working with solder requires precision, especially regarding temperature. Deviations from the optimal range, whether too low or too high, compromise electronic assembly reliability.
• Too low temperature / insufficient heat: Insufficient heat prevents solder from reaching its proper melting point, leading to poor wetting and weak joints. The most common outcome is a cold solder joint, characterised by a dull, lumpy or grainy appearance. This occurs when solder doesn ' t fully melt or flow, often due to an inadequately hot iron or premature heat removal. Low temperatures also hinder flux activation, preventing effective cleaning of oxides and proper solder bonding. Solder may ball up and fail to adhere. In through-hole applications, insufficient heat can prevent solder from filling the hole completely.
• Too high temperature / excessive heat: Excessive heat, whether from an iron or reflow process, can cause irreversible damage to components, PCBs and degrade solder joint quality. o Component damage o Burnt flux and solder joint integrity o Solder bridges and splatter: Overly hot and liquid solder is prone to forming unwanted bridges or splattering. o Shortened equipment life: Consistently running soldering equipment at maximum temperatures shortens the lifespan tips( due to rapid oxidation) and heater elements.
• Select the right tip and tool: Match tip size to the joint; larger tips( more thermal mass) at moderate temperatures are better for large joints. Avoid compensating for small tips by increasing the heat. For fine work, use specialised tips that concentrate heat efficiently.
Jaco, I hope that this assists you with your work and your
understanding of soldering and soldering temperatures.
Grant Laidlaw
grant @ acra. co. za
REFERENCES:
• ACRA
• SAFETY STANDARD EN378-1:2017-03
• IPC-7352: 2023 voices
RACA Journal I July 2026 www. refrigerationandaircon. co. za 35