Soldering: soft soldering and hard soldering
A joining method without melting the base material
With welding the base material itself melts and flows together. With soldering only the filler metal — the solder — melts while the base material remains solid. This gives a joint that is based on the fact that the melted solder penetrates between the two workpieces by capillary action and hardens there. The method is far older than electrical welding and is still widely used today among other things in pipe installation electronics and finer metalwork.
§What determines whether solder is soft or hard
The distinction between soft soldering and hard soldering lies in the solder metal's melting temperature. In soft soldering the solder melts at a working temperature below 450 degrees while hard soldering takes place at temperatures above 450 degrees typically up to around 1000 degrees depending on the solder alloy. The difference in temperature also gives different properties in the finished joint.
| Soft soldering | Hard soldering | |
|---|---|---|
| Temperature | Under 450 °C | Over 450 °C typically up to about 1000 °C |
| Strength | Lower rent | Significantly higher strength |
| Typical use | Electronics, tube assemblies with low requirements. | Construction joints, tools, pipe systems with higher requirements |
§When do you choose soldering instead of welding
Soldering is particularly useful when you need to join different metals that are not easily welded together, when the item is so thin or heat-sensitive that welding would distort or damage it, or when you want a joint with a neat, smooth transition without post-processing. In return, a soldered joint typically withstands less mechanical load than a welded joint of the same material, and it is more temperature-sensitive because the solder can melt again if the item is later exposed to sufficient heat.
§Preparation of the surface
Just as with welding, a clean surface is essential. Grease, oxide and dirt prevent the solder from wetting and bonding to the base material. Therefore, the joint surfaces are cleaned mechanically or chemically, and a flux is often used, which dissolves remaining oxide layers during heating and prevents new oxides from forming while the solder flows.
§Impact soldering in practice
A widespread form of hard soldering in the workshop is impact soldering with gas flame where the smith evenly heats the part until the solder flows into the gap by capillary action. The technique requires that temperature is controlled so the base material just becomes hot enough that the solder wets and penetrates without the part itself melting or warping.