Soldering is a method for chemically joining items using heat. There are many types of soldering, but by far the most common is using a tin alloy to join copper parts. This is a staple of electronics manufacturing, and the focus of this article. The "Basics" section will give you some very condensed core insights to get started with soldering without missing important information.

See also: Soldering Equipment

Basics

• Do not use the soldering iron to exert physical force. Use pliers or other appropriate tools if you have to hold something in place or apply pressure.
• There is a substance called flux that is a component of solder. Basically this serves to make the liquid solder flow correctly and distribute itself onto the component pins/pads. Flux is completely essential to soldering, and incorrect usage of flux is the most common source of error and misery for novice solderers. 
  • Flux is single use, meaning that it will only function for a single heating. When initially soldering a joint, the flux in the solder thread will be sufficient. If you have to reheat a joint, additional flux must be added externally. At Omega Verksted, you can find additional flux in the Poison Fridge.
  • When adding external flux, don't be afraid to add too much (although please don't waste excessively)
• There should be a small amount of tin on the soldering iron when it is in use, for any soldering operation. This will increase the contact surface area and improve thermal transmission.
• If there is oxide on the soldering iron (brownish-black stains), clean it off on the brass wool on the workstation while the iron is hot.
• In general, soldering a joint should take around 2-5 seconds.
• In general, the solder joint should have a "nice" slope. It should not be too concave or convex.
• If the solder joint is not heating up fast enough, almost universally the thing to do is to use a bigger soldering tip. A very coarse rule of thumb is that the soldering tip should be around the same size as the pad on your PCB. You can ask a board member if you can't find a bigger tip.
  • The Metcal soldering stations at Omega Verksted do not allow for adjusting the tip temperature, because they automatically use the optimal tip temperature for the lead free solder stocked by Omega Verksted.
• Use a claw stand ("helping hand") to align the items you are soldering if necessary. Alternatively you can ask someone to help you hold something.
• Component legs should be cut before soldering.
• When finished, add some tin to the soldering iron. This will extend the lifetime of the equipment. 

Desoldering

• When using a solder wick, you should add some flux to the solder joint you want to desolder.

Advanced

Although the following information could be useful for a beginner as well, you shouldn't be too concerned with it if you are just starting out. Soldering is the best way to learn soldering. 

Intermetallic Layer

The goal of optimal soldering is to create an intermetallic layer (the layer of bronze between the copper and the tin) of the optimal thickness. This has been empirically determined to be around 1μm. For most applications, this will be achieved when the solder is heated to the correct temperature, the solder flows across the copper to cover all the areas of the joint ("wetting"), and the joint is heated for around 2-5 seconds. Heating the joint for too long will result in an intermetallic that is too thick.

• If the intermetallic is too thin, the joint will be mechanically weak.
• If the intermetallic is too thick, the joint will become brittle.

Over time, the intermetallic will passively become thicker on its own. This can contribute to the deterioration of old electronics.

Wetting

In soldering, wetting is a the property of the liquid solder to flow over and cover the desired surfaces of the components, wires and PCB pads. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Good wetting is achieved when the solder is heated to the correct temperature, there is sufficient active flux and the surfaces to be soldered are not contamined with dirt, fat or oxides. Active flux will to a large degree remove impurities, but it doesn't have infinite capacity to clean. 

Flux

Flux is a chemical cleaning agent, flowing agent, and/or purifying agent (Wikipedia article). One of the roles of flux is soldering is to prevent and remove copper oxides on components and PCBs. Tin based solder attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. By preventing the formation of metal oxides, flux enables the solder to adhere to the clean metal surface, rather than forming beads, as it would on an oxidized surface. There are various types of flux:

• Resin based: 
• Synthetic
• Water based

Stress relief

Many components have stress relief "built in". In Through Hole components (TH), the legs serve this function. The same goes for IC packages with for instance "seagull" legs, such as QFP packages.

Temperature

With surface mounted components, it is generally recommended that the core of the component (generally the IC) should not experience a temperature rise of over 3°C per second.

Reflow Soldering

The most common technique in industrial electronics manufacturing. Instead of using a soldering iron, the entire circuit is heated with a precise temperature profile (a function over time). Generally gives better results for small or fine-pitch components unless you are extremely good at hand soldering. Omega Verksted has facilities for doing reflow soldering.

See Loddeovn.