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Protective Equipment

Use protective eyewear while soldering! Position the fume extractor close to- and over your working area!

High quality, fairly expensive soldering equipment is available at Omega Verksted. Please be careful with it, and ask a board member if you are unsure about something. 

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.
  • Use a soldering tip of appropriate size. If the solder joint is not heating up fast enough, usually 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, about 412°C.
  • 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. 
    • 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.
  • 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 the ability 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 forcesGood 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 contaminated 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 (norwegian: Fluss) 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

After soldering is complete, there will often be some residue of burnt flux left. This may be cleaned, for example with Isopropyl-alcohol (some components may be damaged by this) or using an ultrasonic cleaner. If you are selling the electronics, or you want to make a good impression with it, you should clean it.


Left to right: Decreasing quality of wetting

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. No-lead packages such as QFN, do not have this advantage. Putting too much solder onto the leads of a component will negatively affect the stress relief ability, and should be avoided.

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.

The soldering temperature is the target temperature of the solder during soldering. It should be 60 - 90°C higher than the melting point of the solder. In turn the tip temperature should be 72-92°C higher than the soldering temperature.

Inspection

Various errors can be detected by closely inspecting the solder joint. A microscope will come in handy.

ErrorDescriptionCauseAction
Cold solderingSolder joint displays wrinkles, stripes or lines. Solder may be clearly separate from the component and/or the copper PCB.Soldering temperature was too low. Usually thermal conduction is too poor (tip too small or not clean), or the joint was not heated for long enough.Remove solder, clean and resolder. Make sure to solder for ~2-5 seconds with an appropriate tip.
DelaminationPCB becomes deformed. Layers of the PCB may come apart.PCB was overheated. Usually the joint/circuit was heated for too long.(Irreparable)
Overheated jointSolder has a finely grained, crystalline appearance.Solder was overheated. Usually the joint/circuit was heated for too long, or there is a mismatch between the tip temperature and the solder used.Remove solder, clean and resolder. Make sure to solder for ~2-5 seconds with an appropriate tip.
Porous solderSolder joint has many small holes and/or grainy surfaceSolder was overheated, combined with contamination. Usually the joint/circuit was heated for too long.Remove solder, clean and resolder. Make sure to solder for ~2-5 seconds with an appropriate tip.
IciclesIcicle-like structures or spikesSolder was cooled down too fast.Add plenty of flux and reheat for better wetting.
Solder bridgesConnections (short circuits) between pads or pinsToo much solder on the tip, often in combination with insufficient flux.Remove solder bridges, add plenty of flux and reheat.
DewettingSolder has not flowed to all regions of the padPoor wetting, caused by contamination or insufficient flux.Remove solder and clean. Preapply solder ("fortinn" loddestedet) before resoldering. Make sure to solder for ~2-5 seconds with an appropriate tip.
DrossCharred areas on the surface of the solderContamination in solder, on components or on PCB.Remove solder, clean and resolder. Make sure to solder for ~2-5 seconds with an appropriate tip.
CrackCracks in the solder jointMovement in the joint before solder has cooled.Remove solder, clean and resolder. Make sure to solder for ~2-5 seconds with an appropriate tip.

Reflow Soldering

The most common technique in industrial electronics manufacturing.

Instead of conventional solder wire, solder paste is used. Solder paste has the same ingredients as solder thread, but instead of being a solid wire, tiny beads of solder are suspended in a viscous flux, resulting in a gel-like substance. In addition to being solder, it also weakly glues the components to the PCB even before soldering.

Instead of using a soldering iron, the entire circuit is heated with a precise temperature profile (temperature as a function over time). Done correctly, reflow soldering generally gives better results for small or fine-pitch components unless you are extremely good at hand soldering. Either way, it also gives much better repeatability and process control, which are the main reasons it's the most common method in industrial manufacturing.

The components are free to move with the flow of the solder, and adhesive and cohesive forces (surface tension) will actually move the components towards the correct placement to some degree, although this effect has its limits. See gifs to the right.

Omega Verksted has facilities for doing reflow soldering, see Reflow Oven.

(Click for satisfying gifs)

Solder paste is a bit excessive here, but illustrates a point. Notice adhesive and cohesive forces moving the component as well as the solder.


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