Soldering defects-Electronics-Custom Design Circuit Board Pcb​

- Feb 08, 2017-

Soldering defects-Electronics-Custom Design

Circuit Board Pcb

Soldering defects

Electronics

Various problems may arise in the soldering process which lead to joints which are nonfunctional either immediately or after a period of use.

The most common defect when hand-soldering results from the parts being joined not exceeding the solder's liquidus temperature, resulting in a "cold solder" joint. This is usually the result of the soldering iron being used to heat the solder directly, rather than the parts themselves. Properly done, the iron heats the parts to be connected, which in turn melt the solder, guaranteeing adequate heat in the joined parts for thorough wetting. In electronic hand soldering the flux is embedded in the solder. Therefore, heating the solder first may cause the flux to evaporate before it cleans the surfaces being soldered. A cold-soldered joint may not conduct at all, or may conduct only intermittently. Cold-soldered joints also happen in mass production, and are a common cause of equipment which passes testing, but malfunctions after sometimes years of operation. A "dry joint" occurs when the cooling solder is moved, and often occurs because the joint moves when the soldering iron is removed from the joint.

An improperly selected or applied flux can cause joint failure. If not properly cleaned, a flux may corrode the joint and cause eventual joint failure. Without flux the joint may not be clean, or may be oxidized, resulting in an unsound joint.

In electronics non-corrosive fluxes are often used. Therefore, cleaning flux off may merely be a matter of aesthetics or to make visual inspection of joints easier in specialised 'mission critical' applications such as medical devices, military and aerospace. For satellites, this will also reduce weight, slightly but usefully. In high humidity, even non-corrosive flux might remain slightly active, therefore the flux may be removed to reduce corrosion over time. In some applications, the PCB might also be coated in some form of protective material such as a lacquer to protect it and exposed solder joints from the environment.

Movement of metals being soldered before the solder has cooled will cause a highly unreliable cracked joint. In electronics soldering terminology this is known as a 'dry' joint. It has a characteristically dull or grainy appearance immediately after the joint is made, rather than being smooth, bright and shiny. This appearance is caused by crystallization of the liquid solder. A dry joint is weak mechanically and a poor conductor electrically.

In general a good-looking soldered joint is a good joint. As mentioned, it should be smooth, bright, and shiny. If the joint has lumps or balls of otherwise shiny solder the metal has not 'wetted' properly. Not being bright and shiny suggests a weak 'dry' joint. However, technicians trying to apply this guideline when using lead-free solder formulations may experience frustration, because these types of solders readily cool to a dull surface even if the joint is good. The solder looks shiny while molten, and suddenly hazes over as it solidifies even though it has not been disturbed during cooling.

In electronics a 'concave' fillet is ideal. This indicates good wetting and minimal use of solder (therefore minimal heating of heat sensitive components). A joint may be good, but if a large amount of unnecessary solder is used then more heating is obviously required. Excessive heating of a PCB may result in 'delamination' - the copper track may actually lift off the board, particularly on single sided PCBs without through hole plating.


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