What is the main aim of soldering?

 What is the main aim of soldering?

For electrical/electronic use, the main aim is to prevent movement and prevent corrosion between two conductors.

A lot of people mistakenly think solder is primarily to provide a conductive path, but that is not the main purpose. And it does end up partly fulfilling that purpose. Like heat sink paste is meant only to fill the microscopic voids as it is more heat conductive than air, solder is meant only to fill the voids as it is more electrically conductive than air. In both cases of heat and electricity, the best conduction is to have the metals in contact.

With surface mount, the ideal situation is with the metal of the part sitting on the copper PCB pad, with just enough solder to mechanically hold the part and seal the contact surfaces in. Although this image from Example analytical investigations is meant as an example of a cracked ceramic capacitor, please note that the bottom of the metal on the end of the capacitor is in contact with the PCB pad, and there is just enough of a solder meniscus to hold the part in place:

For high reliability electronics and PTH components, clinched leads are much more reliable than merely poking the component wire through the PCB hole and soldering it in place. Clinching is when the leads are bent over partly or fully so as to come into contact with the copper PCB trace. Thanks to Electronic Techniques--PCB HARDWARE AND COMPONENT ASSEMBLY (part 1) for the image:

Please note that NASA uses the Western Union Splice aka Lineman’s Splice, which was originally done without soldering. NASA includes soldering. This makes a very secure mechanical and electrical connection.

Image from How-To: Splice Wire to NASA Standards | Make:

More on NASA’s soldering standards, see chapter 8:

https://nepp.nasa.gov/docuploads/06AA01BA-FC7E-4094-AE829CE371A7B05D/NASA-STD-8739.3.pdf

Also note that when crimped connections are used, the proper crimping tool will completely crush the wire so as to drive all air out, and the wire must NOT be tinned with solder. Image from Crimp Photo Gallery

There are exceptions. There is a type of surface mount IC called a Ball Grid Array or BGA that relies on uniform balls of solder to connect a grid of connections on the bottom of the IC with a matching grid of contacts on the PCB. BGAs are a bit of a problem. Remember all those XBoxes with the Red Ring of Death? BGA problems exacerbated by inadequate cooling.

Thanks to BGA - O que é isso ? for the image of a cross section of BGA solder balls:

Solder isn’t that great a conductor. It is much better to have two pieces of copper in intimate contact. However, copper corrodes. Moisture in the air will get on and between the copper pieces (component leads, wires, PCB traces) and cause corrosion accelerated by current flowing between them. And any vibration has the risk of making the connection intermittent. Oils and dust can work their way in and also cause problems.

Solder fills the gaps, keeping the copper in contact, and preventing contaminants and corrosion out. The surface of leaded solder rather quickly develops a film of corrosion, but it has no appreciable effect on the resistance of the connection.

I have seen this illustrated as I’ve worked on a lot of electronics over the decades. ICs in even very high quality sockets will eventually develop problems and the IC must be “reseated”, ie, pulled out and reinserted. This physically wipes corrosion and contaminants away and restores the copper to copper connection.

The only time I’ve had to resolder a soldered connection is when some outside force like heat causes the solder to partly melt and become granular (a type of “cold solder joint”), or fracture due to a lot of vibration. Even then, I suspect the vibration was at work on an already somewhat faulty solder joint.