PCB Assembly — Why Soldering joints corroded after cutting?

PCB Assembly Process - SMD and Through Hole DIP Components Assembly. Why Soldering joints corroded after PCB soldering, leads cutting and cleaning?
In this soldering tutorial, you’ll see a mixed Electronic PCB assembly technology with SMD and DIP components — applying no-clean solder paste, mounting SMD components, and reflowing on an infrared bottom preheater with hot air.
You will see how to manual assembly of through-hole DIP components and solder using Diamond Flux FN231 and FN232 soldering fluxes. The PCB assembly process continues with cutting the long leads, re-soldering, and simulating moisture exposure.

I explain what happens when cut leads are left untinned and exposed, how oxidation and corrosion form on open metal surfaces, and why properly tinned solder joints remain clean. The video clearly shows how corrosion affects different lead metals — copper, steel, and Fe-Ni alloys — after exposure to saltwater.
How to prevent such soldering failures in manufacturing and repair environments to keep printed circuit boards protected from corrosion and oxidation?

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Timestamps:
00:00 Why Soldering joints and Through-Hole Leads Corrode?
00:21 What is Mixed Assembly Technology? SMD and DIP assembly
00:44 Applying Solder Paste Sn62Pb36Ag2 %
01:09 Mount SMD components to the PCB
01:50 Temperature for Bottom Preheating Table
02:03 Why does FR-4 PCB delamination occur?
02:46 Reflow PCB on Infrared Bottom Preheating Table with Hot Air
03:35 Assembling DIP components to the PCB Through holes
05:33 Explaining THT - Through-Hole Technology and DIP — Dual In-Line Package
05:54 Soldering DIP Through-hole components to the PCB
06:12 Cutting DIP Components Leads and Tinning
07:05 Cleaning PCB after Soldering PCB with Flux Off
07:39 Salt Water Experiment Simulating Environmental Effects
08:32 DIP Through hole Components Leads Corrosion, Oxidation and Rust results
09:00 What are the Through-Hole Leads made of?
09:31 How to prevent Through-hole leads corrosion on PCB
10:00 Conclusion

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