Q: Why are there long, thin lines of solder on a PCB? I had a failed Chinese charger. When I opened it, I saw a melted wire of solder that caused a short circuit, burning and damaging the board. The wire was thin and printed upon copper so when melted it touched other solder wires.
This is the component side of the charger with the damage marked with a red circle:

This is the soldering side:

The arrows point to other solder wires and the long line is the path of the melted solder wire which was removed. The solder wires are placed on copper.

A: Such tin on the trace is for making the current capacity larger.
Normal routing traces on cheap PCBs have 1oz thickness copper.
With the given width, to increase the current capacity, the designer may need to make the current capacity larger.
If the trace copper exposed to the out without soldermask layer,
in the wave soldering process, such thick lines are made.
The melting temperature of such solder is more than 200°C.
Thus, for normal operation, solder traces are safe enough.
If it is melt, there is a certain heat source that was already broken before the solder is melt.
Edit - my personal preference
(as several comments are made for more concerns, I add this "Edit" section.)

*

*Why is this (whole PCB) in low quality? - I don't expect high quality when using a cheap product.

*

*Design costs to meet many standards are expensive.

*Testing costs for such standards are also expensive. e.g. consider the cost of CAT II or CE certification testings.

*PCB fabrication and assembly with better materials are expensive.

*By doing all these, it becomes not a cheap product anymore.



*Does it (solder on/along the trace) really help current flowing? - Yes, it certainly makes the path resistance smaller and helps overall safety.

*

*(normal) Solder conductivity is 9~13% of copper's.

*The most common thickness of copper in PCB production is 1oz = 34um.

*0.34mm thick solder has same resistance as 1oz copper.

*PCB fabrication cost gets much higher for thicker copper layer.

*PCB assembly cost is same, no matter whether it has solder-trace or not.



When I am buying a product or reviewing one,
I concern more on the price-quality or price-performance ratio, instead of quality only or price only.

A: because the pcb's are pulled on a chain through a bath of solder and your looks like it.
https://en.wikipedia.org/wiki/Wave_soldering
"Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave."
Ofcourse, the soldermask and how it's applied is a factor too.
