What are the reasons why the pads of the double-sided PCB board are not tinned?

What are the reasons why the pads of the double-sided PCB board are not tinned?

　Our common computer boards are basically epoxy glass cloth-based double-sided PCB boards, one of which is a plug-in component and the other side is a component foot welding surface. It
　　can be seen that the solder joints are very regular. We call it the PCB board pad for the discrete soldering surface of the component feet. Why are other copper wire patterns not tinned?
　　Because in addition to the pads that need to be soldered, the surface of the rest has a solder mask that is resistant to wave soldering. Most of the surface solder masks are green, and a few are yellow, black, blue, etc., so the solder mask oil is often called green oil in the PCB circuit board industry. Its function is to prevent bridging during wave soldering, improve soldering quality and save solder.
　　It is also a permanent protective layer for the printed circuit board of the PCB board, which can prevent moisture, corrosion, mildew and mechanical scratches.
　　From the outside, the green solder mask with smooth and bright surface is a green oil for film-to-board photosensitive heat curing.
　　Not only does the appearance look good, but the most important thing is that the precision of the pads is high, which improves the reliability of the solder joints.
　　There are several situations in which the PCB circuit board is not tinned.
　　1. The PCB circuit board is oxidized, and the PCB board is not tinned.
　　2. The temperature of the furnace is too low, or the speed is too fast, and the tin is not melted.
　　3. Solder paste problem, you can try another kind of solder paste.
　　4. The problem of battery cells, this is the most common problem, because the cells are generally made of stainless steel, and a layer of chrome must be electroplated to be tinned.
　　If this plating layer is oily or poorly plated, it will not be tinned, you can try it with a soldering iron to see if it can be soldered. Or put some solder paste on the battery, and then use a desoldering station or an electric heating station to heat it to see if the battery can be tinned.