Electronic – Contacts corroding – why

Flux. You're missing flux, and lots of experience. I regularly solder with a tip that easily covers 10 or more pins at a time, but by carefully controlling how much solder is on the tip and using lots of flux I can get perfect solder joints. When soldering a TQFP or TSSOP package here's what I do:

1. If the pads on the PCB are not super flat, then use some solder braid and suck up some of the solder on the pads to make them flat.
2. Put the part on the PCB and carefully align the part. Don't rush this step, because if you mess this one up then nothing else will work correctly.
3. Put a drop or two of liquid flux on the part. I use water soluble liquid flux, but others are successful with other liquid fluxes.
4. Clean the iron tip and put a little bit of solder on it.
5. Carefully touch a corner pad (NOT the pin itself) and let the solder wick up and tack down that pin.
6. Repeat 4 & 5 for 2 to 4 corner pins. Reapply flux as required.
7. Put some solder on the tip and run the tip down one complete side of the chip. Ideally you won't have bridges, but if the last 2 or 3 pins are bridged then you did good. Ignore the bridges.
8. Add more flux (you can never have too much flux).
9. Repeat 7 and 8 for all sides of the chip.
10. Add flux, clean the tip on the sponge, and touch the tip to the bridge. Solder should stick to the tip. Repeat until bridges are gone.
11. In bad cases, careful use of a solder wick can help. But be careful because it is easy to bend the pins and then you're, um, hosed.
12. Clean the flux off. Some flux can be conductive, like the water soluble flux, so make sure it's clean (and dry) before powering up.

I should mention that my soldering iron has a straight, but angled tip. It's often called a chisel tip. This tip makes it easy to do everything from 1206's, 0402's, TQFP's, TSSOP's, etc. It just doesn't work for BGA's, QFN's, and some of the weirder things.

It all depends on what you mean with safe.

To cut it short: if what you are doing is some sort of professional work, the answer is not at all. As @Jodes said in a comment, the right thing to do is to use some specialty chemical made for the job.

If, however, you are doing some hobbyist work and you are going to take some risks then your approach may work, but keep in mind that the results may not be very repeatable.

There are at least two things that may go wrong with your approach:

• Electrostatic discharge (ESD) issues
• Mechanical stress

Anytime you rub two objects against each other you have the chance of generating static electric charges on them. This phenomenon is called triboelectric effect. A problem with ESD is that it may not kill your chips rightaway (many chips today have protection circuitry that will handle a small amount of ESD), but it may degrade the performance of your devices (e.g. leakage currents or offset voltages may become permanently worse because of microscopic damages inside the semiconductor device). Therefore a RAM module, to take your example, may still work, but with intermittent failures or glitches (e.g. it may exhibit a higher error rate or it may work reliably only well below its rated maximum speed).

Whether your approach will work consistently in this respect will depend, besides sheer luck, on many parameters like ambient humidity, eraser and PCB materials, operator grounding, etc.

Mechanical stress is maybe a lesser concern, but it should be taken into account: if the eraser is hard, if you press too much while erasing, if the PCB tracks or pads are too thin, you may risk detaching one of these latter from the PCB and this will ruin your day most likely (repairing a multilayer PCB is not easy, is often expensive and sometimes is not even possible).