A 40 watt iron is generally used for small electronics. Most of these cheap soldering irons are actually really bad because the temperature is unregulated. Even the iron I linked you to is unregulated. The danger here is that some sensitive electronic devices can be damaged if over heated. But what we are soldering is just wires and connectors so its a moot point. When choosing a soldering iron you want an iron that can heat up the connection faster than the material you are trying to solder can disappate the heat away. Basically the wire you are trying to solder acts like a heat sink. The larger the guage wire the bigger the heat sink. In order to solder it you need to dump a lot of heat into it very quickly. I do not like the pencil style irons for this type of work because they take a long time to heat up. They use the iron itself to store heat and have a very small heating element. That's why when you unplug them they take a long time to cool down. The instant soldering irons dont store much heat in the iron that is why the tip is so small. They are dumping all the energy into the tip instantly. This is the ideal type of iron to solder wires.
For soldering electronics you really want an iron that regulates temperature and keeps it in the 600-700 degree range. The eutectic point of the particular solder you are using is what determines what temperature to use.
Before I was an engineer I was a tech and reviewed a lot of soldering equipment before it was brought into the company I worked for. You can keep fighting that pencil iron or get a better iron and be less frustrated.
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