Velleman MK131 - 9V LED Educational Traffic Light Project Kit

[jbarcia]

So I started to work on this project kit and after soldering three resistors I didn't like the amount of solder on the connectors so I switched to a thinner solder. That worked much better. I decided to remove the there resistors and use the different. I tried to remove the resistors using my brand new reflow workstation hot air gun, I figured this would melt the solder and I would be able to pull the resistors up with tweezers. It didn't work out as planned. All of a sudden I smelled plastic burning and I heard some popping/sizzling. The resistor was super hot and the board was bubbled.
What is the best way to test this? I was thinking of removing the resistors by my soldering iron this time and check the resistance with a multimeter. Is there a way to check the board? Could the excessive heat damage the board? The kit was cheap so I can always buy a new one, I'm just glad it was a cheap one and not one of the expensive ones.

I can upload pictures if needed.

 

[Militoy]

Heat from a heat gun will definitely carbonize a circuit board. I'll guess you are working with surface mount parts - though you don't indicate that specifically - since you tried hot-air reflow. Using a heat gun to change a SMT resistor will be a little bit like killing a fly with a sledgehammer - It might work, but you are likely to cause some collateral damage.

I use mostly 0805 resistors. The method I use for removal is to add a bit of solder to both sides, so there is more than usually needed. I then alternately touch each side with the tip of the iron, switching back and forth quickly. After a few touches to each side, the resistor will stick to the iron tip, and come off. I then clean off any excess remaining solder, using very fine solder wick. When laying down a resistor, i'll add a drop of liquid RMA flux and solder to one pad only - then place the resistor using a tweezer. A touch with the iron, and one side is secure. I then carefully add solder to the other side.

One important note is to not use too small an iron tip. You'll do less damage to parts; soldering more quickly using a 1/32 chisel tip.

 

[(*steve*)]

Hot air works well on surface mount components that have fairly low thermal mass and where there is nowhere for the heat to escape.

It works less well where the component can leak heat through the other side of the board (e.g. through-hole component)

Use the right tool for the job.

For removing solder from a through-hole component, solder wick is a good option, although the solder wick will suck heat from your soldering iron tip, and the best tip for using solder wick is one that is significantly larger than the one you might use for soldering. As a guestimate, use solder wick that is about the same width as your soldering iron tip (I tend to use 2.5 mm wide (ish) solder wick)

 

[jbarcia]

Heat from a heat gun will definitely carbonize a circuit board. I'll guess you are working with surface mount parts - though you don't indicate that specifically - since you tried hot-air reflow.
...
One important note is to not use too small an iron tip. You'll do less damage to parts; soldering more quickly using a 1/32 chisel tip.

Can you elaborate on the damage to the board, I don't understand what you mean when you say carbonize. I checked the continuity of the traces with my multimeter and it looked good.

On the components, I am not sure if they are surface mount or not, the contacts go through the board and are soldered on the reverse side. I tested the resistance and they are good too.

Thank you both for your help.

 

[(*steve*)]

Here's a hint. The parts are either "surface mount" or "through-hole". You figure it out

Carbonise = burnt, blackened, cooked until well past crispy.

Take your hot air rework station, pack it up, put it away, and don't take it out again until you are working on surface mount components. The right tool for this job is a traditional soldering iron.