Sparks could occur with loose connections. If the wire is close, but not close enough, electrons will jump the small gap. Higher the voltage, more likely it will happen. Add more solder or use a different connector.
Overheating occurs when you use traces that are too small. Wires are rated by Gauge. Lower the Gauge, the more current the wire can handle. If I remember right, a typical 22 Gauge wire can handle 2 amps. Consider using thicker wires.
Does your circuit still work? If not, you may have a short, meaning power and ground of your power source are somehow connected when they shouldnt - that will definitely cause overheating and sparking