A SMbus temperature sensor using a Surface mount IC.

A friend saw a description at www.madhacker.org of a add-on temperature sensor for his computer that used the SMbus connection that is common on most computer motherboards.

The project used an integrated circuit that was only available in the QSOP surface mount package. These packages are now the only way that some components are made and the pose a problem for the home constructer as can be seen in this comparison of the 16 pin QSOP with a 14 pin DIP.

When my friend produced some sample chips I decided to take a trip down memory lane and have a go at construction using my method of 30 years back.

The first step was to lay out the circuit on 1/10 inch graph paper and the internet provided the necessary free programme to print some out. The layout was then transferred to the printed circuit board with a centre punch.

Next the pattern was drawn on the board using a etch resistant ink. There are special pens for this but I went with what I knew and searched for the lump of road tar that I used to dissolve in turps to fuel the mapping pen I have kept since school days. It was not to be found but a 4L can of roofing tar was, so I was all set.

My first thought was to try to make a pattern that the tiny chip could be directly soldered to but the required accuracy was beyond me. Instead I decided to mount the chip on it's back and connect each leg to a pad with wire. This turned out to be very successful as the wire could be first soldered to the board then bent and cut to sit on the IC leg ready for the light touch of a soldering iron with the simultaneous application of solder. Shorted pins could be corrected by using a needle to separate the heated solder and steady progress could be made with each step reversible if problems occurred.

To etch the board I bought some pre-mixed Ferric Chloride solution that can be used cold and would not to soften the tar pattern..

The result was a complete success and my friend has 5 more temperatures that he can display on his computer. The board was placed so the sensor inside the chip would read the incoming air.