So today I have the day off and so I decided that I would complete more of the Softrock RXTX ensemble II kit that I recently received. Two nights ago I completed the +12V and +5V power supply module and everything tested perfectly fine. Today’s modules would include:
- USB power supply
- Local Oscillator
I saved the bulk of the assembly for when I knew that I would be well rested and that I did not have any pressing matters to attend to. The reason for this decision was simply the need to solder surface mount components (SMT) and in particular, the LP2992AIM5-3.3V regulator which is extremely small.
On Saturday, I stopped by the local electronics store and picked up some supplies to aid in the assembly of this kit. The purchases included:
- Hakko FX-888 soldering station (70W, temperature controlled)
- fine bevel soldering tip (for SMT pads)
- rosin flux pen
- 0.020″ diameter solder (no flux)
- “third hand” stand for holding PCB
- solder wick (for removing excess solder)
- cheap ($12) head-mounted magnifying visor
I am really impressed with the FX-888 station as its soldering iron is a very small, light-weight pen style iron with interchangeable tips. For the low price of $140 this unit has a regulated variable temperature range and put out 70W which is enough to do lead-free soldering! The tiny beveled tip I bought ($12 extra) really does a fine job of extremely small pitch / pad size components. It has made the soldering experience so much more enjoyable! My only criticism is that the tip does not have the thermal mass of other similarly sized tips I have used in the past, and so it is not as versatile for parts which are more than twice the tip’s diameter. Having said that however, it is extremely easy to switch between tips, so this is only a minor point. Oh, and does this baby ever heat up fast! From off & cold to 700°F in less than 10 seconds!
Above you will see a macro shot of the portion of the board I finished this morning. This region provides a regulated 3.3v for a Si570 Programmable Oscillator which can be controlled via I²C (serial interface) using a micro-controller such as the ATtiny 85-20 PU. You will notice a binocular ferrite on the board – this was the fist time ever I’ve wound an inductor (in this case, a bifilar transformer) and I can honestly say that it was a breeze. I have no doubt however that the later transformers will present more of a challenge for me.
The LP2992AIM5 3.3V regulator was very easy to solder onto the board. I first cleaned the pads with spectroscopic grade isopropyl alcohol and then applied a small amount of rosin flux to the pads. Next, I tinned one of the pads in order to tack the IC in place once it was positioned to my liking. Next, the SMT IC was positioned using my wife’s tweezers (thanks sweetie) under 2x magnification and tacked in place by simply touching the soldering iron (bevel) tip briefly to the IC’s foot which was on top of the tinned pad. The IC was then soldered in place using the flux-free 0.020″ diameter solder and then the work was inspected.
The component seen to the right of the 3.3V regulator is the Si570 programmable oscillator (which can be programmed to a frequency from 10-1400MHz). I found that this component was more difficult to solder as it did not have any external leads. The oscillator has indents which have a spacing of less then 700µm at the ends! This can be a huge source of troubles if you are not extremely careful and use very little solder to make the connection. The trouble spot is pointed out in the image where the red arrows indicate. Solder bridges are common when working on a package like this, and according to the datasheet, a solder bridge at the ends would render pins 7 and 8 (the I²C data and clock pins respectively) useless. I used very little solder and was able to avoid solder bridges. This is where solder wick could save you if you were overzealous with the solder.
Next up is frequency divider module and perhaps the RF I/O and Switching circuitry.