This is in part a follow-up to the CC1110 IC Friday post from a few weeks back. TI has released a USB-dongle based development kit featuring their CC1111 system-on-chip radio communications IC. The board features an msp430 and the CC1111 on a self-contained USB stick and is about 50USD, look a very reasonable price. For those that want something cheaper, unhealthy the samples for the family of low-power RF chips are already available and shipping. Finally, the board reference design can be found here and is also mirrored below.
( swrr049.zip )
As a small follow-up to my post about package footprints, seek here is a Packaging Databook from Intel. About half of the book is on the physical specifications of various packaged employed by Intel, discount such as physical constants of package materials. There are also some good sections on electrostatic discharge considerations and reflow soldering methods.
What to do when you need to mount a ball-grid array (BGA) package on a circuit board without sophisticated equipment? One popular option is to create something called a “reflow oven” which is able to control your circuit boards temperature with respect to time. The idea behind reflow soldering is that we may want to apply a thin layer of solder paste (solder with flux) over the exposed pads on a printed circuit board, then place all of the surface-mount components on that side, and then heat the board so the solder melts and the components become electrically attached. This is pretty much the only method for attaching components whose pads are completely on the underside making them inaccessible to soldering irons. The temperature profile is fairly standardized (here, here and here) and consists of first removing any excess moisture from the packages, then ramping up to the temperature required to melt the solder, then to cool off in a safe manner that prevents component or joint damage. It should be noted that these temperature profiles aim to limit the time components spend at elevated temperatures (>250C) to minimize the risk of damage due to heat.
What I am proposing is something much simpler: lets use a hot plate to heat the PCB and achieve the same sort of reflow process. The main disadvantage is that the process is much less controlled and the dimensions of the board must be small enough to fit on the hotplate. The primary benefits are its simplicity. I am fortunate enough to have a hotplate which has a thermocouple to the surface and can measure the surface temperature with some degree of proficiency, so an alternate method will be required for other types. Some kind of infra-red measurement method would probably work well.
The idea is that we first apply solder paste to the board, when necessary. In this example, I am mounting a MICROSMD8 package where there is ample solder on the board and the chip to achieve connection. It is often a good idea to put some clean-free flux on the board in any case. Everything is first pre-heated for ten minutes at 50-80C to get rid of some of the moisture. The assembly is then heated to about 230C. At this point, the chips should already be aligned over the target pads. The reason for this temperature is that unlike the oven, the top surface of the PCB is exposed to air and thereby creates a thermal gradient. We need to control the heat on the top surface so that the solder just barely melts. This can be noted when watching the PCB under a microscope or with a magnifying glass as the solder will become very shiny when it melts. As the solder melts on the chips and PCB, the surface tension will pull the chip into alignment. The whole assembly can then be slowly cooled and tested electrically. When populating larger projects, it is best to put on the larger chips first and then place something to act as a heat-sink on top. I have had success with larger DSP chips where I placed inverted bolts on top to radiate away some of their heat while adjusting other components. Finally, don’t forget that a cold PCB looks the same as a hot one, so be sure to avoid burning yourself.
( an081.pdf ) ( an353.pdf ) ( xapp427.pdf )
I have recently stumbled upon a power-related mistake that I made which may be educational. The basic setup is that I designed an analog amplifier with some digital controls and did not power the digital circuitry correctly which then resulted in some very infrequent errors. The power supply for my amplifier was +/- 5V and a ground reference. This was just fine for the analog circuitry, however, some digital potentiometers needed a single +5V supply to operate. Not wanting to contaminate the reference ground with current from the digital components, I decided to create a second “power” ground for the circuit. I did this by placing a 7805 +5V linear regulator between the +5V and -5V power rails and called the new position the power ground. I also placed some LEDs in series with resistors between the power rails and the power ground. Finally, I hooked up the digital electronics between the +5V rail and the newly created power ground.
At first this may seem to be a reasonable idea since the 7805 regulator should provide a voltage that was close to the ground reference. The problem is that the linea regulator essentially acts as a serial impedance between the power rail and the load (see On Semi’s linear regulator guide or the one from National). To be more specific, the 7805 in the previously described configuration acts as a variable impedance between the +5V power rail and power ground which varies to ensure that the voltage between the power ground and -5V would be maintained at 5 volts. So if the load (digital circuitry) is between the +5V rail and the power ground, the regulator cannot really do its job since it cannot drive current between its output and the -5V (its ground) rail. The right way to solve this specific problem would have been to use a 7905 negative 5 volt regulator which would provide the same approximate voltage for power ground and would have no problems driving the current to the -5V rail. The reason the circuit worked most of the time but failed sporadically was because of the LEDs between +5V, power ground and -5V. The amount of current that the digital components typically required was small compared to the LEDs current and therefore it was easily sourced, by the LED, not the regulator.
( hb206-d.pdf )