Breakout and Expansion Boards
Early Prototypes
The Gumstix mainboard exposes 60 I/O lines but those can only be accessed with a breakout board. The breakout-gs available from Gumstix is ideal for early-stage prototyping. We used the breakout-gs to communicate with sensors in a display-less prototype.
Custom Expansion Board
Later on, when space becomes an issue, manually soldering individual wires to the breakout-gs is disadvantageous for a number of reasons. First, the order of the pins most likely do not match the components being connected, leading to wires crossing. Second, discrete wires are relatively thick compared to flat flex cables. The first two issues work together to produce a thick, tangled mess of wires. Third, certain parts of the circuit require special components (i.e. pull up resistors for the I2C signal lines). Finally, when discrete wires break or disconnect themselves, tracking the problem down becomes extremely difficult.
The solution is to build a custom breakout pcb that then allows the use of flat flex cables, and holds most of the supporting components. A single breakout board containing all the additional components required for the prototype is ideal, because the entire system will consist of the Gumstix mainboard and the breakout board beneath it. However, for our device, we split the components between a main breakout board, and a secondary sensor and actuator board because the cables for our controls would not stretch to where we wanted to put the breakout board.
PCB Design
The custom expansion board needs to be designed in a PCB design program. Although the interface takes time to get used to, we think Eagle PCB works very well. The free version limits the size of the PCB, but the limits should still be more than adequate for holding the different components. Below is a picture of the finished PCB (without components). The Hirose connector for interfacing with the Gumstix mainboard is on the left. The space for the LCD interface connector is on the left end of the bottom edge. The holes are for discrete wires to the backlight inverter (BACKLIGHT) and to the microcontroller that is processing sensor input (PIC). The Eagle files can be obtained in the downloads section.
Caveat emptor: at the moment, there are some issues with the current PCB design. Namely:
- Noise from the high frequency LCD clock interferes with the SPI signal. We manually soldered on some 10pf ceramic capacitors where the SPI lines enter the board (labeled PIC) to clean the signals up.
- I2C for the touchscreen may or may not work for the same reason as above.
- USB is unstable. One of the boards we built works, while the other doesn't. We did not respect the USB layout and impedance rules. The LCD may also have something to do with it.
To interface with the Gumstix, you need the Hirose connector available directly from Gumstix. Digikey sells the same thing with a different number (newer version, possibly) under item number H5221CT-ND. Eagle comes with the layout for the Hirose connector in its libary. However, make sure to choose the one that has holes on the board. One thing to watch out for is that the Gumstix board will stack on top of the expansion board. Therefore, be sure that any components in the middle area are not too tall.
SMT Stenciling and Soldering
SMT soldering without the specialized reflow ovens may seem daunting, because it certainly seemed so to us, even though others have written tutorials about the process. Sparkfun has an excellent guide on using a hot plate. The Seattle robotics has a page describing how to use a toaster oven. Another toaster oven resource is Bill Shaw's page. We like the Sparkfun skillet method because we can watch what is happening. Also, a $50 non-contact IR thermometer is very helpful for determining when the hotplate reaches the correct temperature. We tried the temperature indicator stick method but it did not work very reliably. Plus the IR thermometer is so much fun to play with.
Applying solder paste for the 60-pin hirose connector and 30-odd pin LCD connector can be a real chore if done by hand using the syringe method the above sites show. Sparkfun suggests ordering mylar stencils, but if you have a laser cutter available, it is easy to cut your own. We bought 3 mil (0.003") mylar sheets at our local Pearl Art Supply store. We then exported the Cream mask from Eagle to EPS and opened it in Adobe Illustrator (although and other drawing program will also do). To compensate for the width of the laser, we put a negative offset on all the edges (essentially contracting all the shapes). Finally, we set the outline of the mask to the correct color and thickness for performing a vector cut. Depending on your laser, the mylar may warp from excessive heat. We found that setting the power to something very low and cutting using multiple passes minimizes the warping. Applying the paste using the stencil is somewhat of an art. Sparkfun gives an intro here, but it took practice. Since we didn't have spare pcbs lying around, we used very large clamps from Home Depot to hold the stencil in place. Also, for a single PCB, it works better to have a small scraper (we cut a strip of a plastic notebook cover) instead of the big squeegee that Sparkfun uses.
Placing the components is actually remarkably simple. The only thing to watch out for is to make sure there are no areas with excessive amounts of solder paste that angle a component so that some leads are no longer touching the pad. Generally, we've only seen this problem on connectors that have gobs of solder paste on the metal anchor points. The populated and soldered board in our device is below. Notice the blue wire fix where we attempted to fix our non-functional USB connection. We also put too much solder paste on the 60-pin Hirose connector (you can see it oozing up the sides of the contacts) but, since it still works, there is certainly room for some error.
