Introduction

Accelerometers from the MMA84XX family are regularly used in cell phones, laptops and other high end electronics. They are very powerfull devices capable of measuring at high speed (800Hz max), high resolution (4096 counts/g), and are easy to use as they provide all their data on a conventional I2C bus. Best of all, they are not expensive at all priced at about 1€/piece in small quantities.

The problem

Despite accelerometers being very useful to hobbyists, these accelerometers are rarely used because of their unusual 16-VFQFN package:

A package with a size of only 3mm x 3mm and a lead spacing of a depressing 0.5mm. To make matters worse the solder pads are not exposed and only accessible under the package itself. Finally the datasheet mentions the device not being hand solderable.

The solution

The PCB

This device deserves at least an attempt to be used for hobby use. Using the free version of eagle a single layered pcb was made using the regular "toner transfer" method. Many people have already described this wonderful method, and everyone has their own preference in paper, but the general idea is that the cheapest publicity paper can easily produces PCB's with the required resolution.

Soldering the chip

The chip cannot be soldered conventionally, not only due to its small size, but also because it has no exposed leads. The only way to solder the chip is to heat all the pads of the PCB at once, and then drop the accelerator chip onto the molten solder. Heating all the pads can be done succesfully either using a Butane Gas powered soldering tool, or even a hot air gun. While the first is more expensive, it is recomended because it gives more localized heat, and you will not burn your hands as much. Although both methods work equally well.

The key to success is to let the solder position the chip for you, when the solder is sufficiently fluxed it will pull the chip in the correct position automatically, removing the need to position the chip very carefully up to 0.25mm accurately. This also implies that solder flux is absolutely essential. Furtunately the method appears to work even with S-39 flux. In general S-39 is not suited for electronics since it is mildly conductive. However due to absence of other fluxes it has been tested successfully as well. So to summarize the method:

  • Apply sufficient solder to the PCB pads.
  • Apply solder flux to the pads. Not too much since bubling S-39 will move the chip.
  • Put the chip more or less accurately on its pads.
  • Heat until the solder melts and 'pulls' the chip in position.
  • If necessary slightly push the chip downwards if no pulling appears.

The result:

Being careful, and not heating the chip too long is essential! Please note the black burning marks at the edges of the PCB, showing my considerable skill and carefullness ;-)

Testing

Testing the chips is trivial once you are the proud owner of a bus pirate. Off course testing can be done with any I2C capable tool or µC, but in my experience nothing is as easy as the pirate. The following screenshot shows the succesful communication the the chip, asking for its WHO_AM_I register located at 0x0D.

From the 5 chips soldered on the board 1 malfunctioned due to either the heat or bad soldering. However keeping in mind the low cost of the chips, even one functioning chip on 5 attempts would have been a success.

Conclusion

This chip can be relatively easily soldered using standard equipment and some patience. Making this device a very attractive chip for tinkerers. Have fun!