The FXOS8700 used as magnetometer overflows start sending -32768 or 32767.

[dimitrov]

Hi, my project includes a few boards (each board has FXOS8700 on it) communicating with a main board. The FXOS8700 is used as magnetometer only. Sometimes one of the sensor overflows on one of the axes and start sending -32768 or 32767.

Any suggestions?

[TomasVaverka]

Hi Dimitar,

In order to better understand why it happens, could you please elaborate a little bit more on your application?

Does it happen randomly or only with other parts or disturbing objects appearing around the board?

How many FXOS8700CQ devices are you using and on how many of them are you experiencing this behavior?

Are you using an SPI or I2C interface?

Regards,

Tomas

[dimitrov]

I have 16 sensor boards connected to a main one. Each sensor board has two FXOS8700 and a micro. The MCU use I2C to communicates with the two sensors. From the 32 sensors total one of them was sending 32767 and one -32768. When a put a magnet close to them they change with few thousands. When I remove the magnet they go back to 32767 and -32768. We had to replace them both.

Could it be a PCB manufacturing problem? We had to reflow few of the sensors to make them work. The boards have HASL finish and are hand populated which I don't think is best combination for that kind of package :smileyhappy: Do you think we should use ENIG finish next time?

[Martin35804]

Low planarity of HASL process compared to ENIG may cause same issues in this case. Low planarity makes this surface finish unsuitable for use with fine pitch components and on plus higher thermal stress during HASL process may introduce defects to PCB's. What is the highest temperature during your HASL process?

[dimitrov]

We use Hot Air Gun Rework station to solder the sensors. I don't know what is the temperature. I've also noticed that the datasheet recommends no solder mask under the package, and we have. Is that too critical? What is the recommended temperature for HASL soldering?

[Martin35804]

For FXOS8700CQR1:

	Peak Package Body Temperature (PPT)(°C)	260
	Maximum Time at Peak Temperature (s)	40
	Number of Reflow Cycles	3

http://www.freescale.com/webapp/search.partparamdetail.framework?PART_NUMBER=FXOS8700CQR1&buyNow=tru...

-furthermore general SOLDERING PRECAUTIONS can be found in following useful document DL200:

http://cache.freescale.com/files/sensors/doc/support_info/DL200.pdf?fsrch=1&WT_TYPE=Supporting Information&WT_VENDOR=&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation&sr=2

and app. note AN3111:

http://cache.freescale.com/files/sensors/doc/app_note/AN3111.pdf?fsrch=1&sr=1

GENERAL PCB AND ASSEMBLY NOTES:

3.Recommended surface finishes are Organic Surface Protection (OSP), Electroless Nickel Immersion Gold (ENIG), or white tin (Stannous). Hot Air Leveling (HAL) can cause uneven surface issues.

[dimitrov]

And there should not be solder mask under the package or between the pads?

I want to make sure that I'm understanding the diagram in the datasheet correctly.

Thanks for your support.

[Martin35804 · Accepted solution]

With the correct footprint, the packages will self-align when subjected to a solder reflow process. It is always recommended to design boards with a solder mask layer to avoid bridging and shorting between solder pads.