Small SMD Soldering Problems (BMP390)

[Neon]

Hi,
I’m Leonardo Di Bartolomeo and i am part of the “La Sapienza” University of Rome Rocket Team, where we assemble small rocket models.

We started developing our own boards this year and started a partnership with Aisler. We received some amazing PCB from them, but we are now having problems soldering some components.

The culprits are the BMP390 barometric sensor from Bosch and the LIS3MDL magnetometer from STMicroelectronics. While the two LSM6DSO32 present on the same SPI bus solder (and work) perfectly.

We first tried soldering the board trough a heating plate (from Adafruit), resulting in a loose connection between the sensors and the board. We then started focusing on the barometer, trying to reflow it with a heating gun and large amounts of flux.
While the reflow resulted in a stable physical connection, it still won’t communicate.

We are sure that the boards from Aisler are perfect, as we have analyzed them thoroughly under the microscope.

Has anybody tried packages so small (2mm x 2mm with pads 0.25mm x 0.27 mm)? Are there any suggestions or good practices?

Pad of a different board:

Pad after the heating plate (and the sensor broke off):

Sensor after reflow (phisically connected, but not responding to the spi channel):

Thanks a lot in advance,
Leonardo

[Thea]

The routing underneath the MEMS sensor may cause this issue.
MEMS devices are sensitive to mechanical stress, most manufacturers forbid the routing of traces, vias etc. underneath the package.

See this info from ST:

Flux and Cleaning Agents can also cause issues, especially with environmental sensors which have openings.

At a recent event, I’ve used a WE-HIDS MEMS sensor and had a near perfect success rate of all assembled PCBs by following the reference layout and by using a low-temperature paste which reduces the thermal stress on components and PCB.