Can PCB design (not following all guidelines) really affect the performance (precision) of the LPS25HB sensor when using it at highest precision ? Can the LPS25HB works properly at best precision without vent aperture ? in a waterproof ip65 case ?

[VLeme.2256]

Hello,

I work on a wearable solution with your pressure sensor LPS25HB.

I have a problem with the precision of the sensor. Using the LPS25HB adapter board (STEVAL-MK1165V1), I have sufficient precision to detect a high difference of 50cm (I use the biggest precision possible but with only a pressure reading frequency of 1Hz to have a low power system).

But when I use the sensor LPS25HBTR placed on my PCB, I don't reach the same performance. The sensor can not detect with good reliability a fall of 50cm ... (of course using the same code)

The sensor is used in the same environment and with same vent conditions (not in a case, see PCB3D view attached).

Could it be due to PCB design guidelines that are not strictly followed ? For example, I have a trace and a via below the sensor, maybe to big tracks,...

Could it be due to heat propagation regarding my design (see attachments)

Second question, in my final design I will put the sensor in a waterproof case (OKW body case IP65 https://www.okw.com/en/Body-Case/B1604217.htm?ref=8fda6890-c3c4-11e5-96da-8eba63e66ed5).

I read in the documentation (AN4672) a guideline saying that the sensor can work without vent aperture. Is it true for design where we use the best performance of the sensor (to detect 50cm height differences, so highest precision). The application has the purpose to detect the height difference before and after falling.

Do you know if the vent aperture is mandatory if I use the sensor at its maximum precision ? Does the vent aperture only have an impact on response time or also on precision ?

Thanks for your consideration,

Waiting for your answers,

Regards,

Valentin Lemenu

[Eleon BORLINI]

Hi @VLeme.2256​ , if I well understand the problem, I'm afraid that you are not able to detect a pressure variation if you enclose your LPS25HB pressure sensor in a sealed case, because the air cannot flow into the case. If you cannot use another (waterproof) pressure sensor like the LPS33HW and the guidelines, you can design your gasket/case with an hole covered by a pressure vents (I used like these ones in the past). They can block the humidity and let the air flow at the same time. Regards

[VLeme.2256]

Hi @Eleon BORLINI​ ,

Thank you for your reply. The LPS33HW does not fit in the case. I think I will have (as you say) to add holes in the case and use membranes that can block water but let air flowing ...

For my first question, my pressure sensor is not in a case. Do you have any idea why I don't reach the same performances with the LPS25HB on my PCB than with the adapatater board ? Is it really due to the PCB design that does not follow the guidelines ?

Regards,

[Eleon BORLINI]

But can you measure well the static pressure? I mean, the 50cm-above-the-floor pressure and the floor pressure are distinguishable? if so, the problem is in the free fall transitory, and you can improve it changing the aerodynamics of your board by shrinking it or inserting mechanical through holes in your pcb. Regards

[VLeme.2256]

In fact, I measure a relative pressure. But yes the he 50cm-above-the-floor pressure and the floor pressure are distinguishable. I measure the pressure before a fall (more than 50cm from the floor) and the pressure after the fall (at 0cm from the floor) and I check the pressure difference between the 2. (I also take an average on several samples for each case to have a reliable value). This works very well on a breadboard prototype with the adaptateur board STEVAL-MKI165V1 (when I let it fall from 50cm), I have approximately 100 bit pressure difference (I use raw bit value as I want relative pressure, I don't need to convert it). When flash the code on the PCB board (see above), I don't get the same result, the accuracy seems lower ... what is weird because I use the same chip LPS25HB (same as on the STEVAL-MKI165V1) ... I hope this is because PCB design issue (I have a trace and a via under the chip).

My PCB is OPEN so I don't see how the aerodynamics can be the problem ? The air can easily flow on top of the board (like on the breadboard prototype which works very well) ...

[Eleon BORLINI]

My question about the aerodynamics was related to the case you had to measure the transitory of the pressure during the fall and you wasn't able to get the desired accuracy. Btw, I don't believe the layout should impact the performances of the sensor in this way, because those guidelines refer mostly to the enclosure of the pressure sensor in a case. Did you had any chance to measure your sensor before and after the soldering on your pcb? Did you follow the correct guidelines for the part soldering Regards

[VLeme.2256]

Ok I understand, I don't measure the transitory of the pressure during the fall. I just check before and after the fall.

No I didn't measure the sensor before and after soldering on the PCB. I am not sure if the production followed the soldering guidelines. I see only this line that looks important for soldering process : In order to reduce residual stress on the components, the recommended ramp-down temperature slope should not exceed -3 °C/s.

I'll check with my colleagues. A shift of performance could indeed come from a wrong soldering process.

[Eleon BORLINI]

Yes please check also the top surface of the soldered sensor package if it is well preserved or has some contamination. It is better to use a microscope, if you have one. Regards

[VLeme.2256]

What kind of contamination could I see ? do you mean bridges ?

Can I find the recommanded soldering profiles of the LPS25HB ? We use lead-free alloy to solder or PCB and the oven is configured for the solder paste we use, which is the ALPHA OM-338 CSP.

Do you use lead alloy for soldering ? that would mean smaller soldering temperature. Maybe the sensor may not be soldered at high temperature required with lead-free solder paste ... But I can't find the information in your documentations.

Thank you very much for the help !

Regards

[Eleon BORLINI]

The top of the LPS25HB package has 4 little holes, that allow the outer pressure to be equalized with the inner pressure and if the soldering process is not "clean" (any kind of contamination) one of the risks is that these holes can close. About the soldering profile, I suggest you refer to this soldering guidelines, p.10: they are for MEMS microphone but being the this sensor one of the most "gentle" sensors for the soldering p.o.v. they can apply also on other sensors. Ps: no we use the lead alloy no more =)

Regards