BME280 Humidity measurement showing variation board to board.

[Manmohan]

Hi Team.

I have designed a board for temeprature humidity and pressure measurement. to acchive this I have used BME280 Sensor.

After the assembly when I tested the boards the Temperature and Pressure value of all the boards varing in simmilar manner but Humidity in all the boards are not varing in similar manner. drastic change in humidity sensor is obsereved board to board.

Conditions:  All the boards assembled at same time by considering the handling_soldering_mounting_instructions/bst-bme280-hs006.pdf.

                          All board has same code running.

                          All boards tested in same enviornement and at same time and end results captured after 30mins.

                          We followed Reconditioning procedure of a sensor after reflow as a Ambient rehydration 25deg  at >40% rH for 5days.

below are the recorded data.

Actual Humidity: 30.729303 %RH
Actual Temperature: 40.936070 deg C

Board. 2
Actual Humidity : 18.498017 %RH
Actual Temperature: 40.616042 deg C

Board. 3
Actual Humidity : 22.974170 %RH
Actual Temperature: 40.614773 deg C

Board. 4
Actual Humidity : 36.645458 %RH
Actual Temperature: 40.193592 deg C

Please suggest If there is any calibration techniques.

attached is the layout snap.

[BSTRobin]

Hi Manmohan,

What is the surrounding environment like when you read the temperature, humidity, and pressure values of BME280?
Do you mean that you have designed 4 circuit boards, and all 4 circuit boards have different humidity values in the same environment?
Has the humidity value of the four circuit boards always been different at all times?

[Manmohan]

Hi Robin,

Below are the clarifications.

Que. What is the surrounding environment like when you read the temperature, humidity, and pressure values of BME280?

Ans. Surrounding temeprature 27- 28deg / humidity is 55-60%  but due to board temeperature raise caused by other heat sourcing elements like controler and converter whic is contributing 12deg rise in temperature from the ambient 28deg temeperature which is coming around 40deg. the temeprature sensor giving correct 40deg reading as I compared it with fluke temeprature measurement device.

Que. Do you mean that you have designed 4 circuit boards, and all 4 circuit boards have different humidity values in the same environment?

Ans. The design is same but no. of boards are 4. and all 4 boards are showing different humidity values in the same environment. where as temeprature is showing nearly same.

Que. Has the humidity value of the four circuit boards always been different at all times?

Ans.  Yes Kind of, but one poin I observed after 3 4hrs of time the error in the humidity decreseas like it will con to 6 7% variation board to board.

[Manmohan]

Hi Team,

Please help me on the above issue.

[BSTRobin]

Hi Manmohan,

Usually, we do not recommend routing wires on the bottom layer corresponding to the BME280 solder pad. This area needs to be cleared and copper should not be laid.
The wiring with high current needs to be far from BME280. The BME280 itself also needs to be kept away from heat sources.

What are your heat sourcing elements? What is the maximum ambient temperature that they may experience while working?
How far is the heat sourcing elements from BME280?

Additionally, when the heat sourcing elements are not working, i.e. not heating, what are the temperature, humidity, and air pressure of the BME280 on the 4 boards?