OXIGEN SENSOR: MANUFACTURING THE SENSOR

05-05-2019 - Reports

 

Oxygen sensor. Manufacturing the sensor

In this report we will talk about the three types of sensors that we manufacture in FAE and the evolution that this product has followed over the years.
 

1. Introduction to the lambda sensor

1.2. Structure of the lambda sensor

MANUFACTURING PROCESS

2. Barbotine: Formula and mixing
3. Cast
4. Cutting into sheets
5. Hole punching
6. Ink
7. Filling tracks
8. Screen printing
9. Profilometer
10. Stacking
11. Laminating
12. Protection
13. Personalising
14. Sintering
15. Verification

 
 
1. INTRODUCTION TO THE LAMDA SENSOR

FAE uses multilayer planar technology to manufacture its Oxigen sensor sensors. The sensor is made up of different layers of a ceramic substrate. Each of these layers has a different function; each layer is configured separately, and they are then joined together one by one to form a set with complete functionality.



1.2. STRUCTURE OF THE LAMBDA SENSOR

Lambda sensors work by comparing two atmospheres: exhaust gas and external air. The difference in the oxygen concentration of the two gases generates a voltage in the sensor which is read by the car's Electronic Control Unit (ECU).
In order to do this, within the same sensor there have to be different parts in contact with the exhaust gas (measuring electrode) and with the clean external air (reference electrode). The sensor also needs a resistant Heater, as well as connective paths and insulating parts to avoid interference.
Each and every one of these parts is located in a custom layer specific to its function. These layers will be stacked and compressed to form a sensor. So, let's see how these layers are formed and what's used to make them.
 
                                                                                                                                                                                                                                                                                                                                                                                              FAE sensor.


 

                      Basic structure of a sensor.













 
Arrangement of the layers in a sensor and their function.




2. FORMULA AND MIXING

The mixture is made using a monitored manufacturing protocol. The homogenisation is carried out using mixers and mechanical filtering systems, which guarantee the viscosity and homogeneity of the product, and is controlled by a rheometer.
 
Raw material used for manufacturing the ceramic substrate.
Rheometer for measuring the viscosity of the ceramic substrate.

 
 
3. CAST

The ceramic substrate is created in this machine, which forms a continuous tape with a thickness of 200 microns. The ceramic paste is deposited into a transporter material of siliconised PET, which advances at the speed of 1 linear metre per minute through a system of valves, which ensure that the ceramic is deposited homogeneously. Immediately afterwards, the barbotine cast is dried at a controlled temperature and develops a flexible consistency similar to paper. This material has flexible, plastic properties which come from the organic additives. In this state, the ceramic is known as "ceramic in green".

 
      The ISO7 White Room with a controlled atmosphere.

 
        
 
The roll which is made in the casting machine is sent to the White Room for processing. The controlled atmosphere of the room is essential for manufacturing multilayer planar technology. It ensures there are no dust particles trapped between the ceramic layers which could compromise the quality and functionality of the product.

 
Drying of the barbotine.
The barbotine is cut and rolled for further handling.

          
 
 
 
 
 
4. CUTTING INTO SHEETS

The ceramic substrate roll enters the machine and is cut very precisely into sheets of the appropriate size needed for its automatic processing by all the relevant equipment. Each roll of ceramic substrate makes the number of sheets necessary for manufacturing about 3000 sensors.
Once each sheet has been cut, it is then weighed, and its thickness and homogeneity are controlled by artificial vision cameras. These can detect any pore, crack or irregularity in the substrate.
Holes or fiducial markers are made on the sheets in order to facilitate the control of their position by artificial vision cameras.
 
              


 
                                                                                                                                                                                             The tapes are cut and marked one by one.

 
5. HOLE PUNCHING

This machine customises each sheet for the function it will need to perform in the sensor. Depending on the type of sensor being made, some layers will be used to house the heater, while others will be used for electrolytes, insulators, etc. For this reason, it's necessary to make different types of holes or cuts in the sheets. These holes have a maximum size of 300 microns.
These holes or cuts will later be filled with special conductive inks.
The fiducial markers assure that the holes made in each of the tapes will coincide perfectly when they are layered on top of each other to form the layers of each sensor.




                                                                                                                                                 
                                                                                                                                                                              Close-up of a tape which has been punched and                                                                                                                                                                                                        marked with its barcode.

              
 
6. INKS

One of the secrets to manufacturing this type of sensor lies in the development of the inks. These inks have a specific formula for each function, and contain metals such as platinum which provide the sensor with its required functionality.

 
 



 
                                                                                                                                                                                 Each ink is formulated for a specific function.
                     
 
 
 
7. FILLING TRACKS

This machine fills in the previously made cuts or tracks with the conductive inks that have been specially created for each function.

 
 
 
 

 


 
 Tape with its tracks filled with
the specially formulated ink
. 


8. SCREEN PRINTING

Screen printing is used to create the electronic circuits. This is similar to the printing used on t-shirts, but done with extreme precision.
Through the control of the artificial vision camera, the punched tape is perfectly lined up with the printing plate in order to pass the ink. Then, it's weighed, and the thickness is checked. Depending on the required thickness of the ink, up to 3 passes may be necessary.
Afterwards, the inks are dried in an oven.




 
                                                                                                                                                                           Special printing plate for screen printing a heater.              



9. PROFILOMETER

To control the thickness and homogeneity of the ink, the printed tape is passed through a Profilometer. This machine uses a laser to make a micrometric scan of the screen printing, in order to verify that it has been printed correctly.
 
 
 
 

 
 
 
 
                                                                                                                                                                       Perfilómetro para el control de las tintas impresa.

 
10. STACKING

Depending on the sensor, the sheets outlined in the design are stacked in order, according to their function in the sensor. Everything is controlled by a bar code system.
In order to stack the sheets, the transporter PET that has accompanied the ceramic substrate during the entire process must first be removed. The different sheets are layered on top of each other and centred using the fiducial markers, which are controlled by the artificial vision cameras, to create a substrate with all the necessary layers. This houses between 60 and 90 sensors.
The stacking is done by a robot arm to optimise the work.


 
Tapes ready to be stacked.
The robot arm manipulates the tapes for stacking using air suction.


 
          
 
 
11. LAMINATING

The substrate, which is made up of 10 layered sheets, is still a soft, malleable material and the layers are only provisionally bound.
The isostatic laminator uses water at a temperature of 70°C to apply a constant, homogeneous pressure of 300 bar to the sheets for 30 minutes in order to completely bind the sheets together.
.
 
 
 
 

 
 
                                                                                                                                                                                                                                                                                                                                                                                   Bag for isostatic laminator FAE.
 
 
 

12. PROTECTION

Using another screen printing machine, a resistant layer is applied. This protects the electrodes and the parts of the sensor that will be exposed to the exhaust gases, in this way lengthening the lifespan of the sensor.

 
 
 
 

 
 Interior de la máquina de serigrafía FAE
que aplica la capa protectora a los
electrodos de platino.



 
13. PERSONALISING

Prior to the sintering of the ceramic, and while the substrate is still flexible, the sensors are pre-cut into units of 10. This helps to facilitate their subsequent handling and the verification of their functionality.

 

 
 
 
 
 
 
 
                                                                                                                                                                                                             Pre-cut sensors.
 
 


14. SINTERING

The pre-cut sensors are placed in an oven where they will reach a temperature of up to 1500°C for 2-3 hours, using a special temperature control system.
During the sintering process, the molecules of organic materials in the ceramics and the solvents in the inks evaporate, and the materials harden.


 
Sensors ready to be baked and sintered.
 

Interior of the oven where the sensors are subjected to a temperature of up to 1500°C  for 3 hours.


                
 
 
 
15. VERIFICATION

Each group of 10 pre-cut sensors is subjected to a check on the sensor and the resistance of its heater. The sensors are brought to high operating temperatures and their strength and functionality are checked.
The sensors are now cut one by one, and are ready to be assembled into their sensor group, and then placed into the lambda probe of whichever car they're intended for.
 
 
 
 
 
 
 
 
FAE sensor group.