From 16.09. – 25.09.2019 I had the opportunity to get an impression of the production of radiosondes at Graw Radiosondes GmbH & Co. KG in Nuremberg in the form of an short-term internship. I experienced almost every step in the manufacturing process of a radiosonde myself and was able to gain first-hand impressions.
Two things surprised me the most. On one hand how much manual work is in the manufacturing process of each radiosonde. Each sonde goes through many individual steps from the pre-check of the circuit board to the assembly and calibration all the way to the packaging and, by the time it is finally on its way to the customer in the packaging, it has probably been touched two dozen times. The other thing is that what we mainly deal with, GPS, microcontrollers and radio, is hardly important compared to the know-how that goes into the development, production and calibration of the sensors.
Graw currently produces three different types of radiosondes: the DFM-09, the DFM-17, its successor, and the PS-15, a pilot sonde without TU sensors. Apart from the calibration of the sensors, which of course is not necessary for the pilot sonde, the production is relatively similar for all sondes. The production of the sensor booms is also done in-house and is very similar for DFM-09 and -17. The ground stations are assembled in the production facility of the sister company Noris, which is located in the same building complex.
First the production of a sensor boom will be described. The basic material is a flexible PCB that is supplied externally. The PCBs are supplied individually punched out. Ten PCBs are each placed and clamped in a rail-mounted frame. Solder paste is applied to the pads for the thermistor of the humidity sensor (which will not be fitted until much later) using a time-pressure dispenser, followed by manual fitting of the thermistors measuring size 0402. Under the microscope, the thermistors are soldered with hot air and aligned manually.
In the next step, the thermistors for the temperature measurement, which are THT components, are placed in a similar jig over the corresponding pads on the sensor booms, solder paste is applied and the thermistors are soldered under hot air. The sensor booms are then cleaned from flux residues in an ultrasonic bath with rubbing alcohol.
A thermosetting insulating paste is then applied to the solder joints under the fume hood and then cured on a hot plate. In addition to mechanical protection, this also serves as insulation against the reflective coating that will be applied on the sensor boom later. In this step, a piece of kapton tape is also put over the footprint of the humidity sensor so that it will not be coated. The reflective coating of the sensor carrier and the cap covering the humidity sensor is performed by an external specialist company. This process includes not only aluminum deposition, but also the additional application of a protective lacquer. Finally, the coated sensor booms are tested in a test jig for insulation/conductivity of the respective contacts and the (coated) kapton tape is removed.
For the sondes, the manufacturing process begins with a pre-test. The assembled PCBs arrive at the factory unflashed, but separated from the panel and with a soldered switch/pushbutton in reusable boxes from two differnt PCB assembly companies. The PCBs themselves come from two different manufacturers in China.
During the pre-test, the sondes are electrically tested, flashed and in part adjusted and/or calibrated. In addition, each sonde is assigned its serial number and the first small label at this stage.
For the DFM-09, both PCBs, which are still connected with perforated bars, are first electrically connected with a ribbon cable. The sonde is then placed in a test jig that automatically tests basic parameters such as power consumption. The quartz and oscillator frequencies are set via two trimmer capacitors, which are operated via two stepper motors with shafts and screwdriver heads attached to them. In addition, the sonde is flashed and the GPS and 403-MHz-TX are contacted and tested via coaxial test connectors. For the DFM-17 the programming is done via the groundcheck connector by bootloader in temporary lack of such a jig.
The sensor booms are then inserted into the connector located on the PCB and the sondes are moved onto a production cart. The temperature calibration can then be performed. To do this, the sondes are inserted into a carrier frame and then calibrated in temperature baths at different temperatures. The liquid used is ethanol at the higher temperatures and a special coolant at the lower temperatures. During these calibrations, the sondes are connected to the control system via pogo pins, and if calibration is successful, the control system stores the correction data on the EEPROM of the sonde.
After this step, the humidity sensor can be soldered on. A reflow process is also used here, but due to the sensitive nature of the sensor, it is placed with a vacuum gripper and soldered on a hotplate. The humidity sensor can then be calibrated. Therefore, the sensor booms of sixteen sondes are positioned at the same time in a duct through which air of variable humidity is circulating. In this channel, air can be introduced which can be controlled in its water vapour content by drying agents or by passing over a water surface. The calibration takes place at different humidity contents, but only at room temperature. Again, the correction data for each sonde is calculated and stored on the EEPROM after successful calibration.
Now the manual soldering can take place. In the case of the DFM-09, this includes the antenna, the batteries and the coupling of both circuit boards through the RF shield and the GPS antenna, the latter can be omitted in the case of PS-15 and DFM-17. This is followed by the final inspection, in which the sondes are tested in a second test station similar to the one used for humidity calibration. Here various parameters such as plausibility of PTU values, telemetry transmitter and GPS reception are verified. For this purpose a GPS repeater system is in use in the factory, which receives the GPS signal through a roof antenna and distributes it to transmitters on each test stand. Each sonde that successfully passes the final inspection is visually inspected and the protective cap is placed on the humidity sensor.
The finished electronics are then mounted in the housing. With the DFM-09, this is done in two steps. First, the sondes label is printed out and attached to the enclosure, the sonde is then placed in the enclosure. In the second step, the cover is glued to the enclosure, again using a time/pressure dispenser. With the DFM-17, the batteries are inserted and the sonde is checked for function (by switching it on). The maximum run time of the sondes is one minute when leaving the factory. The sonde is placed in the polystyrene housing, the lid is put on and the sonde is locked with a cable tie, before the sondes label is stuck on.
The last step is to pack the sondes. In general, customers can choose between vacuum and cardboard packaging, which has hardly any disadvantages under normal storage conditions. With the DFM-09, in this case ten sondes are packed in one cardboard box. Before this, however, the sondes are inserted into the plastic sleeves, which are assembled in an adjacent workshop for the disabled. The unwinder, which is delivered fully assembled from a workshop for the disabled in Belgium, is inserted into the part of the plastic sleeve that positions the sensor boom in flight. The DFM-17 is placed in a box with a cardboard inlay that keeps the sonde and unwinder in place. With the DFM-17, the stick is already pre-assembled on the unwinder and only needs to be clicked into position by the user inside the cover of the sonde around the cable tie. Furthermore, the unwinders no longer have a plastic sleeve, but a cardboard disc on the upper side and are exclusively equipped with green balloons.
During my internship I gained a lot of insights into production and development and probably held more radiosondes in my hands than I can collect in the next years and decades. For this I would like to thank the whole team of Graw and Noris, especially Mrs. Christina Flohry, Mr. Florian Schmidmer and Mr. Norbert Traeger!