Special Manufacturing Equipment
Inert-Gas Spot-Welding System
Probably the biggest challenge when manufacturing batteries for space, is to ensure that they will withstand the strong random vibrations and pyrotechnic shocks that occur during the launch. A neuralgic point w.r.t. vibration or shock damage is the joint between the cell terminals and the tabs that connect the cells.
The standard method to join cell and tab is spot welding. The main advantage of this method is that the risk of overheating the cell cups is low: The weld heat is delivered in a short pulse and is focused on the small contact area of the welding electrodes, so a small total heat input suffices to create the joint.
But unfortunately, the weld joint quality can vary greatly from one spot pair to the next, depending on how thoroughly the weld parameters are controlled. Also, with conventional spot welding the weld joints will always oxidize to some extent as the molten steel comes into contact with air. This can compromise both the conductivity and mechanical strength of the joints.
In order to overcome these deficiencies, we developed our inert-gas spot-welding system which is depicted above. Before every welding pulse, the air around the weld area is replaced by the inert gas. This prevents the oxidation of the weld spots described above.
Our system is also capable of precisely controlling the contact pressure of the electrodes and of monitoring the true weld energy Eweld. The latter is the fraction of the energy drawn from the capacitor bench that actually reaches the weld area and heats it up (a significant amount of energy is lost on the way due to ohmic resistance of the lines). Eweld is determined as follows: The voltage drop between the electrodes is measured over the entire pulse length. Together with the current curve that is also recorded the true weld power curve is calculated as product of the two. Finally the integration of the power curve yields Eweld. The figure below shows an example of these curves that are recorded for every weld spot pair.
Thanks to the features described above, our inert-gas spot-welding system produces high quality weld joints with an excellent repeatability. We are convinced, that they will always pass their random vibration and shock qualification tests with flying colors.
We use our own vapor phase soldering (VPS) system for the assembly of all our PCBs. As heat transfer liquid we use a perfluoropolyether (PFPE) variant with a boiling point of 200 °C, this TMAX lies about 20°C above the liquidus temperature of the solder paste. Since the condensing vapor on the PCB transfers the heat very effectively (much faster than e.g. the air in a convection oven), the most critical section of the temperature profile between 150 °C and TMAX can be programmed as narrow peak with a width of less than 60 s, and a time above liquidus (TAL) of about 30 s. This way we are absolutely sure that no component on the respective PCB receives too much heat during the soldering process. This is a very important aspect, as a heat damaged component can be tricky to identify. And if such a damage does not surface until late in the qualification process of a product, it can become a costly problem.
After each completed VP-soldering action, the recorded temperature curve is checked and filed as part of our quality assurance.
Vibration and Shock
The random vibration test ensures that the device under test (DUT) will survive the random vibrations that occur aboard the launcher. They are most intense during lift-off and the maximum-dynamic-pressure phase of the ascent.
For each newly designed product a qualification model is built and tested on an electrodynamic shaker. The power spectral density (PSD) function for this random vibration qualification is shown in figure below. The DUT is tested in all three axes with a duration of two minutes per axis.
In the series production phase, the individual units then only need to be acceptance tested, as their design is identical to the qualified one. In that case the acceptance-level PSD-function shown in the next figure applies (with a test duration of one minute per axis).
If a customer already knows the specific random vibration requirement from the launcher that will be used for the mission, the acceptance PSD-function can be adapted accordingly.
The shock test accounts for the pyrotechnic shocks that will occur due to separation events during the launch. This qualification test is performed on a drop table shock machine. The parameters are the following:
- Shock Type
- Acceleration Amplitude
- 1000 g
- 100 Hz
This test is performed in all three axes in plus and minus direction, six shocks in total.
Typically shock testing is only applied to the qualification model, so there is no acceptance-level shock test for the individual units.
For our BP3-SN battery the described concept of qualification and acceptance testing is applied straight forward: For example a a BP3-S5 battery unit only undergoes an acceptance-level random vibration test, since its design has been qualified in the past (with a dedicated BP3-S5 qualification model).
For the PCU-SB7 the situation is different: Since every PCU design is tailored to the requirements of a specific mission, each will contain different PCBs and will in turn feature a different mechanical design. Consequently, each requires its own individual vibration and shock qualification.
Our recommendation to our customers is therefore to buy two PCUs with identical design and use one as qualification model. This way the flight model is again only exposed to the acceptance-level random vibration and does not have to go through the shock test.
But in order to save time and money, the customer can choose to omit the qualification model and to apply the tests directly to the flight model. This is possible, because the qualification model is not needed for radiation qualification tests (see also Radiation section).
Please also note that, since the PCU-SB7 requires individual qualification, the customer has the option to chose his own qualification PSD-function and shock test parameters.
The aim of this test is to show that the DUT will not overheat in a high vacuum environment where the heat transfer is limited to thermal conduction (via PCB and heat bridges) and thermal radiation.
The vacuum chamber is evacuated to 10-5 mbar and its temperature is set to the specified maximum operating temperature of the DUT. Then all thermally critical operating modes of the DUT are tested consecutively. Each mode is upheld until all temperature sensors show a steady state reading. Finally, these readings are compared to the tolerated maximum temperatures.
The thermal vacuum test is not exclusively used for qualification, it is also a part of the functional verification tests, which are performed for every flight model we build. The reason is, that this test is the most effective way to verify the proper implementation of all heat bridges. It also does not damage the DUT or increase its failure risk in any way, which cannot be said for the vibration-and-shock qualification or the radiation qualification.
For the battery also the cold case is examined to prove that it performs well at the specified minimum operating temperature. This qualification test does not require a vacuum environment and is therefore performed in a simple climate chamber.
To evaluate the total dose effect of the radiation environment we irradiate the respective qualification model with a cobalt-60 source. The DUT is exposed to a total ionizing dose (TID) of 20 krad. This corresponds to approximately 150 % of the dose that the DUT would collect over 5 years in a 700 km sun-synchronous orbit, assuming no shielding contribution by the satellite's structure (only the shielding effect of the DUTs own enclosure is taken into account). The DUT is powered and operated in a representative mode for the entire duration of the exposure. The extensive functional (health-) test is performed after the irradiation in a separate test setup.
After a total dose qualification test the respective PCBs cannot be used in a flight model anymore since the accumulated material damage caused by the ionizing radiation is permanent. So, effectively the qualification model is lost, aside from the enclosure and other mechanical parts. This is especially bad for tailored devices that need individual qualification.
This is where the standard building block approach we use for the PCU-SB7 pays off once more: The standard building blocks have all been TID qualified separately (each with a dedicated test setup to enable to operate it in a representative mode). And since all taylored PCU-SB7 units consist exclusively of these qualified standard blocks, they do not require an individual TID qualification.
It is also worth mentioning, that the battery electronics and most of the PCUs standard building blocks have already been verified in orbit. This in-orbit experience is very valuable with respect to single event effects (SEE). Especially the potentially destructive single event latch-ups (SEL) can not be adequately investigated with proton testing on earth: The protons used for those tests usually have energies below 200 MeV. At the same time the (shielded and orbit averaged) flux for protons above this level is about 15 cm⁻2s-1 in our 700 km reference orbit. The total shielded proton flux in that orbit is about 90 cm⁻2s-1. This means that on average every 6th proton exceeds the 200 MeV.
On top of that, even in LEO there is a small number of heavy ions with much higher energies, that are especially effective in creating SEL. So in order to completely characterize the SEL hardness of a component with testing on earth one would need to include heavy ion testing which is simply not affordable for most small satellite projects.
We consider it best practice to use proton testing for new components, in order to filter out extremely vulnerable candidates and to otherwise stick to components that have been successfully used in previous missions.
In addition to relying on in-orbit experience we use software functions and dedicated circuits in our PCU-SB7 design to contain SEL when they happen, either in the PCU itself or in a connected device.