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Overview
Overview
The solar cells used on Iris are the Azurspace triple junction Solar cell 3G30C - Advanced. The analysis for deciding the solar cell is found here. The team will be procuring CICs (3G30A) from Azurspace that are equipped with an external bypass silicon diode, interconnectors as shown in Figure 1 and coverglass. The CICs in Figure 2 are supported and electrically interconnected to form an array. For Iris, there are 7 arrays with 3 body mounted arrays. 4 arrays on 2 wings are deployable. Each array has 3 solar cells and at any point of time, 5 arrays are sun-facing during normal operations.
Figure 1: Azurspace's TJ solar cell assembly 3G30A with Kovar Interconnectors [1]
Figure 2: Dimensions of the 3G30A CIC in mm [1]
CIC Design Properties
CIC Design Properties
GaInP/GaAs/Ge on Ge substrate triple junction solar cell assembly
Coverglass and AR coating (TiOx/Al2O3) on the front side of the cells and bypass silicone diode on one side.
Cell area: 26.51 sq. cm
CICs come with Interconnections that are done using Kovar and a blend of Ag (reduces resistivity losses). Kovar interconnectors form the front side interface (Cathode) and Ge-substrate with Ag/Au contact metallization form the rear side interface (Anode).
Average efficiency at BOL: 29.6%
Bonding the CICs to the Substrate (PCB)
Bonding the CICs to the Substrate (PCB)
Mechanical and electrical connections are required to form proper bonds between the solar cells and the substrate.
Mechanical and Electrical Interfaces
Mechanical and Electrical Interfaces
It is important to note that the back side of the solar CICs is the anode and the negative tabs will be exposed. EPO-TEK-H20E (Datasheet), a strong adhesive with high thermal conductivity and provides damping of any disturbances is chosen to adhere the solar cell assemblies to the PCBs. This adhesive forms an electrical as well as a physical bond and is a common solar cell stringing adhesive and is compatible with fiberglass used to make PCBs. Some of the properties for EPO-TEK-H2OE are:
Thermal conductivity: 2.5 W/mK
Volume resistivity @ 23 C: <=0.0004 Ohm-cm
Suggested operating temperature: < 300 C
Density: 2.6 g/cc [2]
The storage life of this silver epoxy is one year at room temperature , so manufacturing plans have to be made according. It has a pot life of 2.5 days which is limited. Pot life refers to the length of time that an adhesive retains its acceptable properties after preparation for use.
Epoxy Thickness
The minimum bond line thickness for a silver containing, thermally conductive, optical and UV cured and polyimide epoxy is 10-20 micron. The recommended range is between 0.5 to 3 mils. Less than 0.5 mil might make the epoxy electrically insulating [3]. Handling smaller thicknesses is difficult and the team might end up using more epoxy. The selected thickness is 100 microns which may be changed as required during manufacturing.
Total Volume of Epoxy
Epoxy layer thickness: 0.01 cm
Total CIC area: 26.51 sq. cm
Volume: 26.51 * 0.01 = 0.2651 cc
Mass: 2.6 * 0.2651 = 0.7 g
Electrical Bonds between CICs
Electrical Bonds between CICs
The Kovar interconnectors on the CICs are used to make electrical connections between individual cell assemblies. These interconnectors face stress due to mechanical vibration or thermal cycling. Hence, it is important to make sure there is no breaking of the interconnectors. Kovar has suitable thermal expansion properties to avoid stress cracking. The interconnections between the cell assemblies are held by solder.
Testing
Testing
Visual inspection of the flight solar cells is undertaken to check for cleanliness, cracks, alignment, interconnectors, damage etc. The dimensions are also cross-verified with the datasheet.
Three extra CICs are ordered for testing purposes such as creating a string, doing vibration tests, practicing bonding methods, and current generation for one string.
As part of the full spacecraft level tests, vibration and thermal cycling will be conducted.
For the detailed test plan, see the power assembly and manufacturing subpage.
Handling and Storage
Handling and Storage
Contains semi-conductors which can be harmful if not handled properly. Use hand protection during handling. Place cells in appropriate chip trays or polystyrene boxes given with shipment. Must store under clean room conditions at moderate temp. and humidity, only in dry conditions. In operation, must be protected with a bypass diode. Soldering temperature should not exceed 300 C for short time. For detailed handling instructions, see here.
Documents and Test Reports
Documents and Test Reports
The following figures are the documents and test reports that AZUR SPACE provided upon delivery of the solar cells.
Cell Dimension measurements
Cell Dimension measurements
The following shows the measured dimensions of the purchased CICs that we received from the manufacturer. The measurements are not compliant with the verification activity V-POW-0050 (Verify through inspection of the actual solar cells that solar panel cells have the nominal dimensions of 69.1 x 39.7 mm (+/-0.05mm) ). However, this could be because of measurement error and calibration issues. The test results done by the manufacturer shows that all the cells dimensions are within spec (see page 7, acceptance test report).
width measurement of the solar cell. 69.02 mm
height measurement of the solar cell. 39.83 mm
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