Chapter IV: Large Scale Flexible Arrays
A.5 Mechanical and Thermal Design of Circuit Boards
Figure A.7: (a) Sources representing MAPLE reference oscillators are DC-coupled, risking complete system failure if one fails such that its enable/disable digital pin no longer works. (b) Sources representing MAPLE reference oscillators are AC- coupled, allowing individual oscillators to fail as shorts to VDD or GND without effecting the other oscillators.
rated from their current limit, creating concern and a time-sink that could have been avoided.
mechanical design choices are often intertwined. The chassis thermal connection to the custom RFICs of MAPLE’s flex array was a substantial concern during EM design and was the only part to experience failure in preliminary environmental testing. While the custom part and custom heatsink proved challenging, there are several conventional approaches which should be employed in most designs.
1. Wires that interface with a circuit board should have strain relief designed into the PCB. The strain relief ensures that solder joints do not bear mechanical loads. Fig A.8 shows the strain relief used in MAPLE.
2. Circuit components which dissipate power should have clear thermal paths to the thermal sink. Staking, use of small amounts of epoxy to fix components to PCBs, and potting, complete coverage of components with epoxy, bolster mechanical and thermal connections. While potting can cause thermal issues by insulating components from convection in terrestrial applications, no such concern exists in the vacuum of space. A thick ground plane (2+ oz copper) and exposed conductor around the perimeter of the PCB help get heat out of the board. Thermal epoxy can then be used to connect the PCB to the chassis frame, creating a wide, low thermal resistance path. The epoxies used in MAPLE are documented later in this subsection.
3. Circuit board substrates with superior thermomechanical properties should be used when possible. Low coefficient of thermal expansion (CTE) and strong mechanical adhesion of traces even after many temperature cycles is desired. Polyimide is the go-to space grade substrate, selected over woven fiber glass composites such as FR4. For multi-layer PCBs consisting of different substrate materials, CTE should be matched to prevent warping or delamination of traces at temperature extremes.
4. Ensure that the minimal width traces and vias on a board are mechanically viable. MAPLE’s polyimide boards used 8 mil minimum trace width rather than 4 mil (which is within the capability of the board house manufacturing equipment) to guarantee robustness.
5. A polymer coating, typically parylene, should be applied to populated circuit boards when possible. The polymer coating provides electrical insulation in the event of loose conductors or debris and prevents moisture damage or
corrosion. The MAPLE team elected not to coat areas with 10 GHz radio frequency signals as the parylene was not present in simulation or testing.
6. For bare die integrated circuits or BGA parts, underfill epoxy should be used.
Figure A.8: (a) Strain relief consisting of two holes and a solder pad. Wire emerges for use on the camera side of the board. Thermal damage is evident in the stranded wire insulation. In later assemblies, insulation with better thermal property was used. (b) Wire strain relief seen from the opposite side of the board. (c) In later versions of the board the solder was enlarged and moved further from the holes.
Additionally the ground pour around the wire holes was covered with mask to reduce risk of shorting.
While not strictly thermal or mechanical in nature, there are several general design principles which be followed. Vented screws should be used for blind-tapped holes to prevent failure in vacuum. All stand-alone conductors should have a DC current bleed path to chassis ground. In MAPLE, all circuit boards share “PCB ground” which connects to chassis ground at a single point through a mega-ohms sized resistor. The bleed path prevents large potential difference build-up and its accompanying plasma/arcing issues; use of a single resistor to connect to chassis ground avoids unintentional current loops.
Payloads require a variety of epoxies during assembly. Spacegrade epoxies with proper outgassing and temperature behavior are necessary. Below is a list of epoxy’s used in the MAPLE payload.
1. For mechanical purposes, 3M DP2216 Gray, was used. DP2216 has a 90 minute work life, curves over several days at room temperature, and has no special application requirements.
2. For the critical thermal connection between the custom RFIC and the alu- minum PCB frame which acts as a heat sink, Loctite Stycast 2850 and CAT 9 were combined at a ratio of 100:3.5 by weight. Boldline control beads
were also added to the mixture. The mixture was vacuum de-gassed prior to application.
3. For creating thermal bonds between PCBs and the aluminum components they are mounted on, a mixture of Solithane 113 and Catalyst 300 at a weight ratio of 100/65 was used. For potting and encapsulation of components on circuit boards a mixture of Solithane 113, Catalyst 300, and CAB-O-SIL at a weight ratio of 100/74/10 was used.
4. For the custom RFIC underfill, NAMICS KT11276-1-54 was used. The epoxy must be stored below -40𝑐𝑖𝑟 𝑐C. Prior to application, the parts must be pre-baked, and after application the underfill must be cured at temperature.
Improper application can lead to voids forming. For the FM parts the MAPLE relied on Palomar Technologies for application.