Few months ago a customer asked me to develop a portable spectroscopy system with the following requirements:
- Portability: 25x25x50mm 7.5% CsI(Tl) hand polished and enclosed in proper special reflector.
- 100% Solid state: no PMT! Four 6x6mm MicroFC SiPM are used.
- Should be compatible with BecqMoni, Theremino, PRA: audio CODEC from T.I. PCM29xx was choosed.
- No tune: no complicate controls, just plug in the USB-C cable and set line-input level.
- Elegant design: classical “black monolith” geometric shape 3D printed in Nylon carbon reinforced by Xoometry service. I’m not affiliate with them, high prices but also high quality of parts and support.
The prototype realization
The construction of this device followed these steps:
- Design of enclosure with FreeCAD around the dimensions of the crystal.
- Design and draw on Eagle CAD of the electronics, schematics and PCBs.
- Commit to Aisler the manufacturing of PCBs.
- Assembly of PCBs.
- Assembly of SiPMs on daughter board and coupling with crystal.
- Assembly and test of the system.
- After full validation of the system, commit to Xoometry the 3D printed enclosure.
- Final assembly and testing.
FreeCAD is impressive! I stronly recommend it, give it a try! At first is not intuitive but after a couple of hours of giggling with it you can get this kind of results.
The schematic have some annotations on it that where determined empirically during testing of the circuit.
4. Assembly of PCBs
It’s standard SMD tecnique: I’m using stencil and solder paste. After the application of solder paste on the SMD pads I place components than “cook” the PCB in an infrared solder reflow oven. In alternative I use hot air blower.
5. SiPMs assembly on daughter board
This is a triky job! Soldering a 75€x4 (300€) worth of PCB is a task not to be undertaked. SiPM should receive the less heat as possible, it’s optical surface should remain clean and free from flux!!! For this reason I’ve designed SiPM package in Eagle CAD with solder pads long enought, 2mm excess from all sides of SiPM in order to apply heath with my soldering station tip from there without even touching the SiPM. In practice after I’ve covered the pads with the right ammount of solder paste, I place carefully the SiPM. Touching the pad with the soldering iron causes the paste to melt and reflow under the SiPM contact soldering it correctly in place.
I’ve not used hot air or IR reflow oven for this task!
The coupling with crystal is simply made with a special optical coupling, thick, double side tape. In alternative you can use optical coupling grease. The important thing here is to cover the PCB around the SiPM with PTFE to complete the optical reflector layer.
Finally, the connection wire where soldered on the back of the SiPM daughter board and the whole assembly was sealed by a layer of aluminium tape and a layer of 0.1mm thick mu-metal foil. Some automotive grade gray silicone was used to block in place the silicone-insulated connection wires and prevent that a force applied to the wires could break the soldering pads. A layer of kapton tape insulates the soldering pads from the aluminium tape.
6. Assembly and testing for validation
The first test after connection of the crystal module to the main PCB board was with BecqMoni MCA.
I’ve proceeded as always: using a Cs137 source at first and then with Uranium natural ore sample, I’ve calibrated the audio gain of line in and let BecqMoni to sample the pulse shape and draw some spectra.
Something more interesting was testing the device with USB OTG of my old rudged mobile phone with Atom Spectra APP. It works remarcably well.
The mobile phone, thanks to the USB Master capabilities (USB OTG) feeds current to the device and uses it as external audio card. This allows the use of my new spectroscopy device everywer. This is why I call it “Ultra portable”. Size, resoluzion, high efficency and use of composite materials makes it as an innovative product.
8. Final assembly and testing
The device was extensivelly tested with BecqMoni, Theremino and PRA audio software-MCA.