We recently reviewed UbiSwitch Module and UbiSwitch Baseboard Mini for an application involving permanent immersion in MIDEL 7131, at 150 metres seawater depth and an ambient temperature no higher than 30°C.
Our component review identified no obvious incompatibility with MIDEL 7131. However, permanent immersion compatibility of the complete assembly has not been qualified in this fluid. There is stronger evidence for pressure tolerance than for long-term chemical compatibility.
At 150 metres seawater depth, the expected pressure is approximately 15 bar gauge, or 16 bar absolute. Our published pressure testing lists UbiSwitch and the full-size UbiSwitch Baseboard operating in Shell Diala transformer oil at pressures up to 9,380 psi, approximately 647 bar.
The UbiSwitch Baseboard Mini is not explicitly identified in that published test list, but it shares the core components used on the UbiSwitch Baseboard, so we consider that pressure test applicable to the Mini. This assessment does not extend those results to permanent immersion in a different fluid.
We reviewed the component BOMs and available material information. The findings were:
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Capacitors: Both boards use ceramic capacitors. Neither BOM contains wet aluminium electrolytic capacitors with rubber seals, which is encouraging for immersion and pressure tolerance.
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Board-mounted cable connectors: The connector manufacturer specifies nylon housings. MIDEL lists nylon as compatible in its material guidance. This is favourable evidence, although it does not qualify the exact resin formulations used in the connectors.
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Ethernet transformers: The manufacturer’s material declaration identifies polyurethane wire coating and epoxy adhesive, alongside ferrite and metals. MIDEL lists polyurethane wire enamel and various epoxy materials as compatible. The exact adhesive formulation remains unverified for MIDEL immersion.
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Board-to-board stacking connectors: The connector supplier has published immersion-test results for the same connector family, including 250-hour exposure sequences in ElectroCool EC-130 and Fluorinert FC-43. These provide useful supporting evidence, but the tested connector variants differ from those fitted to our boards, and neither test fluid was MIDEL 7131.
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Common-mode chokes: We have not established the exact winding-insulation, coating and adhesive chemistry.
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Power inductors: The available specifications do not establish MIDEL compatibility of their insulation and binder materials.
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Clock oscillator: UbiSwitch Module uses a MEMS oscillator rather than a conventional quartz crystal. Its construction is relevant to the pressure assessment, but the component datasheet does not establish a hydrostatic-pressure or MIDEL immersion rating.
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IC/LED packages and PCB materials: The component BOM does not establish compatibility of every package moulding compound, solder mask, assembly residue or thermal-interface material. These remain assembly-level verification items.
MIDEL’s material-compatibility guidance notes that compatibility can vary between formulations and additives within the same material family. A favourable material match therefore does not, by itself, establish the lifetime of a complete board in the fluid.
Based on this review, UbiSwitch Module and Baseboard Mini appear to be reasonable candidates for this application. However, without testing the assembly ourselves in MIDEL 7131, we cannot fully qualify its suitability.
Users should test the complete assembly under their intended operating conditions. An initial functional test establishes immediate operation; long-term immersion performance requires ageing evidence.
The boards can be operated without a heatsink when immersed in oil, provided component temperatures remain within their specified limits. Oil is a far better thermal conductor than air, but heat must still leave the enclosure, so temperatures should be checked under load in the final installation.