From 868bbf6883cfde25bacb911cd41b65aa221d2177 Mon Sep 17 00:00:00 2001 From: FerretCode Date: Sat, 11 Jul 2026 21:54:43 -0700 Subject: [PATCH] add tradeoff document for backplane connector standard --- backplane/documentation/bom/bom.md | 0 .../bom/tradeoffs/backplane_connector.md | 78 +++++++++++++++++++ 2 files changed, 78 insertions(+) create mode 100644 backplane/documentation/bom/bom.md create mode 100644 backplane/documentation/bom/tradeoffs/backplane_connector.md diff --git a/backplane/documentation/bom/bom.md b/backplane/documentation/bom/bom.md new file mode 100644 index 0000000..e69de29 diff --git a/backplane/documentation/bom/tradeoffs/backplane_connector.md b/backplane/documentation/bom/tradeoffs/backplane_connector.md new file mode 100644 index 0000000..a4b4902 --- /dev/null +++ b/backplane/documentation/bom/tradeoffs/backplane_connector.md @@ -0,0 +1,78 @@ +# Backplane Connector Trade Study + +## Document Control + +- Version: 1 +- Author: Ethan Eggert +- Approver: +- Last Revised Date: 07/05/2026 +- Approval Status: Unapproved + +## Purpose + +This document records the connector standards considered for the primary backplane connector, the evaluation criteria used, and the rationale for the final selection. It supports the connector-selection deliverable in the Backplane Subsystem Info doc and should be referenced from the eventual backplane ICD. + +## Decision Criteria + + +| Criterion | Why it matters | +| :---- | :---- | +| Contact type | Pin-and-socket vs. card-edge/gold-finger: affects vibration robustness | +| Flight heritage | Reduces risk relative to an unproven connector; ideally on a currently-flying CubeSat | +| Mounting technology | Through-hole (THT) vs. surface-mount (SMT): THT joints are mechanically anchored through the board. THT parts are better suited for high-vibration environments than SMT joints. Also easier to assemble and rework | +| Pin density / pitch | Determines how many connector instances are needed per board, per the pin-budget sizing methodology in the Backplane Subsystem doc | +| Cost & availability | Team budget and procurement lead time | +| Mechanical retention | Locking/latching options vs. relying entirely on external retention hardware | + + +## Candidates Considered + +### 1. Card-edge / gold-finger connector (e.g., desktop PCIe-style slot) + +The original starting point. This connector family is friction-fit and was designed for benchtop/industrial use, not launch vibration. A NASA review of the similar CompactPCI card-edge family found it can experience a "contact spreading" effect (intermittent loss of contact pressure) under vibration. No THT option applicable: this connector type mates directly to board copper fingers, not through-hole pins. **Rejected** + +### 3. Harwin Gecko-SL + +Lightweight (~1g), screw-locking mechanism directly addresses vibration retention at the connector level, flight heritage. Primarily an SMT-mounted product line. Locking mechanism is attractive, but the lack of no specific flight-heritage citation for this exact use case weighed against it. **Rejected for this decision, worth revisiting for secondary/debug connectors.** + +### 4. Samtec ERM8/ERF8 (2mm pitch) + +Considered for higher current-per-pin needs on power-heavy rails. Larger footprint than a 1.27mm-pitch part, and not confirmed to have the same flight-heritage track record for a full backplane role. **Rejected as the primary standard; may be revisited for a dedicated high-current power connector if the single-connector-standard approach can't meet a specific rail's current budget.** + +### 5. Samtec SFM/SFH series (as used on OreSat's backplane) + +1.27mm pitch, pin-and-socket, vertical socket connector. Has flight heritage: this is the connector family flying on OreSat's 1U and 2U spacecraft today. Available in through-hole (THT) mounting variants, which gives the mechanical robustness and reworkability benefits described above. Widely stocked, moderate cost, available in a range of pin counts suitable for the pin-budget-per-instance approach already defined for our backplane. **Selected.** + +## Comparison Table + + +| Candidate | Contact Type | Flight Heritage | THT Available | Locking | Pitch | Selected | +| :---- | :---- | :---- | :---- | :---- | :---- | :---- | +| Card-edge (PCIe-style) | Card-edge | None found for backplane use | No | No | N/A | No | +| UWE-3/Kyutech pattern | Pin-and-socket | Yes (multiple missions) | Pattern only, not a specific part | Varies | Varies | No (informed approach) | +| Harwin Gecko-SL | Pin-and-socket | Not confirmed for this use | Not confirmed | Yes (screw-lok) | Small | No | +| Samtec ERM8/ERF8 | Pin-and-socket | Not confirmed for this use | Yes | No | 2mm | No | +| **Samtec SFM/SFH (OreSat)** | **Pin-and-socket** | **Yes (currently flying)** | **Yes** | **No (external retention needed)** | **1.27mm** | **Yes** | + + +## Decision & Rationale + +**We are adopting the Samtec SFM/SFH connector family** (the same family used on OreSat's backplane) as our connector standard for the backplane, per the single-connector-standard approach already defined in the Backplane Subsystem doc. + +The two deciding factors were: + +1. **Flight heritage:** this is a connector family with a track record on a currently-orbiting spacecraft, not just a component with generic aerospace-adjacent marketing. +2. **THT availability:** through-hole variants give more robust, more reworkable solder joints, and are generally more tolerant of launch vibration than surface-mount-only alternatives. + +## Risks / Open Items + +- Mechanical retention is not built into this connector (unlike Gecko-SL's screw-lok): retention hardware (card guides, staking, clamps) remains a required deliverable, per the Backplane Subsystem doc, and should be vibration-tested rather than assumed adequate. +- Exact pin count/variant within the SFM/SFH family, and the resulting connector-instance count per board, is still to be worked out using the pin-budget methodology already defined. +- If a specific power rail's current budget exceeds what this pitch/contact size can support even with paralleled pins, a dedicated higher-current connector (e.g., ERM8/ERF8) may need to be reconsidered for that rail specifically. +- Procurement lead time and stock availability for the specific variant should be confirmed before this is finalized in the ICD. + +## References + +- OreSat backplane design: https://github.com/oresat/oresat-backplane +- NASA review of the CompactPCI card-edge connector family in spaceflight applications: https://nepp.nasa.gov/docuploads/1B921377-60B3-423F-82CDD7CC44080A3E/CompactPCI%20Report,%208-3-07.pdf +- Backplane Subsystem Info doc