Day 2 - Digital Modeling & Architecture⚓

📌 Course Day Overview⚓

Theme: From concept to CAD. Building parametric models for HSP-PCB.

Learning Objectives: - Translate threat model into hardware architecture - Create parametric 3D model of PCB form factor - Design block diagrams for HSP-PCB subsystems - Apply Design for Manufacturing (DFM) constraints - Establish CAD workflows for precision fabrication

🏛️ HSP-PCB Architecture Overview⚓

System-Level Block Diagram⚓

┌─────────────────────────────────────────────────────────┐
│              HSP-PCB Security Coprocessor                │
├─────────────────────────────────────────────────────────┤
│                                                           │
│  ┌──────────────────┐         ┌──────────────────┐      │
│  │ MCU + Crypto     │────────→│  Tamper Detection│      │
│  │ (STM32L0 + ATECC)│         │  System          │      │
│  │                  │         │                  │      │
│  │ • 32-bit ARM     │         │ • Pressure       │      │
│  │ • AES-256        │         │ • Continuity     │      │
│  │ • ECC keygen     │         │ • Temperature    │      │
│  └──────────────────┘         └──────────────────┘      │
│          │                             │                 │
│          └─────────────┬───────────────┘                │
│                        │                                 │
│              ┌─────────▼──────────┐                     │
│              │  Secure Storage    │                     │
│              │  (Flash + EEPROM)  │                     │
│              │                    │                     │
│              │ • Crypto keys      │                     │
│              │ • Device cert      │                     │
│              │ • Firmware         │                     │
│              └────────────────────┘                     │
│                                                           │
└─────────────────────────────────────────────────────────┘
        │                              │
        │ I2C/SPI                      │ Power & GND
        │                              │
     (To host MCU)            (3.3V battery or 5V USB)

Major Subsystems⚓

Subsystem	Purpose	Key Components
Security Processor	Verify, attest, decrypt	STM32L0 ARM MCU + ATECC crypto chip
Tamper Detection	Detect physical attacks	Pressure sensors, continuity traces
Secure Storage	Protect cryptographic keys	Encrypted Flash + EEPROM
Power Management	Low-power + secure shutdown	DC-DC converters, watchdog timer
Communication	Interface with host systems	I2C/SPI isolation, optional NFC

🎨 CAD Workflow: FreeCAD + KiCad⚓

Step 1: Parametric Outline (FreeCAD)⚓

Goal: Create a credit-card form factor PCB outline with tolerance stack-ups.

Constraints & Parameters⚓

Parameter	Value	Notes
Length	85 mm	±0.5 mm (manufacturing tolerance)
Width	54 mm	±0.5 mm (ISO/IEC 7810 ID-1 standard)
Thickness	1.6 mm	FR-4 PCB standard thickness
Corner Radius	3.0 mm	Soft corners for durability
Mount Points	4× M3 holes	For enclosure attachment
Port Openings	SMA/JST (2×)	For external antenna & power

FreeCAD Model Steps⚓

Create 2D sketch with constraints (parametric)
Extrude to 1.6 mm thickness
Add corner fillets (R3.0 mm)
Define mount points with hole diameter constraints
Export as STEP for KiCad import

Key Design Decision: Parametric model allows quick iteration if form factor changes.

Step 2: PCB Layout (KiCad)⚓

Goal: Route signals with security-aware trace layout.

KiCad Design Rules⚓

Trace Width: 0.25 mm (10 mils) for power, 0.15 mm (6 mils) for signals
Clearance: 0.15 mm (6 mils) minimum (conservative for hand-tracing if needed)
Via Size: 0.3 mm drill, 0.8 mm pad (through-hole vias for thermal relief)
Layer Stack: 2-layer design (Top + GND; minimize layers for verifiability)

Security-Aware Layout⚓

Principle: Make attack surface visible through layout.

┌─────────────────────────────────┐
│       Top Layer (Signals)        │
│                                 │
│  [MCU]  [ATECC]  [Flash]       │
│    │       │         │          │
│    └───────┼─────────┘          │
│        [Tamper Sensors]         │
│        (continuity traces)      │
│                                 │
└─────────────────────────────────┘

┌─────────────────────────────────┐
│   Bottom Layer (GND + Power)     │
│                                 │
│  GND plane (continuous)          │
│  + Power distribution traces     │
│                                 │
└─────────────────────────────────┘

Tamper Detection Traces⚓

Perimeter Loop: Continuous trace around device boundary
If trace is cut, micro detects open circuit → triggers secure deletion
Visible on PCB (not hidden) for auditability
Component Continuity Check: Traces under each IC
Verify no components are cloned or replaced
Regular self-test during operation

Step 3: Mechanical Enclosure (FreeCAD)⚓

Goal: Design aluminum enclosure that supports tamper detection casing.

Enclosure Features⚓

Feature	Purpose	Design
Sealed Case	Faraday cage + physical integrity	Aluminum 5052-H32 (2mm) with gasket
Port Glands	Sealed pass-throughs for power/comms	M5 glands with O-ring seals
Pressure Cavity	Room for pressure sensor	Sealed air pocket with vent to sensor
Attachment Points	Modular integration into vehicles/homes	¼-20 tapped holes on sides
Bottom Access Panel	For battery replacement	Tool-free latches (security screw)

🔧 Design for Manufacturing (DFM)⚓

Manufacturing Constraints⚓

PCB Milling (Day 3)⚓

Min trace width: 0.15 mm (6 mils) for 1/32" bit
Min clearance: 0.15 mm between traces
Via aspect ratio: ≤ 1:10 (depth:width)
Undercuts: Avoid; mill from top-down only

CNC Routing (Day 5)⚓

Min feature: 2 mm diameter (tool path)
Undercuts: Require custom bits or multi-axis milling
Draft angle: 0–5° (vertical walls acceptable)
Material: Aluminum 5052-H32 (easy machine)

3D Printing (Day 6)⚓

Wall thickness: Min 1.5 mm (Ultimaker FDM)
Overhangs: Max 45° without support
Resolution: ±0.3 mm (0.2 mm layer height)
Material: PLA (biodegradable, easy post-processing)

📊 Day 2 Deliverables⚓

CAD Files Generated⚓

✅ STEP Model — Parametric PCB outline (FreeCAD → KiCad import)
✅ KiCad Schematic — Preliminary component list (netlist)
✅ Enclosure CAD — FreeCAD assembly drawing
✅ Block Diagram — System architecture (PNG + Visio)
✅ DFM Report — Manufacturing constraints checklist

File Structure⚓

UR-ACEIoT/
├── docs/
│   ├── day_2.md (this file)
│   ├── images/day_2/
│   │   ├── block_diagram.png
│   │   ├── pcb_outline.png
│   │   ├── enclosure_assembly.png
│   │   └── dfm_checklist.pdf
├── cad/
│   ├── HSP_PCB_Outline.FCStd     # FreeCAD model
│   ├── HSP_PCB_Outline.STEP      # STEP export (import to KiCad)
│   ├── HSP_PCB.kicad_pcb         # KiCad layout (preliminary)
│   ├── HSP_PCB.sch               # KiCad schematic
│   └── Enclosure_v1.FCStd        # Mechanical design

🧠 Reflection: From Concept to Model⚓

The Bridge⚓

Yesterday, we established why HSP-PCB matters (threat model, design principles).

Today, we translate that intent into how to build it (CAD, DFM, architecture).

Tomorrow, we verify it can be manufactured (PCB milling specifications).

Key Insight⚓

Parametric design is sovereignty too.

When a design is parametric (form factor, trace width, constraints all defined as variables), anyone can: - Audit the design decisions (parameters are transparent) - Modify for their context (smaller form factor? Change one variable) - Reproduce consistently (same parameters → same output)

This is how open-source hardware scales across communities.

Challenge⚓

The models we create today must be: - Verifiable — Every line and hole justified by threat model or DFM - Auditable — Git history shows why each design decision was made - Replicable — A technician in Rwanda, India, or Brazil can follow the same CAD file and fabricate the same device

🔗 Resources⚓

CAD Tools⚓

FreeCAD: freecadweb.org (open-source 3D CAD)
KiCad: kicad.org (open-source PCB design)
Fusion 360: autodesk.com/fusion (CAM for toolpaths)

Design Standards⚓

Status: Day 2 Complete ✅
Next: Day 3 - PCB Milling & Design for Manufacturability
Last Updated: January 31, 2026

Course Overview⚓

Digital modeling transforms design ideas into precise, fabrication-ready representations. Today I create the detailed CAD model for the HSP-PCB security module.

Project: HSP-PCB Enclosure & Component Layout⚓

Design Specifications⚓

Objective: Create a production-ready 3D model of the HSP-PCB enclosure Software: FreeCAD (Part Design Workbench) + KiCad Final Dimensions: - Credit-card form factor: 85mm × 54mm - Enclosure thickness: <2.5mm - PCB thickness: 1.6mm - Component clearance: 3mm maximum height - Border width: 12mm minimum

Precision Modeling and Scale Control⚓

Understanding Precision⚓

For the CO3 nameplate, precision is critical because: - Letters must be legible and properly proportioned - CNC toolpaths depend on exact dimensions - Tolerances affect the final carved depth

Key Principle

Every dimension in the CAD model directly translates to the physical nameplate. If I model the oval as 150mm wide, the CNC will cut it exactly 150mm wide.

My Approach to Precision for CO3⚓

Use constraints: Applied dimensional constraints for exact oval shape (150mm x 90mm)
Work in millimeters: All measurements match final fabrication requirements
Verify letter spacing: Ensured proper visual balance between C, O, and 3
Consider tool radius: All internal corners have 2mm radius for 4mm end mill

Design for Manufacturing (DFM) Applied to CO3⚓

DFM Principles for CNC Milling⚓

DFM for CO3 Nameplate

"Letters must be machinable with available end mills, depth must be achievable without multiple tool changes, and the design must be efficient to fabricate."

Critical DFM Decisions for CO3 Nameplate⚓

1. Tool-Accessible Geometry - All internal letter corners: 2mm radius (matches 4mm end mill) - No sharp internal corners that would require special tooling - Letters designed with clearance for tool entry and exit

2. Carving Depth Strategy - Selected 4mm depth for optimal visibility - Deep enough for dramatic shadows and visual impact - Shallow enough for single-pass milling with appropriate feeds/speeds - Maintains structural integrity of 10mm base plate

3. Material Considerations - Base thickness (10mm) provides rigidity - Letter walls have 2° draft for easier milling - Surface area optimized for stable workholding during CNC

Modeling Workflow: Creating the CO3 Nameplate⚓

Step 1: Oval Base Creation⚓

1. Setup & Sketch2. Padding (Extrusion)

I started by creating a new Body in the Part Design Workbench and selected the XY Plane.

Tool: Ellipse CreateEllipse
Action: Drew an ellipse centered at origin (0,0)
Constraints:
- Major axis (width): 150mm
- Minor axis (height): 90mm
- Centered at origin for symmetry

Parametric Logic

By fully constraining the ellipse, I ensure the oval shape remains perfect even if dimensions are later adjusted.

Once the oval profile was defined, I extruded it to create the base plate.

Tool: Pad PartDesign_Pad
Parameter: Length = 10mm (base thickness)

Extruded Base: Demonstrating precision modeling and parametric constraints in FreeCAD - the oval base with 10mm thickness showing dimensional accuracy * Result: A solid 10mm thick oval base plate

Step 2: Creating Letter Geometry⚓

3. Letter 'C' Sketch4. Letter 'O' Sketch5. Number '3' Sketch6. Carving the Letters (Pocket)

Created the letter C on the top face of the oval base.

Select Face: Top surface of oval base
Sketch Tool: Circles and arcs to form 'C' shape
Dimensions:
- Outer diameter: 40mm
- Inner diameter: 28mm (12mm stroke width)
- Opening gap: 15mm wide
Position: Centered horizontally, upper third of oval

Created the letter O below the C.

Outer diameter: 38mm
Inner diameter: 26mm (12mm stroke width)
Position: Middle section, 5mm gap from C
Constraint: Horizontally aligned with C

Created the number 3 at the bottom.

Height: 40mm
Width: 24mm
Curves: Two semicircles with 12mm radius
Position: Lower third, 5mm gap from O
Constraint: Horizontally aligned with C and O

Used the Pocket tool to carve all three letters into the base.

Tool: Pocket PartDesign_Pocket
Depth: 4mm (removes material downward)
Important: Added 2mm fillet to all internal corners for tool clearance
Result: Letters carved 4mm deep into the oval base

7. Outer Edge Chamfer

Added a subtle chamfer to the outer edge for professional appearance.

Tool: Chamfer PartDesign_Chamfer
Angle: 45°
Distance: 1mm
Result: Smooth transition from base to edges

Fabrication Considerations for CO3 Nameplate⚓

Design Validation Checklist⚓

Feature	Specification	Fabrication Logic
Oval Dimensions	150mm x 90mm	Fits standard sheet materials, easy to clamp
Base Thickness	10mm	Provides rigidity, accommodates 4mm carving depth
Letter Depth	4mm carved	Creates strong shadows, visible from distance
Corner Radii	2mm minimum	Matches 4mm end mill, prevents tool breakage
Letter Stroke	12mm width	Ensures letters remain strong after carving
Edge Chamfer	1mm x 45°	Prevents sharp edges, professional finish

Material Considerations⚓

Tested Materials: Plywood, hardwood (maple/walnut), aluminum
Final Selection: To be determined after material testing (Day 4)
Clamping Strategy: Vacuum table or double-sided tape for secure holding

File Formats for Digital Fabrication⚓

Exporting CO3 Nameplate for Different Processes⚓

Format	Purpose	Export Settings
STEP	CNC CAM programming	Solid body, all features preserved
STL	3D printed prototype (Day 6)	Resolution: 0.1mm, binary format
DXF	Laser cut test template (Day 5)	Top view projection, 2D outline only
3MF	Alternative 3D print format	Color and material data included

Export Process from FreeCAD⚓

For CNC Milling (STEP): 1. Select the Body 2. File → Export → STEP format 3. Save as: CO3_Nameplate_v1.step 4. Verify: Open in CAM software to confirm geometry

For 3D Printing (STL): 1. Select the Body 2. File → Export → STL format 3. Settings: Binary, 0.1mm deviation 4. Save as: CO3_Nameplate_prototype.stl 5. Import into slicer (Cura) for validation

For Laser Test (DXF): 1. Create sketch on XY plane 2. Project outer oval outline 3. Export sketch as DXF 4. Save as: CO3_Outline_template.dxf

Precision Measurements⚓

Dimensional Verification⚓

After completing the CAD model, I verified all critical dimensions:

Oval width: 150.00mm ± 0.1mm
Oval height: 90.00mm ± 0.1mm
Base thickness: 10.00mm
Letter C height: 40.00mm
Letter O height: 38.00mm
Number 3 height: 40.00mm
Carving depth: 4.00mm
Internal corner radii: 2.00mm (all corners)
Letter spacing: 5.00mm between each character

Reflection⚓

Day 2 Achievements⚓

Today I successfully created a production-ready 3D CAD model of the CO3 nameplate with: - Precise oval base geometry (150mm x 90mm x 10mm) - Three carved letters (C, O, 3) with 4mm depth - All internal corners properly radiused for CNC milling - Professional edge chamfer for finished appearance - Multiple file formats exported for different fabrication methods

Key Takeaways⚓

Parametric constraints enable rapid iteration - If dimensions need adjustment, constraints maintain design intent
DFM thinking from the start saves time - Adding corner radii during modeling prevents CAM headaches later
File format selection depends on fabrication method - STEP for CNC, STL for 3D printing, DXF for laser cutting
Dimensional verification prevents errors - Double-checking measurements before export ensures fabrication success
Professional details matter - Small touches like edge chamfers elevate the final appearance

Challenges Overcome⚓

Challenge 1: Creating smooth letter curves Solution: Used combination of circles and arcs with tangent constraints

Challenge 2: Ensuring consistent letter stroke width Solution: Applied parametric dimensions (12mm) to all letter elements

Challenge 3: Previewing carving depth visually Solution: Used FreeCAD's section view to verify 4mm depth looks correct

CAD Design Gallery⚓

Extruded Base: Demonstrating precision modeling and parametric constraints in FreeCAD

Next Steps for Day 3⚓

While Day 3 typically focuses on PCB design, for the CO3 project I will: - Conduct dimensional verification using digital calipers techniques - Create detailed technical drawings with dimensions - Generate quality control documentation - Prepare measurement protocols for validating the final fabricated piece

This session reinforced that digital modeling is not just about creating shapes—it's about creating manufacturable designs. Every decision, from geometry to file format, impacts the final fabricated object.

Key Takeaways⚓

Design with fabrication in mind from the start
Tolerances are essential, not optional
File format selection impacts fabrication success
DFM principles save time, materials, and money
Parametric modeling enables rapid iteration

Day 2 - Digital Modeling & Architecture⚓

📌 Course Day Overview⚓

🏛️ HSP-PCB Architecture Overview⚓

System-Level Block Diagram⚓

Major Subsystems⚓

🎨 CAD Workflow: FreeCAD + KiCad⚓

Step 1: Parametric Outline (FreeCAD)⚓

Constraints & Parameters⚓

FreeCAD Model Steps⚓

Step 2: PCB Layout (KiCad)⚓

KiCad Design Rules⚓

Security-Aware Layout⚓

Tamper Detection Traces⚓

Step 3: Mechanical Enclosure (FreeCAD)⚓

Enclosure Features⚓

🔧 Design for Manufacturing (DFM)⚓

Manufacturing Constraints⚓

PCB Milling (Day 3)⚓

CNC Routing (Day 5)⚓

3D Printing (Day 6)⚓

📊 Day 2 Deliverables⚓

CAD Files Generated⚓

File Structure⚓

🧠 Reflection: From Concept to Model⚓

The Bridge⚓

Key Insight⚓

Challenge⚓

🔗 Resources⚓

CAD Tools⚓

Design Standards⚓

Course Overview⚓

Project: HSP-PCB Enclosure & Component Layout⚓

Design Specifications⚓

Precision Modeling and Scale Control⚓

Understanding Precision⚓

My Approach to Precision for CO3⚓

Design for Manufacturing (DFM) Applied to CO3⚓

DFM Principles for CNC Milling⚓

Critical DFM Decisions for CO3 Nameplate⚓

Modeling Workflow: Creating the CO3 Nameplate⚓

Step 1: Oval Base Creation⚓

Step 2: Creating Letter Geometry⚓

Step 3: Edge Refinement⚓

Fabrication Considerations for CO3 Nameplate⚓

Design Validation Checklist⚓

Material Considerations⚓

File Formats for Digital Fabrication⚓

Exporting CO3 Nameplate for Different Processes⚓

Export Process from FreeCAD⚓

Precision Measurements⚓

Dimensional Verification⚓

Reflection⚓

Day 2 Achievements⚓

Key Takeaways⚓

Challenges Overcome⚓

CAD Design Gallery⚓

Next Steps for Day 3⚓

Key Takeaways⚓