Capacitive Electrode for Concrete Slipform

# FastForm Capacitive Sensor Electrode — PCB Design & Order Brief

---

## Overview

I need a **very simple 2-layer PCB** designed and manufactured. Two variants, 5 boards each.

**This board has no components, no traces, no schematic, and no routing.** It is four copper rectangles and three vias. Someone competent with KiCad should complete the layout in under an hour.

**Important context so the design decisions make sense:** this is not a normal circuit board. It is a **capacitive sensor electrode** that will be immersed in wet concrete. The **solder mask is a functional dielectric layer**, not a cosmetic finish. Several instructions below will look unusual for a PCB — please follow them exactly rather than applying normal PCB conventions.

---

## Deliverables

1. **KiCad 7 or 8 source files** (I need these for future revisions)
2. **Gerber files** (RS-274X) and **Excellon drill files**
3. **Order placed on my behalf** with JLCPCB (or PCBWay if cheaper), shipped to me
4. **Order confirmation + tracking number**
5. **One question answered by the fab** — see "Question for the fab" below

---

## Board specification — common to both variants

| Parameter | Value |
|---|---|
| Outline | **100 × 100 mm** square |
| Layers | **2** |
| Substrate | FR4, **1.6 mm** |
| Copper | **1 oz (35 µm)**, both sides |
| Impedance control | **No** |
| Controlled dielectric | **No** |
| Castellations / edge plating | **No** |

---

## VARIANT 1 — `FF-CAP-100` (guarded)

### Front copper — F.Cu

Two **electrically isolated** filled zones, both centred on the board:

| Zone | Geometry |
|---|---|
| **ACTIVE PLATE** | Filled square, **80 × 80 mm**, centred |
| **GUARD RING** | Filled square, outer **94 × 94 mm**, with an **82 × 82 mm** cutout — producing a **uniform 1.0 mm gap** between plate and ring on all four sides |

Result: a solid 80 mm plate, a 1 mm bare-FR4 gap all around it, then a 6 mm-wide guard ring, then 3 mm of bare FR4 to the board edge.

### ⚠ CRITICAL — solder mask on the front face

> **Both front zones must be FULLY COVERED BY SOLDER MASK.**
> **There must be NO mask apertures and NO exposed copper anywhere on the front face.**
>
> The solder mask is the sensor's dielectric layer. If any copper is exposed on the front, the board is **unusable** — it will short directly into the wet concrete.

This is the single most important instruction in this brief. Normal PCB practice would expose copper somewhere; here, do not.

### Back copper — B.Cu

- **GROUND PLANE** — filled zone, **94 × 94 mm**, centred
- Solder-mask covered **except** at the three pads below

### Connections — back side, one corner

Three solder pads, **2.5 mm diameter**, mask-opened, **5 mm apart**, silkscreen-labelled:

| Pad | Net | Connection |
|---|---|---|
| **ACT** | Active | Via to the front ACTIVE plate. **⚠ Place this via at the geometric CENTRE of the 80 × 80 mm plate**, not at its edge. |
| **GRD** | Guard | Via to the front GUARD ring. Via position anywhere convenient within the ring. |
| **SHD** | Shield | Connects to the back GROUND PLANE. |

**All three nets must be electrically isolated from one another.** No connection between plate, guard, and ground plane anywhere on the board.

### Silkscreen

- Back side: board name **`FF-CAP-100`**
- Back side: the three pad labels **ACT**, **GRD**, **SHD**
- Front side: **nothing** (keep the sensing face clean)

---

## VARIANT 2 — `FF-CAP-100-NG` (unguarded control)

**Identical to Variant 1 in every respect, except the guard ring is omitted.**

- Front: **ACTIVE PLATE only** (80 × 80 mm, centred, solder-mask covered)
- No guard ring, no GRD pad (or leave the pad present but unconnected — your choice, just tell me which)
- Back: ground plane and ACT / SHD pads as before
- Silkscreen name: **`FF-CAP-100-NG`**

*This is a deliberate experimental control. It exists so the guard ring's effect can be measured rather than assumed. Please do not "helpfully" add the guard back in.*

---

## Manufacturing order

Please order from **JLCPCB** (or PCBWay if cheaper) using **exactly** these options:

| Option | Setting |
|---|---|
| Quantity | **5 of each variant** (10 boards total) |
| Layers | 2 |
| Dimensions | 100 × 100 mm |
| PCB thickness | **1.6 mm** |
| Copper weight | **1 oz** |
| Surface finish | **HASL, lead-free** |
| Solder mask colour | **Green** |
| Silkscreen | White |
| **Conformal coating** | **NO — leave uncoated** |
| Panelisation | None |
| Gold fingers / impedance control | No |

**Please confirm total cost with me before placing the order.** Expect roughly **US$30–60** for both variants including shipping to Australia.

---

## Question for the fab — please ask and report back

> **"What is the nominal solder mask thickness, in microns?"**

I need this number. This board uses the solder mask as a functional dielectric, and its thickness directly determines the sensor's coupling capacitance. Any value between **15–40 µm** is acceptable — I simply need to know which.

If the fab won't provide it, that's not fatal; just tell me they declined.

---

## Summary of the unusual requirements

Because these will look wrong to an experienced PCB designer, here they are in one place:

1. **No exposed copper on the front face.** Solder mask over everything. Yes, really.
2. **The ACT via goes in the centre of the plate**, not at the edge.
3. **Three isolated nets**, no ground stitching between them.
4. **Variant 2 deliberately omits the guard ring.** This is intentional.
5. **No conformal coating.** (I know it's a wet application — I want the first run uncoated as a calculable baseline.)
6. **No front silkscreen.**

If anything here seems like a mistake, please ask me before "correcting" it.