Capacitive Stack-Up: OD, VA and Sensor Explained

If you come from infrared, the most confusing thing about a capacitive panel is that one size is not one number. A panel is specified by three dimensions, and mixing them up is the single most common cause of a housing that does not fit.

OD — the outline

OD is the outside dimension of the cover glass. This is what your housing is machined to, and it is the number a mechanical engineer cares about. It includes the printed border.

VA — the visible area

VA is the transparent window inside the printed border. Your UI is designed to this, your LCD is centred in it, and if the VA is smaller than the LCD active area you will crop the image. A typical border is 8–12 mm on a 21.5 inch panel, so the VA is roughly 20 mm smaller than the OD in each axis.

Sensor — the ITO pattern

The sensor sits inside the glass, usually 1–3 mm inboard of the border's inner edge, because the routing traces run around its perimeter. If the sensor is larger than the print allows, you get a dead band at the edge of the visible area — a failure that only shows up when a user tries to tap a button at the very edge of the UI.

The tail

The FPC exits from one edge of the sensor, folds behind the glass and connects to the controller. It needs 8–12 mm of clearance behind the glass and a fold radius of at least 2 mm. If your housing has a lip behind the glass on the exit side, the tail will crease — and a creased tail is an intermittent touch failure that is very hard to diagnose later.

Total thickness

1.1 mm glass gives about 1.6 mm total; 2.0 mm glass gives about 2.4 mm. Add 0.15 mm if you specify an OCA bond to the LCD, and 0.5 mm for an air gap with a foam spacer.

The tolerance budget

Glass outline: ±0.1 mm. Print position: ±0.15 mm. Sensor to glass: ±0.2 mm. Housing cut-out: whatever your machinist holds, usually ±0.2 mm. Stack them and you need 1 mm of clearance in the housing — which is why we ask for the cut-out dimension rather than the nominal one.