SLAU857 User guide

SLAU857A May 2021 – July 2021 MSP430FR2476 , MSP430FR2512 , MSP430FR2522 , MSP430FR2532 , MSP430FR2533 , MSP430FR2632 , MSP430FR2633 , MSP430FR2672 , MSP430FR2673 , MSP430FR2675 , MSP430FR2676

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4.2.2 PCB Layout

Some of the goals of PCB layout and mechanical structure design are the same, to improve signal strength and to reduce EMC problems. Table 4-4 is the PCB layout checklist. For specific examples, refer to TI EVM board CAPTIVATE-BSWP (self-inductive) and CAPTIVATE-PHONE (mutual-capacitive).

Table 4-4 PCB Layout Checklist

Number	Component		Recommendation
1	Button (self-capacitance)	Shape	Solid circle or square.
		size	The diameter/side length is 10 mm to 12 mm.
		Distance to ground grid copper	The button is placed on the top layer of the PCB, and the ground copper is 0.5 times the thickness of the cover layer of the button.
2	Button (mutual capacitance)	Shape	Square or round, inner Rx, outer Tx.
		Size	Outer diameter/side length is 10mm and 12mm. RX width is 0.5 mm, Tx width is 1 mm, and the distance between Rx and Tx is 0.5 mm.
		Distance to grid copper	The button is placed on the top layer of the PCB, and the ground copper is 0.5 times the thickness of the cover layer of the button.
3	Slider or wheel (self-capacitance)	Shape	Refer to Automating Capacitive Touch Sensor PCB Design Using OpenSCAD Scripts
		Size	The electrode width is 10 to 12 mm, and the length depends on the required touch area.
		Number of electrodes	More than or equal to 3 electrodes, add electrodes according to the sensitivity and resolution requirements.
4	Proximity sensor (self-capacitance)	Shape size	Solid copper clad, hollow copper clad or wire, the sensing distance is positively related to the enclosed area.
5	Sensor wiring	Width	8 mil or the minimum thickness allowed by the PCB manufacturer.
5		Length	Minimize the length from the sensor to the controller.
		Points to note when routing	Keep 10 mil to 20 mil spacing with grounding copper. No sharp angles. Reduce via holes, via hole diameter 10 mil. Keep away from other touch signal wires, digital signal wires and analog signal wires by more than 4mm or choose vertical routing.
7	Ground copper		Use grid-like copper plating (line width 8 mil, grid width 64 mil, angle 45°). Self-capacitive design, double-sided copper-clad, selective copper-clad directly under the button. Mutual-capacitance type can use copper plating on only the bottom surface of the board.
8	Series resistance, TVS tube, EMC filter capacitor placement		Place within 10 mm of MCU pin.
8	Moisture and liquid resistance		Self-inductive solution: Reduce the grounding copper and improve the touch sensitivity to reduce the parasitic capacitance change caused by water. Mutual-capacitive scheme: Liquid Tolerant Capacitive Touch Buttonpad Reference Design

In terms of sensitivity, buttons and slider wheels that are similar in width to a finger, and sensor traces that are narrow and short can enhance the capacitance change caused by touch. Fewer vias and thinner copper can reduce the capacitance to ground. However, grounding copper also directly affects EMC anti-noise ability. The specific layout and grid density must be adjusted according to test results. Because capacitive touch is both a source of interference and a sensitive device, for EMC considerations, in addition to adding protection devices, route touch signals away from other signal lines or devices.