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HMI PCBA Design Guide: Touch and Display Integration for Industrial Human Machine Interface

Human Machine Interface acts as the essential intermediary for connecting operators and industrial control systems by interpreting human inputs to machine commands and visualizing the process data. Regardless of the application of HMI on the factory floor, process control applications or human machine interface in PLC system, the robustness of an HMI depends entirely upon the quality of the HMI PCBA (Printed Circuit Board Assembly).
 
The task of designing an HMI PCBA for HMI industrial automation applications is one that has many difficulties beyond that of consumer electronics. Industrial settings call for an expanded range of temperatures, EMI resistance, ESD protection, and voltage variations — all while providing a sensitive display and touch functionality in one HMI PCBA, there are engineering requirements in four different but related areas: trace shielding and parasitic capacitance considerations for capacitive touch screens, layout considerations for LCD backlight driver circuits, ESD protection as defined by ICE 61000-4-2, and voltage control for wide-input industrial power supplies.
 
In this regard, the present guide aims to provide a complete solution for the engineers designing HMI PCBA solutions that endure harsh industrial conditions. Human Machine Interface manufacturers who successfully integrate the following techniques, provide a product that offers the best performance at an optimal cost.
HMI PCBA - Human Machine Interface

1. Trace Shielding and Parasitic Capacitance Control for Capacitive Touchscreens

Capacitive sensing represents the basic technology that underlies the operation of modern Human Machine Interfaces. The high sensitivity of capacitive touch allows for easy touch interactions without any mechanical wear and tear. This high sensitivity, however, represents the primary disadvantage of capacitive touch — the high sensitivity to parasitic capacitance, or undesired capacitance between the electrodes of the sensors and other conducting objects, such as the ground plane, nearby tracks, and switch regulators.
 
The capacitance from the finger during interaction with humans is usually at the level of 1-5 fF. This is much lower compared to parasitic capacitance in an HMI PCBA which is several orders of magnitude higher. There are three major sources of parasitic capacitance:
 
  • Capacitance to Ground Plane (C_SG): If there is one layer of ground plane one 0.1mm from a sensor trace, then there will be .05-.02pF/centimeter. This is not a size that corresponds to the touch value.
  • Mutual Coupling Between Channels: All of the PCAP (Projected Capacitive Touch) designs have some form of mutual coupling between the adjacent x/y electrodes, resulting in cross-talk artefacts that must be taken into account.
  • Integrated Display-Touches: There are LCD COM switches for COM Electrode Switching Noise on LCD COM, which generates a very large, periodic and synchronous noise floor.

Hatched Ground Plane Design

The easiest way to minimize too much crosstalk is to switch to a 30-50% air-filled ground plane with the sensor trace paths. Diagonal line hatching should be used as an alternative to orthogonal grid hatching (30 degrees wide, 0.15 mm- 0.20 mm, spaced 0.3 mm- 0.5 mm apart). Diagonal patterns are less likely to coincide with the rectangular electrode matrices that might be found above the hatched pattern. If extremely sensitive, one can use “driven” or bootstrapped shielding. The waveform at the excitation input to the sensor is being used to drive the shielded copper by means of a buffer amplifier. If the shape of the shielded copper is exactly as that of the sensor output, the voltage difference across the parasitic capacitance is cancelled out. This can lead to up to 10 times less crosstalk in comparison to a passive grounding plane due to parasitic capacitances.

Practical Routing Rules for HMI PCBA Layout

  • Keep sensor traces short: The longer the traces, the higher is the parasitic capacitance added and the antenna surface area increased. Try to keep the traces of sensors as short as possible.
  • Optimize the location of touch controller: The optimal placement of the touch controller IC should be near the FPC connector, keeping in view thermal issues.
  • Use separate inner layer for touch routing: Use one half of a specific inner layer of PCB for touch routing with a driven shield plane on both sides of the layer. Do not route touch circuitry on static ground plane.
  • Provide filtering to power supplies: Install a ferrite bead along with 100nF and 10uF capacitors in series in between AVDD and DVDD supply planes to touch controller. The purpose is to filter out any noise coupled from digital side of the touch controller to the analog sensing power supply.
  • Perform touch measurements during LCD blanking: Perform touch measurements using the SYNC output of the touch controller during LCD blanking period only when COM lines are stable.

2. Layout Design for Backlight Driver Circuits Within LCD Modules

Backlight driving circuit of the LCD module is a high-frequency DC-to-DC converter working between 150 kHz and 2 MHz with SW voltages of 10–100 V/ns. Therefore, the SW node becomes a broadband electromagnetic interference (EMI) emitter affecting the compliance of the radiated emissions as well as the touch signal quality.
 
There are two types of interference sources that impact the touch functionality:
  • Conducted noise – transferred via a common power line
  • Radiated noise – transferred to touch sensor electrodes (antennas)

Critical Layout Rules for HMI PCBA Backlight Circuits

  • Decrease power loop area: The fast current loop circuit consisting of the input capacitor, switching MOSFET, and catch diode must be placed on a single copper layer in the board with components kept as close as possible. The trace width must be at least 1.5mm with maximum current of 1A.
  • Stacking of layers for a 4-layer board: Layer 2 must act as the Faraday shield between the noisy switching circuits in Layer 1 and the delicate touch routing in Layer 3. There should not be any route in Layer 1 close to the backlight driver.
LayerTask
L1 (Top)Backlight driver components, short power connections
L2Solid, unbroken ground plane beneath driver
L3Power planes and touch sensor routing
L4(Bottom)Display interface traces, secondary components
  • Establish a local ground moat: The placement of a ground island near the driver in Layer 1 and connection of the island through multiple vias to Layer 2 is needed. It must be connected to the main ground only at the component level to avoid noise current flowing through the ground plane.
  • Filter the LED cable: A common mode choke (CMC) is needed on the board side of the LED connector with 10-22 nF C0G ceramic capacitors placed from LED+ to LED- conductors and from the connector to ground. The ground island of the filter needs to be connected to chassis ground instead of signal ground for highest common mode filtering.
  • PWM dimming details: In order to avoid any visual flickering observed through the camera, high PWM frequencies greater than 1 kHz have to be used. In order to obtain low EMI signature, analog (DC) dimming using the feedback node or spread-spectrum PWM with frequency deviation of ±5 – 10% should be considered.

3. ESD Countermeasures Against Human Body Model Discharge per IEC 61000-4-2

It is possible for personnel to generate static charges of more than 15kV in dry industrial conditions. Each contact to the HMI PCBA could result in a potential discharge. The IEC 61000-4-2 Level 4 test requirement of ±8 kV for contact discharge and ±15 kV for air discharge has become a standard in the market for HMI industrial automation devices.
The IEC 61000-4-2 pulse wave has one of the fastest rise times of less than 0.8 ns and a frequency band up to the GHz region. It can make even those components that operate at frequencies of 1 MHz reasonable at 500 MHz.

Three-Layer ESD Protection Architecture

  • Layer 1 – Mechanical/Panel Isolation: Attach the touch screen (either glass or bezel) to a grounded chassis via its edge. It will ensure the creation of a low-impedance path that will direct ESD current to the ground before getting to the PCB. Extra isolation for the FFC can be provided using cable wrappers or EMI-absorbing foam.
  • Layer 2 – TVS Protection at Entry Points: Apply TVS diodes on all FPC connectors as well as touch sensor lines, display buses (such as MIPI DSI, LVDS) and power lines. TVS diodes must be installed not further than 2-3 mm from the connector on the same layer, preceding any series impedance.
Selection criteria for TVS devices according to IEC 61000-4-2 Level 4 requirements:
  • Capacitance: 0.3–1.5 pF per channel for high-speed interfaces (MIPI DSI up to 1.5 Gbps lanes). TVS diode arrays for multi-channel ESD protection, developed by manufacturers like Semtech, Littelfuse and ST, can be applied here.
  • Clamping voltage (Vc): It must be lower than the absolute maximum rating of the protected IC, e.g. below 12 V for CMOS inputs at test current of 1 A.
  • Response time: Silicon TVS has <1 ns response time. Metal oxide varistor (MOV) cannot be used as primary protection devices as MOVs usually have a delay of 25–50 ns.
  • Bidirectional devices: Should be placed on the touch sensor lines as the ESD event occurs with a dual polarity (positive and negative).
  • Layer 3 – Series Impedance and Local Decoupling: After the TVS device, series impedance in the range of 33 Ω to 100 Ω per sensitive signal line will need to be inserted to limit the peak current entering the ESD structures inside the IC. The remaining ESD impulse will be attenuated by the RC low pass filter which consists of the capacitance of the TVS diode and additional series impedance. 100 nF X7R ceramic capacitor (high frequency decoupling) and 10 µF ceramic/tantalum bulk capacitor (transient suppression) should be placed adjacent to the IC supply pins.

PCB Layout Rules for ESD Protection

  1. Design a short, straight, low impedance path for ESD to ground that doesn’t go near the ICs.
  2. Place the TVS diodes on the top layer between the connector and the IC. Make sure there is only one via (minimum diameter 0.3 mm) from the cathode end of each TVS diode to the ground plane.
  3. Use multiple points of connection to chassis ground (either 0 ohm jumper wires or mechanical standoffs) around the edge of the board to provide an ESD path.
  4. Do not have any slotted or discontinuous ground planes between the ESD entry points and the ICs—they add inductance that will make the design ineffective.

4. Voltage Regulation for Wide-Input Power Supplies in Industrial HMIs

During normal operation, the steady-state voltage variations of an industrial DC Bus designed at 24V DC is ±20% (19.2V to 28.8V), and transient voltage variations is due to other loads switching on/off, and may cause peak voltages to reach 36V or higher. Thus, a hierarchical conversion approach is required for a HMI PCBA power supply design that can operate over the entire range of operating voltage, rather than having a series of linear regulators which dissipate watts as heat.

Conversion Hierarchy

The HMI display system spans multiple voltage domains:
DomainVoltageKey Requirements
Backlight LED string12-28VCurrent accuracy ±5%
MIPI/LVDS display logic1.2V / 1.8VRipple < 10 mVpp
Touch controller AVDD2.8–3.3VRipple < 5 mVpp
Touch controller DVDD1.8VRipple < 20 mVpp
Processor core1.0–1.2VFast load transient
  • Stage 1: The first stage is a synchronous buck converter which converts input voltage from 24 V to intermediate voltage (12 V or 5 V).
    1. Select an IC with an absolute maximum input rating of at least 1.5× the expected peak transient (e.g., 60 V for 40 V peaks).
    2. Implement reverse polarity protection using a P-channel MOSFET ideal diode.
    3. Place a TVS clamp (e.g., P6KE33A) immediately adjacent to the input connector to limit voltage spikes from battery disconnection.
    4. Input capacitor bank: Combine 470–1000 µF of rated aluminum electrolytic capacitors with four 1210-package, 50 V, 4.7–10 µF MLCCs.
    5. Critical note: X7R MLCCs exhibit significant DC bias derating—a 10 µF/50 V ceramic capacitor may measure less than 2–3 µF at 24 V operating voltage. Verify loop compensation stability across the entire 19.2 V to 28.8 V input range, simulating at both voltage limits with worst-case component tolerances.
 
  • Stage 2: Planning for post-regulation for noise critical domains at LDO. Power supply the touch controller AVDD and other noise sensitive supplies from an LDO that takes its power supply from the intermediate rail.
    1. Noise level < 30 µV rms (10 Hz – 100 kHz)
    2. PSRR > 60 dB at 100 kHz: To avoid that the switching noise of the backlight reaches the analog sensor supply.
    3. Provide LDO output with a 1 uF C0G ceramic (less than 0.5 mm separation from LDO output pin) and a 10 uF X5R ceramic. Because of the high humidity content of the wiring system, it is recommended to avoid using Tantalum capacitors as their failure mode (short circuit) can cause failures if shorted in the wiring system.

Input Protection Chain

A full 24V input defence network, from connector to the inside:
  • A MOSFET with reverse polarity is ideal for a diode controller, and has near zero voltage drop.
  • TVS/MOV clamp: 33V clamping voltage, can capture the load-dump transient and EFT transient.
  • For IEC 61000-4-4 (EFT) and 61000-4-5 (surge): Attenuation is provided in both cases.
  • Two X capacitors (100 nF) + ferrite bead (100Ω at 100 MHz) are placed on both sides of the pi filter.
  • The first is called bulk capacitance, which is the sum of the electrolytic and MLCC capacitance.

Power Sequencing and Soft-Start

MIPI DSI displays are required to be powered on in a specific sequence as VCC_IO on first, VCC_CORE on second, VCC_LCD on third and VLED fourth. Implement using a PMIC that has programmable sequencing delays, or by cascading enable pins, where the enable for the next stage is controlled by the previous stage. Use RC timers to add 100-500ms inter-stage delays. Soft start at the first stage buck is a must. If it is not present, inrush current into the bulk input capacitance can be as high as 20 – 50 A peak at the time of power-on, creating stress on the switching MOSFETs and upstream protection devices.
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Conclusion

HMI PCBA design for industrial Human Machine Interface application is an integrative engineering discipline – not a series of stand-alone design issues. The backlight driving switching frequency sets the requirement for the touch controller to reject noise. The series resistance required for ESD protection is defined by the TVS capacitance, which further affects the display data eye diagram. The transient response of the power supply defines how well the LDO will satisfy the touch AVDD ripple requirements.
 
Engineers who see them as different specialties would find integration tough. Engineers who adopt the concept of system co-simulation, coordinated protection architecture, planned stack-up, and proper IEC 61000-4-x qualification would come up with reliable HMI industrial automation solutions that can run in the real world for many years. Human Machine Interface manufacturers who make such engineering decisions even before producing the first board always beat their competitors.
 
What distinguishes a good HMI PCBA from an excellent one is not the expense of components, but rather the engineering judgment used in designing them. This holds true whether the application utilizes a human-machine interface in PLC system within a manufacturing plant or a stand-alone industrial touch panel; these integration ideas are fundamental to Human Machine Interface design.

FAQs about PLC PCBA

An HMI PCBA (Human Machine Interface Printed Circuit Board Assembly) is the core electronic substrate that integrates touch sensing, display driving, power management, and communication interfaces in an industrial Human Machine Interface device. Its design is critical because industrial environments demand high reliability under extreme temperatures, electrical noise, electrostatic discharge, and wide voltage fluctuations. A well-designed HMI PCBA ensures responsive touch performance, clear display output, and long-term operational stability—directly impacting the safety and productivity of HMI industrial automation systems.
The LCD COM (common) signals switch with a high frequency when refreshing the display, thus producing periodic synchronous noise. The noise gets coupled into the touch sensor lines, especially in in-cell/on-cell touch display modules, showing up as a synchronous noise floor in the touch readings. The solution to this problem is to acquire touch data synchronized with LCD blanking periods by using the touch controller’s SYNC output signal.

Use PWM frequencies above 1 kHz to eliminate visible flicker, particularly when cameras are used for inspection or monitoring (flicker below ~1 kHz is visible in video recordings). For minimal EMI, consider:

  • Analog (DC) dimming: Uses the feedback node directly—no PWM switching means zero PWM EMI, ideal for noise-sensitive touch applications

  • Spread-spectrum PWM: Varies the PWM frequency by ±5–10%, spreading energy across a wider bandwidth and reducing peak emissions

The trade-off is that analog dimming may have limited dimming range at low currents, while spread-spectrum introduces slight frequency variation that must be accommodated by the display driver.

Inductance is created in the ESD discharge circuit by a slotted or cut ground plane. At the very high frequencies that exist in an ESD discharge (GHz order of magnitude), even minimal inductance results in considerable impedance leading to:

  • Increased voltage drops on the IC input
  • Currents being diverted through improper routes
  • Possibility of damage even when TVS components are used

The ESD discharge route needs to be kept short and direct with low inductance from the point of ESD entry to the chassis ground via TVS diodes, using multiple vias and with no cuts in the ground plane.

MIPI DSI displays require a specific power-on sequence to prevent damage and ensure proper initialization:

  1. VCC_IO (I/O power) — first

  2. VCC_CORE (core logic power) — second

  3. VCC_LCD (display panel power) — third

  4. VLED (backlight LED power) — fourth

Implement using either:

  • A PMIC with programmable sequencing delays

  • Cascaded enable pins (each stage’s enable controlled by the previous stage’s power-good output)

  • RC timers to add 100–500 ms inter-stage delays

Reverse sequencing during power-off (VLED off first, then VCC_LCD, etc.) is equally important to avoid latch-up or premature display failure.

For a human machine interface in PLC system, the HMI PCBA acts as the physical interface between the user and the programmable logic controller. Important integration points are as follows:

  • Communication interface: The HMI PCBA should have reliable RS-232, RS-485, Ethernet or CAN transceivers to communicate with PLCs within the noisy environment of a factory
  • I/O isolation: Opto-isolation/galvanic isolation should be provided on communication interfaces to avoid ground loops due to voltage difference between HMI and PLC
  • Real-time performance: The HMI PCBA software should give priority to screen update and touch inputs and communicate with the PLC in the presence of network congestion
  • Fail-safe operation: In cases where the HMI is being used as a control interface, it should be able to detect loss of connection to the PLC and put itself into a safe mode of operation

Industrial Human Machine Interface devices are increasingly functioning as edge computing nodes where data collected from PLC is visualized along with analysis carried out locally.

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