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Metal-Substrate PCBs in LED System Design
Metal-substrate PCBs for LEDs are a widely used solution in high-performance LED lighting systems due to their excellent heat dissipation capabilities. When designing a metal-substrate PCB for LEDs, it is essential to consider numerous electrical, thermal, and manufacturing aspects that can affect the reliability, safety, and performance of the final product.
In this in-depth article, we analyze the key factors to consider when designing printed circuit boards for professional LED lighting.
The dielectric layer in metal-substrate PCBs for LEDs
1. Composition and General Properties
As is well known, insulation between the metal substrate and the copper layer is achieved by means of a thin layer of dielectric material.
The exact composition of this layer is not generally provided by the manufacturer.
Among the most commonly used materials for the dielectric layer are epoxy resins, whose thermal conductivity is increased by adding fillers.
Among the most commonly used fillers are:
- Alumina (Al₂O₃)
- Boron Nitride (BN)
- Beryllium Oxide (BeO)
Alumina is the most widely used solution thanks to its good balance of cost, safety, and thermal conductivity.
Recently, to reduce the cost of the ceramic component, materials derived from other industrial processes have been used, such as:
- Caolino
- Talc
- Steatite
If the composition of the dielectric layer is not known precisely, it is not recommended to cut or drill holes in the PCB to avoid inhaling potentially harmful dust.
2. Electrical Properties of the Dielectric
Since the dielectric consists of a mixture of resin and ceramic powders, the final dielectric constant takes on an intermediate value that depends on the proportions of the mixture used.
3. Capacitance formed between the conductive layers
The copper layer and the aluminum layer, separated by the dielectric, form a capacitor with flat, parallel plates.
To obtain reliable data, it is advisable to measure the capacitance of a known portion of the PCB and determine the actual value of the dielectric constant of the material used.
4. Effects of Layout and Photolithography
Building on the analysis presented in the study on determining the thickness of a PCB laminate, we can see how the photolithography process creates sharp edges along the copper traces.
As is well known, the highest density of electric charge is concentrated in regions characterized by a small radius of curvature.
Near these points, the electric field reaches high values, increasing the risk of discharges through the underlying dielectric.
5. Probabilistic Effects and Dielectric Defects
Any defects in the dielectric's homogeneity, such as cavities or a series of cavities, can become initiation points for electrical discharges.
Reducing the area of the copper cladding can improve the likelihood of passing insulation tests, provided that heat dissipation requirements are met.
6. Leakage currents to the ground conductor
Leakage currents are particularly significant in street lighting applications, where an effective ground connection is not always guaranteed.
The trip threshold for 50-Hz alternating current is set at 10 mA. This limit represents the maximum current that could flow through the human body in the event of contact with an appliance that is not properly grounded.
When calculating the value of the leakage current caused by a metal-substrate PCB for LEDs with a dielectric layerof d = 100 μm, a total area ofA = 400 × 100 mm², and an applied line voltageof 500 Vrms, we also assume that the copper coverage area is 25%.
7. Considerations Regarding Electrostatic Discharges
Electrostatic discharges can be applied directly or indirectly.
In the case of a direct discharge, the ESD generator can be modeled as a capacitor charged to 8000 VDC that discharges directly to the terminals of the LED module, as shown in Fig. 6.
Capacity Cd+ will be charged by direct charge transfer using all the charge present in the 150 pF test capacitor.
The voltage applied to the capacitor Cd can be easily calculated if we assume that the LEDs do not significantly interfere with the charge transfer process and that the entire copper circuit reaches the same potential. At this voltage, if the LED string is long enough (140–160 LEDs in series), the parasitic capacitance protects the LED string from the overcurrents caused by electrostatic discharge.
For this evaluation, it is always assumed that the ground circuit is functioning properly; otherwise, the problem does not arise.
Copper Sizing
To correctly dimension the copper layer in metal-substrate PCBs for LEDs, it is advisable to follow a structured procedure:
- Determine the actual value of the dielectric constant of the material used.
- Use the minimum copper area compatible with the heat dissipation requirements.
- Calculate the parasitic capacitance and the resulting leakage current.
- Check the circuit's behavior in the presence of electrostatic discharges.
- Perform insulation and insulation resistance tests to verify the integrity of the dielectric layer.
Note: The regulations require a minimum insulation resistance of 2 MΩ; this is a very low value for any PCB. Typically, values exceeding ten GΩ are achieved, even for very large PCBs.
Article by: Paolo Gabbi
