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Jun 08, 2020

A Comprehensive Introduction to Aluminum PCBs

A regular Printed Circuit Board (PCB) made of FR-4 or CEM1-3 material is not very good at removing heat from components mounted on it and generating heat. Although designer use thermal vias and heat sinks for removal of heat, the result is often expensive and bulky. Using metal core PCBs (MCPCBs) is a more effective proposition for applications requiring thermal management in PCBs mounted with high-power components generating heat.

MCPCBs use a metal as the base, as metals are good conductors of heat. For this, manufacturers mostly use a base made of Copper, Aluminum, or Stainless-Steel. According to their physical properties, Copper has the best thermal conductivity of the three, but is also the most expensive. Aluminum also has a good thermal conductivity, but lower than that of Copper. Aluminum is also less expensive than Copper is. Although Stainless-Steel has the lowest thermal conductivity of the three, and is also the least expensive, Stainless-Steel is the most stable and rigid of the three metals.

MCPCBs are also known as Metal Core Boards, Metal Clad PCBs, Metal Backed PCBs, and Metal Base PCBs. The most popular MCPCB is the Aluminum Core PCB, also known as Aluminum PCB, and its popularity is mainly because of the most economic option Aluminum offers considering its thermal conductivity, rigidness, and cost.

Uses of Aluminum PCBs

Fig 1: Assembled Aluminum PCB

Applications that require effective removal of heat use Aluminum PCBs. These applications include high-power LED lights, Industrial power equipment, automotive applications, high-power audio equipment, thermal printer heads, solar cell substrates, semiconductor refrigeration devices, and many more.

Structure of Aluminum PCBs

The structure of a basic Aluminum PCB is that of a Copper Clad Laminate (CCL) backed with an Aluminum base. A Copper foil and a dielectric layer make up the CCL. The Aluminum base may have an additional Aluminum base membrane as a protection.

Fig 2: Structure of an Aluminum PCB

Copper Foil Layer: Manufacturers generally use the same Copper foil as they use for regular FR-4 boards, unless the applications demand large current carrying capacity. For the latter, they use relatively heavy copper, with thickness varying from 1 to 10 ounces. The inner side of the Copper foil goes through a typical Zinc and Brass plating to improve its peel-off strength, while the top surface undergoes a chemical treatment to prevent oxidation.

Dielectric Layer: The thickness of this layer varies from 50 to 200 um, and forms the core technology of the Aluminum PCB. The dielectric layer has excellent thermal conductivity, low thermal resistance, and high electrical insulation properties. It also has excellent anti-thermal aging properties and can withstand thermal and mechanical stress.

The structure of the dielectric layer depends on the application. For instance, Universal Aluminum PCBs use a pre-preg of epoxy glass fiber as a dielectric layer, special Aluminum PCBs use resins of high thermal conductivity, and High-Frequency Aluminum PCBs use a dielectric layer with a prepreg of Polyimide or Polyolefin resin glass fiber.

Aluminum Base Layer: This is an Aluminum substrate material acting as the supporting component. Along with high thermal conductivity, the base layer must be suitable for regular mechanical manufacturing processes including cutting, punching, and drilling.

Base Membrane: Made of Aluminum, the base membrane protects the outer surface of the base layer from acquiring scratches and during the etching process.

Fabricating Aluminum PCBs

Fabricating methods for Aluminum PCBs are like those manufacturers follow for regular PCBs. The design process is the same and uses the same CAD tools. The only exception is the designer must use only SMCs and place them only on one side. Although single-sided Aluminum PCBs have no vias, some manufacturers make double- and multi-layer Aluminum PCBs, with the design and manufacturing process sharing many similarities with that for FR-4 PCBs. However, some aspects of the manufacturing process call for rigorous and effective control and management.

Etching: Designers typically use Aluminum PCBs for power devices with heavy copper for the foil. For copper foil with thickness of 3 ounces or more, etching requires appropriate compensation for achieving the necessary trace width, which the designer must design appropriately. Additionally, it is necessary for the operator to control etching factors and agent parameters rigorously.

Printing Solder Mask: With thick copper foil on Aluminum PCBs, the top surface is not entirely flat, and there is a large step between trace edges and the base. This makes it difficult to print the solder mask. Some manufacturers get around this problem by printing the solder mask two times, susing an epoxy of high quality. Others use resin filling before printing the solder mask.

Mechanical Manufacturing: Aluminum PCBs often require to undergo mechanical manufacturing processes before they can be useful. This may include drilling, milling, V-scoring, and more. To preserve the high quality of the board, most manufacturers resort to electric milling with professional cutters for low-volume production, and die molding patterns with necessary controls for high-volume production. Drilling requires proper adjustments and controls on heavy copper Aluminum PCBs to prevent generation of burrs.

Advantages of Aluminum PCBs

Use of Aluminum PCBs offers several advantages over that of regular FR-4 PCBs. Compared to standard FR-4 constructions, the Aluminum PCBs offer a dramatically superior heat dissipation. Although only a tenth of the thickness of conventional epoxy-glass, the dielectrics in an Aluminum PCB can be 5 to 10 times more conductive thermally. This makes thermal management in Aluminum PCBs exponentially more efficient compared to that of a conventional rigid PCB. Therefore, designers can use lower copper weights in Aluminum PCBs than those suggested by IPC in their heat-rise charts.

Different Types of Aluminum PCBs

Manufacturers have been successful in innovating different types of Aluminum PCBs such as:

Flexible Aluminum PCBs: These use new flexible dielectrics featuring Polyimide resin systems with ceramic fillers for excellent electrical insulation, thermal conductivity, and flexibility. The metal base uses flexible Aluminum such as 5754 or similar. The combination produces a PCB that the user can bend into place, but not use for regular flexing.

Hybrid Aluminum PCBs: These use a non-thermal material as the sub-assembly, which the fabricator processes independently, and then bonds to the Aluminum base using thermal materials. For instance, a conventional 2- or 4-layer FR-4 PCB bonded to an Aluminum base helps to improve heat dissipation and mechanical rigidity.

Multi-Layer Aluminum PCBs: Fabricators make these from multiple layers of thermally conductive dielectrics. The designer can place one or more layers of circuits buried in the dielectrics, and use blind and buried vias. However, these PCBs are not so effective in transferring heat.

Through-Hole Aluminum PCBs: Prior to lamination, the fabricator must pre-drill the Aluminum base, and back-fill it with dielectric to form the core of a multi-layer PCB. This allows the fabricator to laminate both sides of the Aluminum base with thermal materials and PCB subassemblies. After lamination, the fabricator can drill the assembly like drilling a conventional multilayer PCB, with plated through holes passing through the clearances in the Aluminum. This is the most complex construction of an Aluminum PCB.