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Why Use a Multi-Layer PCB?

Although a multi-layer PCB is more expensive than a single or double-layer board of the same size, the former offers several benefits. For a given circuit complexity, the multi-layer PCB has a much smaller size as compared to that a designer can achieve with a single or even a double-layer board—helping to offset the higher cost—with the main advantage being the higher assembly density the multiple layers offer.

There are other benefits of a multi-layer PCB as well, such as increased flexibility through reduced need for interconnection wiring harnesses, and improved EMI shielding with careful placements of layers for ground and power. It is easier to control impedance features in multi-layer PCBs meant for high-frequency circuits, where cross talk and skin effect is more prominent and critical.

As a result, one can find equipment with multi-layer PCBs in nearly all major industries, including home appliances, communication, commercial, industrial, aerospace, underwater, and military applications. Although rigid multi-layer PCBs are popular, flexible types are also available, and they offer additional benefits over their rigid counterparts—lower weight, higher flexibility, ability to withstand harsh environments, and more. Additionally, rigid flex multi-layer PCBs are also available, offering the benefits of both types in the same PCB.

Advantages of a Multi-Layer PCB

Compared to single or double-layer boards, multi-layer PCBs offer pronounced advantages, such as:

  • Higher Routing Density
  • Compact Size
  • Lower Overall Weight
  • Improved Design Functionality

Use of multiple layers in PCBs is advantageous as they increase the surface area available to the designer, without the associated increase in the physical size of the board. Consequently, the designer has additional freedom to include more components within a given area of the PCB and route the interconnecting traces with better control over their impedance. This not only produces higher routing density, but also reduces the overall size of the board, resulting in lower overall weight of the device, and improving its design functionality.

The method of construction of multi-layer PCBs makes them more durable compared to single and double-layer boards. Burying the copper traces deep within multiple layers allows them to withstand adverse environment much better. This makes boards with multiple layers a better choice for industrial applications that regularly undergo rough handling.

With the availability of increasingly smaller electronic components, there is a tendency towards device miniaturization, and the use of multi-layer PCBs augments this trend by providing a more comprehensive solution than single or double-layer PCBs can. As these trends are irreversible, more OEMs are increasingly using multi-layer boards in their equipment.

With the several advantages of multiple layer PCBs, it is imperative to know their disadvantages as well. Repairing PCBs with several layers is extremely difficult as several copper traces are inaccessible. Therefore, the failure of a multi-layer circuit board may turn out to be an expensive burden, sometimes necessitating a total replacement.

PCB manufacturers are improving their processes to overcome the increase in inputs and to reduce design and production times for decreasing the overall costs in producing multi-layer PCBs. With improved production techniques and better machinery, they have improved the quality of multi-layer PCBs substantially, offering better balance between size and functionality.

What are Multi-Layer PCBs?

Most electronic equipment have one or more Printed Circuit Boards (PCB) with components mounted on them. The wiring to and from these PCBs determines the basic functionality of the equipment. It is usual to expect a complex PCB within equipment meant to deliver highly involved performance. While a single layer PCB is adequate for simple equipment such as a voltage stabilizer, an audio amplifier may require a PCB with two layers. Equipment with more complicated specifications such as a modem or a computer requires PCB with multiple layers, that is, a PCB with more than two layers.

Construction of a Multi-Layer PCB

Multiple layer PCBs have three or more layers of conductive copper foil separated by layers of insulation, also called laminate or prepreg. However, a simple visual inspection of a PCB may not imply its multi-layer structure, as only the two outermost copper layers are available for external connection, with the inner copper layers remaining hidden inside. Fabricators usually transform the copper layers into thin traces according to the predefined electrical circuit. However, some of the layers may also represent a ground or power connection with a large and continuous copper area. The fabricator makes electrical interconnections between the various copper layers using plated through holes. These are tiny holes drilled through the copper and insulation layers and electroplated to make them electrically conducting.

A via connecting the outermost copper layers and some or all of the inner layers is a through via, that connecting one of the outermost layers to one or more inner layers is the blind via, while the one connecting two or more inner layers but not visible on the outermost layers is the blind via. Fabricators drill exceptionally small diameter holes using lasers to make vias, as this allows maximizing the area available for routing the traces.

As odd number of layers can be a cause of warping in PCBs, manufacturers prefer to make multiple layer boards with even number of layers. The core of a PCB is an insulating laminate layer with copper foils pasted on both its sides—forming the basic construction of a double-layer board. Fabricators make up further layers by adding a combination of prepreg insulation and copper layers on each side of the double-layer board—repeating the process for as many extra layers as defined by the design—to make a multi-layer PCB.

Depending on the electrical circuit, the designer has to define the layout of traces on each copper layer of the board, and the placement of individual vias, preferably using CAD software packages. The designer transfers the layered design output onto photographic films, which the fabricator utilizes to remove the excess metal from individual copper layers by the process of chemical etching, followed by drilling necessary holes and electroplating them to form vias. As they complete etching and drilling for each layer, the fabricator adds it on to the proper side of the multi-layer board.

Once the fabricator has placed all layers properly atop each other, application of heat and external pressure to the combination makes the insulation layers melt and bond to form a single multi-layer PCB.

Helping Encapsulated Modules Keep Their Cool

When you encapsulate an active module, you actually cut off air from circulating and removing heat from around the components by the normal process of convection. That forces heat build-up within the active components, including some passive components as well, leading to possible premature failures. Intersil has now mastered the technology of effectively removing heat away from fully-encapsulated modules. Using their unique thermal design, Intersil is able to design very compact encapsulated modules handling up to 50A.

For example, the ISL8240 from Intersil is a 100W analog module, a step-down power supply with single 40A and dual 20A output in the same design. You can parallel up to six of these tiny modules to get a whopping 240A output. Applications involve LTE base stations and data center servers with design architectures built using several FPGAs, ASICs and microprocessors. Only 17x17mm in size, it is extremely difficult to keep the ISL8240 modules cool while delivering full power. Interestingly, Intersil has already announced another module with single 50A and dual 25A module in the same size.

The efficiency of Intersil’s thermal design was evident at a thermal test conducted with the ISL8240 module delivering 40A as output. The fully encapsulated module showed an impressive 99.6°C maximum temperature. Intersil has an evaluation board for users to try their design – ISL8240MEVAL4Z. The tests were conducted using the evaluation board at room temperature without any air flow.

The secret of the Intersil thermal design is a multilayer PC board. The trick is in placing multiple vias strategically to maximize the thermal performance. If this is done correctly, the design need not use any heat sink or fan.

In addition, the IC is mounted thermally on to a copper substrate. This allows attainment of a low thermal resistance of the order of 8.5°C/W. The multilayer board also has two internal copper planes sandwiched in between. These are connected to the top plane with multiple vias, allowing a low thermal resistance design that can remove the excess heat efficiently from the module. The top and bottom layer of the 4-layer board uses 2 oz. Copper, while the inner board layers are made of 1 oz. Copper. Intersil offers Gerber files to speed up your design time.

Intersil makes the PCBs of FR4 grade board material and copper with small additional amounts of solder, nickel and gold. The board uses vias with a finished hole size of 0.012 inches. For making a via, the initial hole drilled is of 0.014 inches. Plating adds a copper wall of 0.001 inches to the hole. Subsequently, the board is plated overall with an ENIG process, adding about 200µ inches of nickel and 5µ inches of gold on to the outer copper surfaces.

If you consider the thermal resistance of one via to that of the copper in the board layers, it will be seen that the via has a much higher thermal impedance for each layer. However, one via occupies only about 1/5000th of a square inch of the board area. The effect of placing N multiple vias in an area is a reduction of the thermal resistance by Nx times.