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Important Considerations While Designing A Multi-Layer Board

By pcbpower      June 07,2017

Apart from the regular Design Rule Checks (DRC) that most Printed Circuit Board (PCB) design software offer and the various standards that one has to follow while designing a PCB, there are other important considerations applicable to the design process. Unless the PCB is designed properly in the first place, issues are going to crop up eventually. Although not complete, these considerations may be summed up as follows:

  • Defining the PCB Stackup
  • Introducing Suitable Via Types
  • Setting Up a Breakout Strategy
  • Checking Signal Integrity
  • Checking Power Integrity

Defining the PCB Stackup

This is the most important step in designing a multi-layer board. As the cost of the board rises proportional to the number of its layers, specifying the stackup at the start is essential, as this defines the optimum number of layers for the PCB. This also helps the designer/engineer to establish the characteristic impedances on the various layers. In actual practice, defining the PCB stackup/layer-count is often a trade-off with the fabrication processes, through which the designer/engineer tries to achieve the desired reliability, cost targets, and yield. For the designer to understand the PCB stackup, it is necessary for them to know how manufacturers build up a multilayer PCB.


Structure of Multilayer PCB

1: Blind via. 2: Buried via. 3: Through-hole via

Image Courtesy: http://techdocs.altium.com/sites/default/files/wiki_attachments/209845/Pcb_Obj-Via_Via_LayerStackup.png

All multi-layer boards are built up of cores, pre-pregs, and copper foils in the form of a panel. The core is essentially a double-sided PCB. It consists of a rigid base laminate with copper foils pre-bonded on both sides. To make up a multi-layer PCB, manufacturers place one or more sheets of pre-preg on each side of the core, each followed by a sheet of copper foil.

In practice, the designer generates Gerber files from the PCB design software on his computer, with one set of patterns for each layer along with the drill file containing details of all the holes in that layer. The manufacturer starts with drilling the core for buried vias (if required), and electroplates them. The next step involves image transferring according to the pattern for the innermost layers, and etching the copper foils on both sides of the core. The copper patterns are usually given a chemical coating to make them suitable for bonding to subsequent layers.

Pre-pregs are formed by pre-(im)preg(nating) glass-fiber cloth with uncured resin. For the additional layers, PCB manufacturers bond sheets of copper foil and pre-preg onto the finished core using pressure and heat. The process cures the resin within the pre-preg, while bonding the panel together. The copper foils on both sides of the build are then drilled for blind vias using depth-controlled drilling machines, and the holes are electroplated. Copper pattern images for the two layers are then transferred to the panel and the sides etched. This process is repeated for all subsequent layers.

Once all the layers of the board have been built-up, the outermost copper layers also receive the same treatment of drilling for blind and through-hole vias, electroplating, image transferring, and subsequent etching. A green mask is applied to the outermost layers to prevent undesired shorting during soldering, and an application of silkscreen helps in component mounting. All exposed solderable copper pads are then given a finishing treatment as required by the customer. Finally, a routing machine separates individual boards from the panel, and cuts the PCBs to their required shape and size. Refer pcb-manufacturing-process for more details.

Introducing Suitable Via Types

With multiple layers on the PCB, there must be some way to connect between them. Designers use various types of plated through vias to interconnect the circuits on multiple layers and components. Mostly, these are Through, Buried, Blind, and Micro type vias, and each has its own function. As the name suggests, through-hole vias provide a means for mounting components with leads. These holes run through the entire stack, and connect the circuit on the topmost layer to that on the bottom layer, and to any other layer in between.

Buried vias are not visible on either surface of the PCB, as they mainly connect circuits on inner layers other than that on the top and bottom layers. Blind vias connect the circuits on the outermost layers to those on any of the inner layers. Therefore, blind vias are visible only on any one of the outermost layers.

Introduction of highly integrated packages such as the Ball Grid Array (BGA), and the shrinking outlines of modern electronic gadgets have reduced the available space for the PCB as well. To pack more circuitry within the limited space designers now use micro-vias. These are extremely small diameter vias, which are often placed on pads and tracks giving more space to the designer to route their traces. Vias placed on pads or tracks are electroplated and often filled and covered with copper.

Designers should ensure the via they have chosen to apply has the desired current carrying capacity. They can parallel additional vias to build up the necessary high current paths.

Setting Up a Breakout Strategy

The designer must ensure it is possible to breakout and route all the signals on high-pin-count integrated circuits that are so common nowadays—as this will affect the stackup of the PCB as well. This may require extensive use of micro-vias and in-pad vias that go deep into the stack. After defining the stackup, the designer must decide on the routing strategy to be used for the board—a layer-based breakout, the traditional East, West, North, and South, or a hybrid style.

Checking Signal Integrity

This is an essential part of designing a good PCB. An engineer will typically consider things such as track lengths, characteristic impedances, and the signal rise and fall times on them. He or she will also consider the drive strengths of the drivers and the resulting slew rates due to terminations. To ensure the best performance, the pre-layout and post-layout signal-integrity simulations are very useful, as is the consideration for the crosstalk budget.

Checking Power Integrity

Modern high-performing devices, especially ASICs and FPGAs, usually work on low voltages but require large currents. Therefore, considerations for the power distribution network and its static and dynamic performance on the multi-layer board assume greater significance, and defining the power and ground layers in the stackup is important to characterize this performance. Placement of the power and ground layers with respect to the signal layers also affects signal integrity, especially for traces that carry high-frequency or high-speed signals.

Conclusion

The above does not necessarily cover all aspects of the design of all types of multi-layer PCBs, but only the most important ones to provide a good starting point. For instance, designers of multi-layer PCBs for power circuits must consider wider traces to withstand higher currents, use an inner layer for control ground, and keep the power and control grounds separate.

Likewise, designers of multi-layer boards for mixed-signal circuits must consider protecting the analog ground from noise, and keep the digital and analog grounds separate.

Finally, there are two very important points every designer of multi-layer PCBs should consider. First, they should engage with a manufacturer of PCBs with proven capabilities in the field. Second, the designer should be in constant touch with their PCB provider to ensure the design they are proposing is manufacturable.