What are PCBs?
PCBs are Printed Circuit Boards and they are the heart of everything electronic. These boards offer mechanical support and create an electrical connection between several electronic components with the aid of pads, conductive tracks, and several other features which are made from copper sheets and laminated onto non-conductive substrates. The PCB is also a housing unit on which several components (resistors, capacitors or active devices) may be soldered onto. Some advanced printed circuit boards are also designed as such that they contain the components implanted in the substrate. In this article, we review an important aspect of PCB design, the multilayer board.
Why does design layer and stack-up matter?
In the ever-evolving world of consumer electronics, people demand thinner, lighter, faster and better multi-tasking devices. These demands on the industry play a huge role in the popularity and availability of multilayered printed circuit boards. Multilayer PCBs are PCBs that house multiple layers of PCBs which are stacked together in a way that is governed by the pre-defined connection and functionality of each board.
Stack-up generally refers to the layout of the copper and insulating layers of the PCB before commencing the board layout design. It is imperative that the multilayer stack-up is well planned for a number of reasons. The most important of these reasons, however, is the determination of the Electromagnetic Compatibility (EMC) performance of a product. Properly designed layer stack-up plays a significant role in minimization of radiation, and also helps in either reducing or eliminating the influence of external noise sources. Furthermore, a well-designed PCB multilayer and stack-up also reduces impedance incompatibility and mismatch, as well as signal cross-talk.
Getting the stack layer right
More often than not, the defects that arise in PCB fabrication stems from issues of mis-ordering and mis-layering. If fabricators are to attain perfection in the artwork and to produce a design of high quality upon the first attempt, it is important that adequate and accurate information is supplied to them.
One of the most effective and proven ways to ensure that layers are accurate and designed accordingly is to input layer numbers into the artwork. This should be done directly in the copper layers of the PCBs.
How is this done?
To begin, simply surround each concerned layer number with a square box; this will serve as an indication to the fabricator that that particular layer has the specified number and order. The layer indicators must be on the boards themselves, as any number that is positioned outside the boundary of the layer will most likely be lost when the fabricator goes to panelize the board. It is also important that the numbers are offset slightly from one another as we progress through the layer stacks.
The main objective here is to achieve a pattern that, when viewed from the primary side of the board, will reveal all the numbers of all the layers. In order to achieve this and a proper final product, it is imperative that the solder mask to be used is relieved by at least 100 mils all the way around this feature. Although this gives no information about the manner in which the board has been ordered, it achieves 2 things; first, it gives the fabricator a reliable indicator to ensure that the artwork is in the right order. Second, by visually inspecting the pattern, they can decipher that the artwork was used for each layer.
Now, this does not give any information about the order in which the stack-up has been arranged, and as such, we need to introduce another feature to the board. This feature involves the stacking stripes.
Stacking stripes are pieces of copper feature that are placed on the edges of the board. The primary side will feature these stripes at 200 mils in length and 50 mils in width. The stripes need to protrude from off the edge of the board so that once the board is routed out of the panel, the copper becomes visible on the edge of the board when visually inspecting the edge. As we progress down the stack, there should be an extension of the stripe over another 100 mils on each of the subsequent layers. This should give a pattern that can show us each of the layers in the right order once inspected visually for Quality Analysis and Assurance purposes. This will also go a long way to ensure that the quality and numbering exists in the right order, which will in-turn enable the fabricator to determine what the artwork is for. This is key to ensuring that the design and product are achieved and properly fabricated on the first trial.
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