Printed Circuit Boards: The Devil is in the Layout

May 15, 2018

Printed Circuit Boards: The Devil is in the Layout

Printed circuit boards (PCBs) are a critical element for the success of your product. For electronic devices, the PCB is essential—by connecting different components to each other using circuits and copper traces the PCB brings the device to life. These factors are influenced by the physical board constraints, such as size, materials, layers, and planes, as well as connectivity. The PCB design takes electrical and mechanical constraints into consideration when laying out a board for its desired form, fit and function.

As devices are getting smaller, PCBs are required to follow suit, which makes designing and debugging a PCB more complex and difficult. Reworking a PCB can be costly, so ensure you have a design-for-test strategy and have designed for manufacturing, consulting both the design and test engineering teams.

Common PCB Pitfalls

The most common issues experienced in PCB manufacturing can be prevented. Here are some common pitfalls to avoid:

Plating voids occur when the copper-coated holes in the PCB have imperfections. These holes allow current to flow from one side of the circuit board to the other and become ineffective if the copper deposition is not perfect, caught an air bubble, was contaminated, or any number of other issues. Typically cleaning the material after drilling the holes can help identify potential issues.
Insufficient copper-to-edge clearance occurs if an edge-piece is trimmed too close, removing the cover and exposing the copper layer. The cover is meant to protect the copper from corrosion. Ensuring there is space between the edge of the copper and the edge of the board reduces risk of creating shorts during depanelization.
Slivers occur in two ways during the etching process and can have seriously negative consequences on the PCB’s function. One way is by strips of too thin copper or solder mask detaching before dissolving and contaminating the chemical bath. Another way is by cutting an area of the board too deep or two thinly. When you do that there is the potential a sliver of material can break away. By designing sections with minimum widths, you can reduce the chances of producing slivers.
An acid trap is an acute angle in a circuit which can trap acid during the etching process. This can compromise the connection and make the circuit defective. Double check your work to make sure you have no acute angles in your design, especially on thin traces.
Electromagnetic interference (EMI) is important to consider to ensure circuits perform optimally. Good grounding, proper stackup design and shielding of sensitive traces from noisy signals are all good practices to employ.
Insufficient soldermask dams can lead to potential assembly issues. Tombstoning, solder bridges and floating parts are all reasons to verify soldermask openings are carefully considered. This can also cause a solder short between fine pitch leads.

Trust the PCB Experts

Companies will often rely on specialized technicians to utilize best practices in laying out the PCB. Additionally, they will use computer aided design and manufacturing (CAD/CAM) software that will help identify errors, design issues, and make the best use of the PCB. While each PCB will be different, there are accepted practices that can be beneficial to any PCB layout in each stage of development, so often the experts and software can provide the first-step (and often finished product) for a template that will work.

Although the best practices can be helpful, the experience of a skilled PCB layout designer using up-to-date design software is the best possible scenario. ACDi’s team of designers has over a century of combined experience on every kind of board, from small 2 layer boards to 28 layer high-speed digital, RF designs, and everything in between.

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