Countering Circuit Design Challenges

Key Takeaways

• The requirements and impact of different design considerations.

• How flex manages to stretch the limits of multi-board and cable assemblies.

• HDI techniques that empower designers to maximize space savings.

In modern PCB manufacturing and design, circuit design challenges are numerous: balancing form factor against board functionality and power, optimizing placement for best signal integrity/thermal performance, and keeping designs malleable enough to withstand assembly changes brought upon by component shortages. Additionally, devices are becoming increasingly communicative in IoT models, and designer teams will need to ensure the proper function of all near and server-bound channels and eliminate any factors that would inhibit network protocols. In general, devices will continue to improve and add to their functionality while getting smaller, lighter, and faster. The pressure already exists to leverage existing design technologies as well as to find new design solutions to create these devices, and that places the PCB designer at the focal point of satisfying design intent while juggling a host of parameters – some of which counteract each other. 

Factors Influencing Circuit Design Challenges

One of the foremost truths for design – whether PCB or otherwise – is that you can’t please all the people all the time. There is no way to simply maximize every aspect of performance without greatly increasing the cost or reducing the yield of a device (in some cases, this is acceptable or even necessary). Instead, design teams must carefully evaluate their product and where it fits within the greater market to best position it for success. For example, it wouldn’t make sense for a board intended for a children’s toy to wring out every drop of reliability; this would be a significant cost adder and the toy would most likely be retired or discarded long before any reliability benefits could be seen. 

Still, consumers are not necessarily mindful of design constraints: there is an overwhelming need to continually improve device performance, increase power and range, and yet reduce size. It falls on the shoulders of design teams to continually push the available technology to squeeze the best performance out of designs at minimal per-board cost.

Even devices that may seem relatively mundane with the march of technology can be bursting at the seams from components and functionality. Consider a smart watch whose design requires packing in a lot of circuitry into a very small space. The major components will already take up a majority of the room: the display, battery, and sensors. In addition, the smartwatch is also going to require a CPU, memory, graphics processing, and wireless circuitry.

Many of these requirements can fit on a system-on-chip (SoC) component that will be designed into the IoT device, but there will still be a lot more that has to be packed in. Wearable IoT devices like smartwatches need to be light enough to be worn by their owners without them being a burden, while at the same time increasing their functionality.

Achieving the goal of smaller, lighter, and faster designs will require PCB designers to work more with advanced design technologies. These will include high-density interconnect (HDI) design methodologies, embedded components, and compact components such as multi-chip modules (MCM) and three-dimensional ICs (3D ICs). Additionally, most devices operate as mixed signal systems – that is both analog and digital components. Because analog and digital components have different design constraints, placing both together means any overlapping circuitry areas must conform to the requirements of both for signal integrity purposes. Once more, design teams are tasked with an extremely difficult – though not insurmountable – challenge of optimizing for a wide variety of performance characteristics to a high standard, even though many of these parameters can be explicitly at odds.

Fortunately, manufacturing has devised a few methods to maximize the given volume in tight designs. Flex PCB assemblies, as opposed to the more standard rigid boards, allow designers to operate unfettered within 3D space, while high-density interconnect (HDI) techniques allow manufacturers to shrink the size of some common board features to free up additional space.

Flex PCBs and HDI

As the demand for smaller devices – particularly IoT-enabled devices – forces PCB designers to fit more circuitry into smaller spaces, the need for substrate materials other than standard FR-4 has become apparent. Flex design technology is not new, and the advantages of flexible substrates for PCBs are quickly proving themselves as the best solution to some of the inherent design problems in IoT devices.

For instance, instead of having a standard PCB that connects to a sensor through a complex wiring harness, all the PCB components and the sensor can be built onto one flex PCB. This minimizes the number of individual boards needed for the IoT device, eliminates the problems related to using a wiring harness, and allows the circuit to be folded and fitted precisely into the device.

HDI is not as radical of a conceptual departure as flex boards; instead, precise manufacturing equipment is used to produce features far smaller than typically possible. One of the most common applications is the use of microvias instead of through holes in the board stackup: blind and buried vias, instead of running the thickness of the board, can be depth-drilled a specific distance between layers, though generally no more than a couple of layers at a time. While this is a significant cost adder compared to traditional drilling/plating techniques, it also affords the designer space savings. Because the aspect ratio, or the ratio between the board thickness and hole diameter, acts as a lower limit on the size of a drilled hole, decreasing the depth allows for a smaller drilled hole without sacrificing reliability or producibility. Further, a microvia does not reserve the space above and below the drilled layers (by definition, a through hole has to reserve the same x,y-position on every layer), meaning that absent any other design constraints such as signal integrity, designers are free to route on the layers above and below microvias.

Flex PCBs along with HDI design practices will improve the performance and reliability of the PCBs used in IoT devices. Components and trace routing can be tighter in HDI designs, and by using flex materials, all the components and circuitry can be located on one PCB design. This combination of technologies will help to improve signal quality while at the same time reducing power consumption and lowering thermal stress, making for a more robust design.

PCB Design for IoT 

As advanced design principles such as HDI design processes and flex PCB materials become more commonplace in IoT devices, PCB designers are going to find that they are ultimately designing the entire product instead of many individual boards. This will require working hand-in-hand with mechanical designers as signal integrity, thermal management, form, and fit of the design become more important than ever.

Not only will PCB designers need to respond to the electrical needs of complex circuit design challenges, but they will also need to take into account the effects of these circuits in the unusual mechanical configurations of a small IoT device. Flex designs are a real benefit in these circumstances, and PCB designers will have to design as much to the mechanical constraints of the device as they do the electrical constraints.

All of this will require the most advanced PCB design tools for simulation and analysis as well as the ability to work with HDI and flex PCB design processes. The good news is that the tools you need to solve your circuit design challenges are already here. OrCAD PCB Designer has the features you need to creat successful designs.

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