Printed Circuit Board Layout - An Executive Summary
ABSTRACT: Creating a PCB design is an essential step in realizing a concept. The PCB board lies at the nexus of chips and hardware, providing a platform onto which a unique mix of components combine to create the internal workings of the electronics. Your engineering team will benefit from the practical guidance that the PCB layout provider shares along the way to a successful launch.
PCB Layout Begins With the Library
Having a consistent space for each type of component involves generating a library that ties the electrical schematic symbol to the physical footprint. Library creation is a first step of many but the yield in assembly can be no better than the geometry that these building blocks provide.
Figure 1. Image Credit: Author - Closeup of an FPGA using a perimeter Ball Grid Array package.
A larger ECAD team would tend to have a dedicated librarian. There are also web based companies that provide a library based on a Bill of Material. A rigorous checking process is good insurance against a fatal flaw from the parts department. Someone needs to be responsible for this aspect and take an active role. Loop this person into decisions involving components with any of the following attributes:
- Fine-pitch, high pin-count or heavy components (Placement/rework issues)
- Memory, particularly DDR or other devices with high edge rates (Aggressors)
- RF antennas or other printed components (Victims)
- High Current devices (Thermal challenges)
- Edge Connectors (Plating and board thickness requirements)
When anything new and different is added to the component mix, the Designer is going to have to get the data sheet and application notes. New parts may take longer than expected to join the library if the information is conflicting or unclear. Reinvent the wheel only if you must.
Using FPGAs, ASICs and Modules
If you happen to have a chip foundry under your roof, congratulations. Your board designer becomes an extension of the chip team by way of the substrate. Your new silicon will require an interposer that expands the pin-pitch to something that can be supported with the PCB technology you plan to use.
The geometry of the device packaging will drive the board technology. The inflection point is whether or not to use High Density Interconnect (HDI) that includes the use of micro-vias and sequential lamination steps. Those cost adders are often the price of admission to today’s portable solutions. Do you want to give up the size and power-consumption bragging rights to the other vendor(s)? If size is less of a concern, the low cost path is using standard plated through-hole technology.
Design Reuse In a Printed Circuit Board Assembly
Any proven circuit block is a good candidate for reuse. Voltage regulators can be layout-dependent circuits while anything called a “sensor” is liable to be very much layout dependent following a reference design from the start. There are often subtle compromises to get to the sweet spot of any circuit design. This is especially true of analog circuits where the output is more than a binary option. We’re always looking for a little extra margin there.
One industry trend is to create hybrid footprints capable of loading two parts that are similar but not exactly matching. Combining the two is often detrimental to one or both options in terms of SMT assembly yields. It’s wise to pick a winner, if possible, somewhere during the pre-production cycle.
How to Proceed When a Product is Mission Critical
Higher reliability printed circuit boards will have larger solderable lands for each pin of the various components. The extra metal lies mostly outboard of the pin increasing the so-called toe-filet. This provides a larger surface for thermal radiation while keeping a greater distance between components.
Figure 2. Image Credit: Author - Part of a LiDAR sensor for autonomous vehicles uses many layers and redundant vias for 12V distribution.
The components themselves are derated by as much as 100% such that a circuit that requires a capacitor with a 25 Volt rating would get a 50 Volt rated cap instead. Traceability that includes a paper trail of a parts origin and qualification processes is essential for Defense and Aerospace and Automotive applications.
The construction of high-reliability PC boards also includes slightly thicker plate-up of the copper layers. Test coupons would be added to the process panel to measure impedance, insertion loss or other characteristics.
Just as the components are derated the current capacity of the board itself is over-engineered with extra stitching vias and so on. The shorthand for such PCB assemblies is to label them as Class 3 according to the Institute for Interconnecting and Packaging Electronic Circuits or IPC.
Commercial Grade Printed Circuits
A less rigorous reliability regime for commercial items where failure to function would not expose people to danger is called Class 2. This is the level of reliability that most electronic products would use. Failure of a Class 2 product could cost a business or otherwise disrupt an end user and is expected to be a rare occurrence in any case.
Figure 3. Image Credit: Author - A sub-panel showing both sides of a single board. The “A-B Flip” panelization allows the same solder paste stencil to be used for either side.
Finally, Class 1 is the lowest reliability tier and is reserved for toys and other items where the impact of a failure is negligible. The market you’re serving will dictate the approach to quality assurance and reliability.
Rigid, Flex, and Rigid/Flex Circuits
There are two types of printed circuit board material of note, rigid and flex. A system can consist of rigid boards joined through connectors to flex circuits or through integrating flex material throughout the board and selectively removing the rigid layers to create a rigid/flex hybrid.
Combining multi-layer rigid and flex circuits is a specialty although knowing each discipline individually is enough so long as you lean into the vendor for the stack-up and design capability. Rigid/flex can be a very powerful solution to a tricky electronics packaging scheme.
A monolithic rigid/flex board works without the board-to-board connectors which increases the performance and reliability compared to a collection of task-specific circuit boards. Meanwhile purpose built boards that use stacking connectors or other appropriate interconnects are easier to manage in terms of improving individual circuits.
Process Gates For PCB Layout
There will be some overlapping and parallel efforts between each transition point.
- Schematic Capture - The abstract logical representation of the circuit.
- Board Outline Definition - Includes connector placement, shields, and keep-out areas.
- Placement - Initial floorplanning of the features based on voltage and primary signal requirements, organizing the passive parts for maximum performance within the outline.
- Routing - Fan-out and connection of the circuit.
- Design Verification - Sufficient power distribution, impedance and length matching etc.
- Tape-Out, Archive - Collecting the data for manufacturing and reconciling any technical questions that may arise with the vendor(s).
Each one of these stages makes a good milestone and a time/place for a low-level review prior to arriving at the Design Verification phase. For example, advice from the assembly and physical design teams regarding the placement is much more useful prior to routing than after. Note that overlap exists while the cost of new information increases as the tape-out time approaches.
If you know your desired end-date, it makes sense to work backwards from that and schedule meetings that coincide with those process gates. If the project is on schedule, then the meetings are about the next gate. If the project is off track, then it becomes a time for a course correction. Either way, a set meeting time stands as a goal that everyone acknowledges going in. Checklists that dovetail into the requirements for each gate are another means of ensuring that everything is considered at the optimum time.
The one constant is change but the plan is still necessary in order to manage feature creep and align everyone’s efforts. The right Designer can help drive schedule and maintain the errata sheet for continuous improvement efforts. Details of this nature are coachable. The combination of good people and equally good processes will get you to those high quality printed circuit boards when you need them.