What Comes First? The Chip or the Board?

SoC Design, PCB Design
Oct 19, 2016 2:11:31 PM

In today’s highly competitive marketplace, System-on-Chip (SoC) based printed circuit boards (PCBs) are becoming increasingly complex as more and more functionality is packed into ever smaller areas. Relentless market demands for more functionality, higher performance, lower power consumption, and faster product cycles combine to create enormous challenges for PCB physical design specialists. They have the difficult task of laying out and routing complex boards, producing systems that can be manufactured to high-quality standards with good yields. They do this while meeting a long list of constraints, including:

staying within budget,
meeting timing constraints,
maintaining high signal integrity,
fulfilling power and thermal management requirements,
satisfying all manufacturability rules, and
working within tight schedules.

Meeting theses constraints is a tall order. Anything that can be done to simplify this process can potentially have a significant payoff to the business.

semiconductor-design-should-take-PCB-design-into-account.jpg We work with clients who face these problems every day. Frequently our clients will outsource an SoC design to us, but retain ownership of the printed circuit board that contains the SoC. Traditionally, they would start the PCB layout process after the chip specification is complete, using substantially the same flow for PCBs using standard off-the-shelf ICs. Following this approach, the PCB layout team extracts the required information from a data sheet and adapts the PCB to the requirements of the standard IC. The burden is on the PCB engineers to meet all of the system requirements and constraints. If the IC specifications don't align well with the requirements of the PCB, there is little choice but to modify the PCB accordingly. Otherwise, they would be forced to choose another IC or worse yet, if the PCB layout cannot be modified, then the IC must be re-floorplanned, and the entire place and route and timing closure process must be updated. This change has a significant cost and schedule impact and possibly cause a missed product release window.

There is a better way

But there is an opportunity to do better with a custom IC. In our experience, the traditional flow almost always results in a suboptimal system – either the PCB must incorporate compromises to accommodate less than optimal chip factors, or vice-versa. It is far better to flip this flow around by addressing PCB place and route issues before completing the IC design and layout process. By working through the PCB physical design issues early in the chip design process, it is often possible to identify physical problems on the PCB that might be readily addressed with relatively straightforward changes to the IC floorplan.

As a simple example, consider what happens if the placement of I/Os on a custom IC is suboptimal relative to the PCB. This could force extra-long traces on the PCB, resulting in reduced performance. Or, if the PCB is congested, it could require the addition of extra layers to complete the route, thereby adding cost to the manufacturing process. Had the I/O placement requirement been known early enough by the IC design team, it might have been a simple matter to swap or mirror the I/Os so that they align better with the layout of the PCB. There would have been no need to sacrifice performance or accept the increased cost, as long as the chip design and PCB layout teams coordinate early in the design process.

There are other ways in which early knowledge of PCB constraints and requirements can benefit the IC design. For example, achieving optimal signal and power integrity often involves modeling sub-circuits that span both the IC and the PCB. These models will take into account multiple PCB and IC design parameters, including wiring routes on the PCB, and I/O buffer parameters, drive information, and layout on the IC. Analysis of these models, usually through simulation, will identify parameter values that can be adjusted in both places (IC and PCB) to achieve higher signal and power integrity. Without this advance knowledge, neither the IC nor PCB design can be done in an optimal fashion.

Early coordination is important

To achieve these benefits requires that PCB engineers are brought into the IC design process early in the life cycle of the project. Essentially, IC designers should view PCB engineers as one of their primary customers. By doing so, it is possible to:

address some of the PCB layout issues in a fairly straightforward manner by making simple changes to the IC,
achieve a more optimal system by carefully coordinated adjustments to the IC and the PCB.

This will require some planning, and a strong commitment by management to bring these two design teams together. But the rewards can be significant. They include reduced schedule risk, cost savings, increased system performance, and improved manufacturability. In today's business environment, this can make a big difference.

If your next project could use a helping hand and you are looking for help, consider downloading the eBook titled “Five Criteria for Evaluation of Semiconductor Design Service Providers” as an aid in your process.