FPGA/PCB Co-Design Increases Fabrication Yields Print E-mail
Written by Yan Killy   
Saturday, 01 March 2008 22:56

When integrating FPGAs into PCB design every signal and pin has a measureable effect on production yield.

PCB designs with field-programmable gate arrays (FPGAs) are often done more empirically than systematically – the board is designed and then “tweaked” by altering component placement, component orientation, PCB layer stack up, PBC signal layer pairs, trace routing, and even manufacturing materials.

As FPGAs have become a more dominant design component, the concept that the FPGA pinout can be optimized for both internal and external FPGA signal integrity (in PCB design) provides a flexibility not available with competing technologies. Taking advantage of that flexibility throughout the design process can measurably increase PCB yields and increase profit. Incorporating FPGA/PCB co-design and paying special attention to FPGA placement and routing can reduce “tweaking” time by 20-50%, increase fabrication yields, and ultimately increase the end-product’s profitability.

FPGA Devices Dominate New Designs

A decade ago, ASICs were the dominant definable component found in design starts. Today the flexibility, rapid deployment, and lower development costs of FPGAs have resulted in the dramatic increase of their use in new designs.

As FPGAs have matured as circuit elements, pin count has risen significantly. The increase in pin count represents a factorial-increased complexity to the PCB design. This complexity increases with greater numbers of traces, vias and tighter tolerances, plus signal integrity and timing constraint issues. The increased pin count also creates the need for additional layers as well, each adding 10-20% to the manufacturing cost of a completed board.

Typical Design Flow

Having evolved from different technology arenas, PCB and FPGA designs have some differences in design flows that have hindered many co-design efforts and negatively impacted the optimization of the FPGA-PCB interface. PCB designs use a schematic-based data entry method. FPGAs, however, are defined by a language-based description. This difference in methodologies coupled with human interface issues has been at the root of most problems.

In a typical design, the FPGA team defines high-level block diagrams comprising the system design. At this point, the logical function of the device has been defined, generally as VHDL or Verilog code, but very little effort has been put into defining the physical connection of the FPGA.

Next, the FPGA team concerns itself with the most critical signals. Typically, these are all the high-speed signals, including the clocks. These signals are defined and locked during the FPGA synthesis step.

The FPGA vendor will then use place-and-route software to assign the remaining logical signals to physical pins, creating a pin map file, which almost always requires several iterations before the pinout is both physically and mechanically optimized.

Only then does the PCB design team become involved. Once the pin mapping is complete, the data is transferred to the PCB team, where it is defined by the librarian for use in the PCB design. This transfer by the librarian is almost always done manually and therefore is also a source of possible error.

Finally, the PCB designer instantiates the FPGA symbol into the PCB schematic, and it then goes to PCB place and route. Because the PCB design has not been a factor in the system design until now, the need to add extra board layers because of complex pin maps are not uncommon.

From here, the design enters the “tweaking” phase mentioned earlier. In this process, the flexibility of the FPGA pinout is often used to optimize the signal integrity and timing of the PCB.

In many designs, the need for tweaking has been exacerbated by the good intentions of the FPGA design team. In an effort to involve the PCB design team, the FPGA locks in the I/O pin assignments early in the process, but later finds that the pin assignments result in PCB layouts that can’t meet signal and timing specifications, or that they must change pinouts for other reasons. The result is that the FPGA pinouts must be reconfigured and the PCB re-spun.

While it’s clear that the flexibility of the FPGA is a great help in solving many PCB signal and timing issues, this flexibility is not typically used far enough ahead in the process to prevent the issues that require tweaking later in the design process.

Increasing PCB Yields

Two ways to increase PCB yields are well known. Decrease the number of layers, and complete the design of the board with as few iterations as possible, and fabrication yields increase. Other techniques, such as shortening traces and minimizing the number and length of vias, can also increase yields. Longer traces not only require more board space but also increase the risk of crosstalk, noise, and signal coupling that can compromise timing. Vias can become problems due to mechanical stresses and vibration.

Certainly, PCB design teams consistently employ the techniques mentioned above, as well as others, to increase the manufacturing yield of PCBs. However, the later the PCB design team enters the FPGA design process, the fewer the number of successful and contributory options there are available. In the worst case, a routable board may not be achievable, or the only viable solution may be to add layers, thereby reducing yields as well as increasing the cost of the PCB.

What is needed is to involve the PCB team earlier in the FPGA design, using a tool that can understand both a PCB schematic representation and the FPGA design language. Further, the tool must be able to quickly translate design changes between both domains.

Concurrent Software I/O Designer

Figure 1 graphically illustrates both the FPGA and PCB design flows. Essentially, this is what has been previously described as a “traditional” design flow. In Figure 1, the center bar represents the co-design software I/O design management tool function previously discussed. This I/O design tool would dynamically manage communications between the two design flows, providing data when required, keeping both design flows up-to-date, and allowing the flexibility of the FPGA to be maximized.

Fig. 1

Contrasting Figure 1 with the earlier design description, it can be readily seen that the PCB team involvement has been moved to much earlier in the overall design process – all the way back to the initial VHDL/Verilog description. Using an available software tool like this one, the I/O design manager can monitor the FPGA flow and update the PCB with the current data.

Beginning with the intial language-based description schematic derived earlier in the process, from the initial language-based description, the schematic is revised as each definition from the FPGA is learned. This early schematic may have all the pins assigned, or only the critical pins. As electrical and physical constraints are defined, their influence is accounted for in the PCB design, and as pins are assigned in the FPGA flow, they become defined on the PCB design.

The benefits of automation in the PCB pin definition process should also not be overlooked. Any manual transcription of data introduces the chance of error. The greater the number of transcription operations, the greater the chance and likelihood of resultant errors.

Looking at the values listed in Table 1, you can see that each signal (S) has S! (S factorial) number of interface connections. Thus, a two-pin package has two interface connections; a four-pin package has 24, and a 100-pin FPGA has 9 x 10157 connection combinations. Microsoft Excel cannot even calculate the connection possibilities for a common 1000-pin device (see Table 1). Clearly, the potential for error when manually transferring pin-map data is extremely high.

Table 1

Also, the tools are not one-way data transfers, and constraints can be defined on the PCB side that reflect to the FPGA side. And of course, the real beauty of the FPGA is that its design can be altered to optimize external connections.

By allowing the PCB design flow to feed back data to the FPGA design process, all aspects of the PCB design can be optimized. Trace lengths are minimized, and the number of layers and therefore the number and length of vias are reduced as well.

The Bottom Line

Without question, increased pin count and decreased pin pitch on FPGAs will continue. As this trend marches forward, design complexity will dictate that the best way to increase PCB yields in the shortest possible design time is through the cooperative and parallel design of the FPGA and the PCB.

Software tools are now available that provide the I/O designer with the tools needed to perform this precise function. As with high pin counts, several variables can contribute to reduce yield, increase design time and negatively affect profit margin. Among these are long design times caused by pin map entry errors and resulting PCB reiteration, longer-than-necessary traces that affect timing and signal integrity, and unnecessary PCB layers and vias that increase manufacturing cost and reduce reliability.

In a PCB design integrating an FPGA, quite literally, every signal and pin has a direct and measurable affect on production yield and profit. Incorporating software that helps manage and mitigate the co-design of the FPGA and PCB is imperative to realizing maximum yield potential. PCD&F

Yan Killy is a technical marketing engineer for Mentor Graphics’ system design division; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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