The PMC, or PCI Mezzanine Card, follows the IEEE P1386.1 standard for printed circuit boards. PMCs combine the electrical characteristics of the PCI bus with the mechanical dimensions of the Common Mezzanine Card, or CMC, format. Within the PMC format single PMC boards measure 74mm × 149mm. While the standard also defines a double-sized card, this format is rare. For PMC cards, as defined by the standard, connector configurations can be:
- 2 bus connectors (P1 and P2) supporting 32-bit PCI signals,
- 3 bus connectors (P1, P2 and P3) supporting 64 bit PCI signals, and/or
- 4th bus connector (P4) supporting non-specified I/O signals.
Originally developed by Intel, PCI was meant to provide a low cost way to add high performance peripheral components to a CPU. PCI delivered a very high performance local bus to provide a means of interconnecting components on a board using no external access.
There are a number of (sometimes interconnected) considerations when planning and implementing a PMC design.
Know Your Manufacturing Process
Most engineers will agree that board design is easier when the manufacturing process guidelines are known from the start. When setting up for your PMC design, contact your manufacturer for some early assistance. Depending upon the complexity of your design, you may need to get behind the manufacturer's published capabilities. Working through the technical parameters for your design with your manufacturer can help identify problem areas early. A well-equipped prototyping solutions provider should also be able to assist by providing the following tools:
Templates: There is value in making sure the template you use is accurate. Given the load-bearing role of the connectors, their exact placement on the board is a critical first step in the design process. There are also numerous "keepout" areas on the PMC board. If possible, obtain a certified template for your design tool from your PCB manufacturer. PCB123™, for example, supplies a PCI Mezzanine template for their design tool. If you use another CAD tool, you may find a template available in your tool or available through the tool's user forum.
DFM Tools: DFM (design for manufacturability) will allow you to interactively check each routing choice against the design rules for your PCB fab. Make use of this knowledge as early as possible in your design process.
Make use of whatever DFM tools your PCB fabricator has available. Full-board DFM run at the end of the design process may not be ideal, but it's better than nothing. Sunstone Circuits offers certified rule decks for EAGLE and Altium, as well as built-in full-DFM from our own PCB123 design environment. Ask your PCB manufacturer about rule decks for your editor environment.
Other Steps in the Manufacturing Supply Chain: Don't overlook your assembly house as a process resource. Sunstone Circuits, for example, works with another company called Screaming Circuits. The staff at Screaming Circuits can provide valuable feedback on their work with components, board designs, and even surface finishes, much of which is also shared openly via their blogs or customer support. For example, with SMT pitch getting tighter and tighter, you may find the need for a more planar surface finish, but which one? Even a simple question over RoHS compliant finishes has a range of answers, but each type has its pros and cons. Partners like Screaming Circuits can offer assistance specific to your design criteria.
When you start planning your timing, signal integrity, and impedance requirements, there are some specifics to think about. First, the PCI bus can run at any speed from 33 to 133 MHz. It will adjust automatically to the slowest card on the bus. If you make a 33MHz card, you will create a bottleneck that may make performance of the entire system unacceptable.
Next, always be aware of your signal return path. It's not enough to simply make sure all of your ground pins are connected; you will need to keep the return path for every signal in mind. If your signals cross a split in the reference plane, the signal return will have to find a way around that split and back to its place under the signal. Every time that happens you will add EMI to your design. Is there a practical way around signal return path issues? Yes. With PCB123, for example, you have the ability to turn off just the traces, leaving the area fills visible. This is a useful technique with 2 layer boards, since they don't have the luxury of dedicated ground planes. With that done, you can ensure that your return path can flow directly to all areas of the board. Use vias to stitch your fills together making a more complete and direct return path for all of your signals. Finally, if your product is going to need controlled impedance, try to figure out how to achieve that impedance level with standard board construction and materials that can be built anywhere. If you get all exotic with your requirements, costs will skyrocket, and while your product may work just great, it will cost so much you can't sell it.
Connectors will require two careful considerations: correct placement on the template, and correct specification of the right part for assembly. The good news is, the connectors are already specified and IEEE will tell you exactly where to put them. They even tell you which signal goes to which pin. In the PCI spec, however, you have the choice of both a 3.3 volt and a 5 volt configuration. Connectors are keyed to prevent it being plugged into the wrong voltage source, so you will need to pick the right version of the connectors depending on voltage requirements.
Typical PMC implementations will require exact connector placement on both ends of the daughterboard's long axis. It is because of these close tolerances that engineers embarking on their first PMC format design should carefully research the initial setup of their PMC template. Or, better yet, use a known-good template supplied from a reputable source like your CAD tool supplier, or your PCB board fabricator.
Component issues in your design can take on one of two common characteristics: landing patterns and thermal issues. Let's look at both.
PCB manufacturing shops will drill, plate and stencil your board to match the landing footprint you used in your design. There is, however, no guarantee that the footprint you used is the exact match for the part you ultimately order. While this seems quite obvious, engineers of all levels of expertise still run into this problem.
An assembly house will tell you to be extremely diligent about double-checking the accuracy of the footprint symbol you use on your board. Duane Benson, Marketing Manager at Screaming Circuits, for example, says "Usually when I talk about parts library issues or footprint issues, I'm referring to tiny QFNs or new exotic chip scale parts or things like that. Well, even bigger, older parts can have library issues.
This thru-hole switch is a good example. It looks like the footprint is on a .1 inch pitch and the switch pins are at .09 or a metric pitch. It's interesting that the tabs on the outside of the part are in the right spot, even though the pins aren't."
Benson goes on to say, "we've found a number of switches and relays with non-standard footprints. If you don't find the exact part number in your component library and substitute something close, make sure to double check the part you selected for fit before sending the boards out for fab or assembly."
Speaking of component selection, one should make sure not to overlook some basic pitfalls. With any board and a myriad of components and footprints to work with, it can be easy to make a library or BOM edit resulting in lost time and/or cost. Parts library and BOM verification can greatly reduce the potential for error.
The 3D Envelope
PMC cards are mezzanine cards. That is, they attach to another board and must be designed to exist in the space between two boards. Physical and thermal issues within this space can be of particular concern.
Carefully map out the thickness tolerances available to you between boards. Give careful consideration to the thermal flows. If you know that there's a heat-generating part underneath your mezzanine card's ultimate location, plan for that. Don't place your main heat generating components over/near the other heat source.
Since this card will be mated parallel with its data-donor card, you will have to be very mindful of your component height after assembly. You will have to use the MAXIMUM dimension from all of your datasheets to come up with this number. Though the keep-out areas defined in the IEEE spec might seem overly conservative, don't try to cheat your parts into those areas. They are there for a reason. You may get by 90% of the time if you hang some decoupling in a keep-out that looks like it will fit, but eventually you will crash into something. One place you can, if your impedance calculations will permit, gain some room in the Y direction is by using a thinner board. Instead of .062" go with .031"; your impedance will be lower with the thinner substrate, and that's a good thing. It won't be as rigid, but you will buy yourself an extra 31mils of headroom.
If you are like most engineers, you like to use hot parts. Since many of your hottest components will be sandwiched between two boards with little or no way to directly air cool them, you will have to take the heat out through an indirect path. If there is room you can use a heat pipe and hang a heat sink. If you live in the real world, you will also have to take the heat out through the board.
There are several ways to make thermal management work, many of which are well documented in articles on the web. Or, you can contact your favorite PCB fab shop and ask if they have any suggestions. Odds are they have seen this before and can offer preventative advice. Your assembly shop will also be able to help here since they see what happens to your components when the solder melts.
Using Assembly on your Prototypes
Designing a PCI mezzanine card is likely to require some assembly services. For engineers making the transition from assembling their own prototypes, it's important to coordinate the assembly services with your design. Assembly houses will offer either kitted assembly services, turnkey services, or both. Kitted Assembly is where you send the assembly house your files, all the parts and the empty PC boards. It's the easiest way to go, and the simplest to quote for the assembly house. Turnkey, on the other hand, is easier for you all around, but the quote process is a bit more awkward. Most companies will quote the labor for the job, and then place the parts on order for you.
Whether kitting or turnkey best suits your needs is ultimately your choice. Using the kitted approach for first prototypes increases the chances that you'll catch a parts mismatch before it's too late, but it does add some time and personal effort to the prototyping process.
The PMC daughterboard, from a manufacturing standpoint, is no different than any number of other boards. Bringing together the information for the overall design is the key. Knowing which connectors are necessary, how the board fits within the "mother", accurate libraries and Bills of Materials, and the requirements/capabilities of your manufacturing partners (both PCB and assembler) will provide you with a smooth R&D process. Always consider the potential for different partners as well. In a lot of instances, one set of manufacturers may be used for prototyping while production may be done elsewhere. In a lot of instances, one small detail is the difference between a successful, on-time, on-budget design, and a project delay.