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.

Thermal

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 heatpipe 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.

Wrap Up

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.

This article was written by Robert Tise, CAM Engineering, and Michael Hebda, CAM Engineering Manager, Sunstone Circuits (Mulino, OR). For more information, contact Mr. Tise at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , Mr. Hebda at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , or click here.