Custom board DM648

IIRC, the thickness of the DM648 EVM was 0.062" because it had a PCI add in card connector.  The 0.062" requirement came from PCI.  If you are not building a PCI card then you don't have to have a board 0.062" thick.  You do have to be cautious that you can hit the impedance requirements of DDR and SGMII and that can have some bearing on PCB thickness, but it should not be a strict limitation.

-Mike

There's no problem electrically with a board significantly thinner or thicker than what we use.  As you mentioned, the default is 62 mils, but electrically it doesn't really matter what the board thickness is as long as the design guidelines for SATA, PCIe, and other high speed interfaces are followed.

The problem is that mechanically there are some complications if a design goes thicker or thinner.    For most designs, a board as thick as 95 mils can be done, but sometimes there are problems making sure the through hole parts and connectors have long enough leads to accommodate that thickness because they have to stick out enough to wick the solder into the hole.  On the other hand, SMT (surface mount) parts don't care what thickness the board is.

Going thinner is easier.  In fact, electrically a thinner board is better in most cases, since the power and ground planes can be brought closer together, which increases inter plane capacitance (extremely useful for decoupling parts).  The only limiting factor for thinner boards is usually rigidity.  The board must have more mounting holes because it's more flexible when thinner.  The long term reliability must be correctly estimated because the flexing will test the solder joints and can cause long term solder failures since the very rigid SMT parts are at odds with the flexible PCB. 

There is a lot of room for change in the board thickness, but a designer needs to know what the limiting factors are when choosing board thickness.  Most assembly houses will also be happy to work with you on the different options.

Keven

Actually my custom board fits in a closed box with no air flow. The board thickness is 93 mils. It is a 12 layer board. In the closed box there is already an antenna and RF circuitry hence the custom board is mounted such that metallic spacers transfer and dissipate all the heat to the heat sink but still the top and back layer of PCB are heated. Would you please suggest any causes or solution?

What you are doing sounds fine.  There is no problem with a 93 mil thickness as long as all the correct impedances are met for the high speed digital signals. 

I'm not sure I understand the question.  Are you saying you have a problem with overheating chips?

Keven