GARTHWILSON wrote:
Quote:
I looked at using posts/receptacles and card edge, and decided on the latter because of theoretically lower distributed capacitance.
If you have any numbers, I would definitely be interested; but just intuitively speaking, since the capacitance is a function of plate width and separation and the dielectric constant between them, I would expect the wider fingers and reduced separation of the board-edge connectors would give more capacitance than the little pins and the tuning-fork-type contacts they fit into do.
I don't have any numbers, but consider this: the "plate width" of adjacent edge fingers is actually the copper thickness, which is only a couple of mils. The dielectric in this case is air with a
K of approximately 1. The wider dimension of a finger on one side is opposed to that of a finger on the other side (if present) but has the relatively thick PCB material as the dielectric. Typical PCB construction produces a
K of around 4.6.
Another consideration of using edge connections as opposed to pins is if the pin header is on the motherboard (as it appears you are planning) then the mating receptacle has to be mounted to the memory module, which might complicate the physical construction. With edge connections, nothing has to be attached to the memory module PCB except the components. With a four-layer module, it should be possible to eliminate decoupling capacitors, since the power and ground planes act as the plates of a large capacitor.
I recall the days when memory was sold as SIPs, which plugged into a receptacle on the motherboard. Aside from the ease at which the pins could be broken off a SIP, it was discovered that the receptacle introduced undesirable reactive effects. You don't see SIP memory modules anymore. I'm sure the industry would have stayed with them if it produced better (and less expensive) results. Either way, a receptacle of some kind will be required.
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I can put GND signals in there it it makes it easier for everyone.
