What are the reasons that VRAM cannot stably run at its rated speed?

For example, I have an old 650M that uses K4G20325FD-FC04 GDDR5 chips, which are rated by Samsung to run at 1.5V and 5GHz. My 650M's memory is 4GHz by default, and I can overclock it to 4.5GHz. But anything above that produces artifacts.

 

The chip is only half of the equation.

If the board can't support a 5ghz bus then the chip won't operate at 5ghz because any data sent/received on the memory bus will be dirty.

There's a lot that goes into this:

-Trace coupling between layers
-Trace coupling on the same layers,
-Accurate equidistant trace routing (tolerances go way up the higher the frequency.)
-Properly designed ground plane to minimize noise (usually requires a large ground plane and/or multiple ground planes.)
-Component placing relative to power and ground planes to eliminate noise inducing loop currents.
-etc.

The higher the frequency you want your memory bus to operate at, the more complex, your board design will become. Usually the only way to achieve really high frequencies is to increase the board size and/or thickness. This is one of the main reasons why the GTX 980 desktop MXM board and current GTX 1080 MXM boards are larger than the standard form factor and cost so much.

There's no demand to go through this excersise on a low-mid level mobile gpu. The difference between a reliable 4ghz bus with high yield numbers and a 5ghz bus is probably not worth it to the designer due to cost and size considering the application and demand.

 

Thanks for shedding some light on this.