Just Add More DRAM
JEDEC is a council of electronic-device manufacturers and design firms that sets industry standards to be universally adapted by its members. Because some DRAM manufacturers decided to exceed the JEDEC maximum DDR3-1600 CAS 11 (and, later, CAS 9) by offering tighter timings and higher data rates, mixing DRAM has not been as easy as the council intended.
Simply put, mixing DRAM from different packages is a crapshoot, even when you have two identical packages of the same exact DRAM model. I would like to add that DIMMs that don’t appear to work well together often, but not always, can be helped with voltage and/or timing adjustments. There are a couple of examples in "DDR3 Memory: What Makes Performance Better?" where two of the companies didn’t market 32GB sets of 2400 MT/s DRAM and sent me a pair of matching 2 x 8GB sets. Neither worked initially, and it took minor adjustments for them to run smoothly.
Why is this such a problem? After all, they are the same frequency, timings and voltage.
DRAM is made up (basically) of memory chips that are soldered to a PCB (printed circuit board), driven by electricity. During the course of a DRAM production run on a given model, the manufacturer might be finishing up a large section of PCB that has been cut to the DIMM size but then might switch to a new PCB from a different production lot, which can result in slightly different properties.
The same can happen with the physical solder; the manufacturer may change to a completely different kind that has ever so slightly different conductivity properties.
Then, there are the dies themselves. When made by the actual chip manufacturer, the chips are "binned" (sorted) according to their quality.
Let’s take a closer theoretical look at this concept. A single production lot may result in, say, 1000 memory chips, which are separated or binned. A manufacturer may classify 200 chips as entry-level, and separate 350 chips that are a little better, 300 chips that are even better and 150 chips that are the best. Then, they sell these chips to different manufacturers.
If you were to go out and buy an 1866 MT/s module from each company, you would likely be getting a different PCB in each, a different solder with various conducting qualities and quite possibly differently graded chips and/or chips from different manufacturers.
Several companies are making memory chips, which further adds to the questions about compatibility, and you might start to see why mixing DRAM can be, and often is, problematic.
We also noted earlier that most newer lines of DRAM use 4Gb densities, whereas the norm with older lines was 2Gb.