I have been involved in stacked DRAM products on the supplier side since the early 80's. While the concept is quite viable, there are some points to consider.
1. Addressing. DRAMS use each address pin for two address inputs. When you double the density, you need a 1/2 pin more addressing. Either the memory module or the system must provide for the.
2. Fanin/fanout. By stacking, you at least double the drive requirements and the output loading of the module. If you add a buffer chip to take care of this, you add delays. For a buffered module, it's usually a clock cycle worth.
3. Cooling. The added height of the DRAM stack may interfere with the air flow over the module.
4. Cost. Cost is more than 2X the original because of the stacking costs, additional testing, any support chips, etc.
5. Source control. Module manufacturers can buy from a plurality of DRAM vendors making the control of vendor source and die revisions more difficult.
6. Reliability. There are more interconnections and 2X the number of die involved.
All that being said, stacking is sometimes the best or only way to go.
I am intimately familiar with the stacked TSOP DDR DRAM. I can say we've seen a high rate of manufacturing defects related to these parts - particularly, loose solder joints and shorts on the pins, and issues with the interposer PCB between the top and bottom chips in the stack. These stacked TSOPs are very hard to rework/repair when closely spaced together.
I hardly see this as a solution to the stated problems but aggravating them. Stacking is indeed the best way to clear out old but well-established products to make way for newer ones. But Nx stacking is Nx cost and Nx interface complexity, and the yield is harder to meet (on a totally new product). If you can put more onto the same chip, that is still more cost effective. Sometimes, that is not possible - then you can stack.
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