Intel Broadwell Architecture Preview: A Glimpse into Core Mby Ryan Smith on August 11, 2014 12:01 PM EST
Putting It All Together: Small Core M
Next to power constraints, the final element of Intel’s fanless challenge is the size of the SoC itself. Sub-10mm thickness doesn’t just put constraints on the heat capacity of the device but it also constrains just how large an SoC and its supporting circuitry can be. As a result Intel has focused on making Broadwell-Y the smallest Core processor yet, making the entire SoC under 500mm2 in size.
As was the case with power, reducing the size of Broadwell-Y is a multi-faceted effort. The 14nm process plays a big part here, allowing for one of the smallest Core CPU dice yet. At 82mm2 the Broadwell-Y CPU die is some 37% smaller than the Haswell-Y CPU die, none the less packing a dual-core CPU and a full GPU slice.
With such a small die Intel was in turn able to reduce the size of the entire SoC package through the combination of the reduced die area and further optimizations to the packaging itself. Haswell-Y’s already small ball pitch of .65mm was further reduced to just .5mm, producing a package with Intel’s smallest solder ball pads yet. Intel considers the reduction in the ball pitch to be the key change that allowed Broadwell-Y to be so small, as they were already pad-limited on Haswell-Y despite having ample excess packaging even after taking the CPU die’s larger size into account. As a result Broadwell-Y takes up almost 50% less surface area (XY) than Haswell-Y.
Intel has also made a number of changes for Broadwell-Y to reduce the Z-height of the Y SoCs, as even 1.5mm for the SoC starts to become a significant design constraint in a sub-10mm device. Again owing to the 14nm process, the Z-height of the Broadwell-Y die itself is down to 170um. Meanwhile the Z-height of the substrate has been cut in half from 400um to 200um, which accounts for nearly half of the total reduction in SoC Z-height.
The final element in reducing the SoC Z-height, and what’s likely the most unorthodox change for Broadwell-Y’s packaging, is Intel’s 3DL inductors. The 3DLs aren’t just to improve energy efficiency as we discussed before, but they are part of Intel’s efforts to reduce the SoC size. For Broadwell-Y the 3DLs are on their own PCB on the back of the SoC, extending well below the back of the package. To accommodate this, logic boards housing Broadwell-Y will have a hole in them where the 3DL PCB would be in order to allow the complete SoC to fit. Because there are no BGA connections here this change isn’t quite as radical as it first appears, but it’s a very good example of just what lengths Intel was willing to go to reduce the package Z-height.
All told then, the combination of these space optimizations has reduced Broadwell-Y’s Z-height by nearly 30%, from 1.5mm on Haswell-Y to 1.04mm on Broadwell-Y (3DL PCB not included). By bringing Broadwell-Y’s thickness under 1.1mm, the SoC is now no taller than the other common components on a logic board (e.g. RAM), meaning the SoC will no longer stick out above the other components, which is useful both for saving space and for allowing simpler (flatter) heatsinks.
Finally, the smaller size of the Broadwell-Y package will also have a knock-on effect on the size of the logic board, further feeding into Intel’s goals to get Broadwell-Y into smaller devices. Intel tells us that the size of a complete platform (logic) board for Broadwell-Y has been reduced by roughly 25% as compared to Haswell-Y, allowing Broadwell-Y to better fit into not just thinner devices but overall smaller devices too.