By Ed Sperling
System-Level Design sat down to discuss 2.5D stacked die with Vassilios Gerousis, senior architect at Cadence; Drew Wingard, CTO at Sonics; Kurt Shuler, director of marketing at Arteris; Kalar Rajendiran, senior director of marketing at eSilicon, and Hamid Savoj, technology fellow at Magma. What follow are excerpts of that discussion.
SLD: It sounds like the biggest hurdle is the business infrastructure. Is that accurate?
Gerousis: Yes. It’s the economics and business that are holding things up.
Savoj: That’s a big challenge, but it’s not the only one. The tools and other things will come over time.
Shuler: What we’re implying is that there’s going to be a platform approach where you have one key part somewhere in the stack you’re going to re-use, like an LTE co-processor. But it’s taken awhile for people to go from everything homegrown to having a platform approach even within a company. The mindset that has to change so you can do it for multiple pieces of silicon—and multiple pieces of silicon they may be purchasing from somewhere else—is huge.
Wingard: The business change is about taking advantage of partitioning. With respect to the way designs are being partitioned today, 2.5D is already practical. You can save cost, form factor, reduce power, apply new thermal properties by doing 2.5D today. If you want to get even more benefit, now you have to deal with other issues. But people do know how to do a lot of this stuff. The basics about taking standard parts and taking that version that isn’t already packaged in medium- to high-volume production has been done for awhile.
SLD: Is it really cheaper though?
Gerousis: The iPhone was built from multiple parts from different companies. My assumption is that the next iPhone will be a stack. It will be DRAM on top of the other parts.
Savoj: It will be a packaging solution.
Gerousis: It will be more like a niche product being sold in the market at a cheaper price. The more consumers can buy it, the more the price will go down.
SLD: Will the risk go up or down in 2.5D?
Savoj: As you create more partitions between analog and digital and memory your risk goes down. In a 28nm [planar] chip, if your analog part doesn’t work it doesn’t yield. But if you can separate that into three different pieces and you’re using an older part for analog, then that’s no longer a consideration. That helps yield and reduces risk.
SLD: But if something goes wrong doesn’t the cost of failure—and therefore the risk—actually go up?
Shuler: The more times you touch it the more risk you incur.
Gerousis: But people do that already today. They take multiple chips and integrate them. You may have additional risk, but it won’t be the same. Putting analog on 28nm is riskier than using analog at 65nm.
Wingard: If I’m a semiconductor company and you, the OEM, agree to pay as much money for a six-die stack as you would have paid for six separate die, it’s a win. Unfortunately, the OEMs know that when things get integrated they get cheaper. That’s where the risk/reward equation needs to be considered very carefully. I don’t pay the sum of the piece parts. There are some application-level exceptions where the benefit of form factor reduction is sufficient to change that equation, but by and large you pay less for integration.
Rajendiran: That’s just one aspect of it. The other aspect is time to market. Form factor affects unit cost. Every chip you do is not going to be cheaper, but the basic assumption is that most of the chip already has been pre-verified. This will save an enormous amount of time in functional verification. One of the challenges is that when you stack die a lot of things will break, so the risk will shift. But if you don’t use pre-verified pieces in a tile-based approach, you will actually increase the risk.
SLD: Verification goes down but validation goes up, right? The challenge is how is all of this stuff being used together.
Wingard: That validation was still required. You shift work from one venue to another. My view of how good system designers work is that they’re masters of abstraction. They don’t even know it. But there’s some part of the system that they did before that worked, and what’s new shouldn’t impact that. You spend all your energy on the new stuff. If you need to go down to the transistor level to figure out what the risk is, then you tunnel down. What’s interesting, though, is what corners were cut the last time? A chip that was cut into a piece with a whole operating system view and drivers so it looks like a plug-in model at the software stack level, then it should work. We have not been good as an industry at that.
SLD: And it may be harder to get to embedded software in a stack.
Wingard: The challenges of debug are daunting.
Savoj: There could be some advantages to that. If you have a part that’s already done and memories are available then you just work on the digital part. You can use an emulator to map your system to an FPGA and then start developing your software.
Wingard: And assuming the emulator works fast enough to make the mixed signal part work. There are a bunch of partitioning decisions you have to make.
Savoj: But even today when people want to do software development they do a chip that’s not as fast. The teams are working to getting the best speed and power, and then they put it all together.
SLD: But if you go into a 2.5D stack with Wide I/O, won’t you get a significant boost in performance and lower power?
Shuler: Yes, you’re removing a constraint on both of those.
Wingard: But even if you use an interposer, with Wide I/O you’re dealing with a 50-micron thin piece of silicon. That’s just fundamentally harder. Most of the rest of the 2.5D is using regular wafers. Wide I/O defines the move to 3D. You have to bite off some of the problems to get there.
SLD: Who benefits from 2.5D within our industry?
Gerousis: System integrators.
Shuler: Yes. They’re getting something that uses less power, less board space, and they won’t have to pay the same for separate chips.
Wingard: But does Foxconn become the assembly house? The original SoC was everything on one piece of silicon. Now it’s everything in one package. Maybe they put the die together. It sure looks like 2.5D reduces pressure on the SoC and the PCB. You can accelerate integration in the same package.