Published by SemiWiki on 10-26-2017
Being an old ASIC physical design guy, I tend to think of ASICs from a “bond-pads-in” perspective. This week however, I had a very eye-opening discussion with Dan Leung, Director of Packaging and Assembly for Open-Silicon, that totally changed my perspective. While I had been exposed many times to the concept of systems-in-a-package (SiPs) I had never thought of it from the view point of an ASIC or IP provider. The point to be made here is that one can’t afford to think “pads-in” ASICs anymore.
The more-than-Moore effect has resulted in a very robust manufacturing ecosystem for SiPs. As a result, ASIC and IP vendors alike really need to be thinking about the full system-in-package solution. In my conversation with Dan, he walked me through a presentation done by Open-Silicon at the 24th annual IEEE Electronic Design Process Symposium (EDPS) on Efficient Design and Manufacturing that was held last month in Milpitas, CA. The presentation was entitled “High Volume Manufacturing Supply Chain Ecosystem for 2.5D HBM2 ASIC SiPs". While the presentation focused on manufacturing for HBM2-based systems, it quickly became apparent that this ecosystem is key to enabling not only high-bandwidth memory applications, but also the quickly growing internet-of-things (IoT) market. SiPs have gone mainstream and how you build your IP and ASICs will be highly dependent upon how you plan to manufacture your SiPs.
As an example, Open-Silicon recently released a HBM2 memory control IP subsystem. When doing this IP, they went through the process of designing their own HBM2 SiP so that they could understand the trade-offs that must be made during the design process.
It turns out that there are many challenges to properly designing a SiP and the number of players in the ecosystem with whom you must work is daunting to say the least. It includes foundries, interposer foundries, OSATS (outsourced assembly and test companies), ASIC and IP houses, known good die (KGD) vendors, package vendors, test companies and EDA vendors.
What Open-Silicon found in doing their IP design is that it is key to understand the manufacturing ecosystem and the impact it will have on your design. You can’t afford to be only thinking pads-in, but instead, you must also be thinking about the constraints the silicon interposer and package will have on the complexity and cost of your design.
In Open-Silicon’s HBM2 memory subsystem, they spent a lot of time optimizing the pad locations and drivers of their ASIC IP so that they could meet the stringent HBM2 interface specs while minimizing the footprint and cost of their interposer. Open-Silicon also had to think about how to make their design IP as agnostic as possible to different design rules from the various interposer manufacturers so that their IP could be readily usable in both foundry and OSAT manufacturing flows.
Open-Silicon also found that it was key to consider your proposed interposer complexity. State of the art manufacturing enables wafer level testing of die so that you have known good die before assembly. The interposer however is another story. Interposers play an important role in the overall yield and cost function of the SiP. Silicon interposers are not bleeding edge technology in terms of printing, however in terms of assembly they are unique in that interposers with through-silicon-vias (TSVs) must go through many more manufacturing steps to thin down the interposer (in some cases the system die as well if you are stacking die on the interposer). These ultra-thin dice are easily deformed and require special assembly techniques and are highly susceptible to yield loss.
Additionally, since the interposers don’t have active devices on them, testing can be problematic. On an interposer die the size of a reticle field, there can be hundreds if not thousands of traces running through a 2 to 3 level metallization. The large die size can negatively impact yield both in terms of manufacturing defects and handling defects. Interposers bigger than the reticle field can be costly to print and the fine pitches even at 65nm are such that it can be very expensive to build probe cards capable of testing every trace through the interposer.
To keep costs down, manufacturers put test structures on the interposer and use those to test the overall manufacturing process. The interposer function however is usually not fully tested until the interposer can be placed onto the package substrate with at least one of the known good die. It’s at that point that you have electrical signals that can be generated by the die along with probe a fixture that can be easily used on a tester. The bad news is that if you have a bad interposer, you likely just wasted an expensive known good die and possibly the package. Having a well thought out test strategy that can be used to check the interposer before adding the most expensive die can save you a lot of money.
So, how do you navigate the fast waters of this new SiP manufacturing ecosystem? The answer is to work with those who have traveled those paths before you. Working with a company like Open-Silicon who has gone through the SiP design, manufacturing and testing process multiple times with multiple different vendors in the ecosystem can mitigate a lot of risk, and save you a lot of time and money, especially if this is your first SiP design.
For ASIC designers who have had a pads-in mentality, it’s time to wake up and start drinking your early morning coffee with companies like Open-Silicon who can you help you navigate the new frontier of the SiP manufacturing ecosystem.
Open-Silicon transforms ideas into system-optimized ASIC solutions within the time-to-market parameters desired by customers. The company enhances the value of customers’ products by innovating at every stage of design — architecture, logic, physical, system, software and IP — and then continues to partner to deliver fully tested silicon and platforms.
Open-Silicon web site
Electronic Design Process Symposium (EPDS)