5 things you need to plan for custom system silicon
Custom silicon can be the right solution for your system for a lot of different reasons, but whatever they may be in your case, there are some basic steps that should always be taken to start on the path towards a successful custom silicon project. Here are five basic steps:
1. Decide where to integrate each type of circuit.
By ‘where’ I mean multiple things. First, I mean what semiconductor process node. For a typical battery operated device, you will have chips for the processor, which like to be fabricated in processes with small lithographic features and which due to semiconductor physics allow only lower(i.e. lower than the battery level) power supply voltages to be applied to the chip. Then there are circuits that will be connected to the battery and the charger input port, and these want to be integrated in semiconductor processes with devices that can withstand typical Lithium-Ion battery voltages in the 4.2V range, and USB charger input voltage which is at least 5V. There may be other power management chips that typically also need to be tolerant to at least the battery voltage.
Second, there is the location that needs to be considered. For example, you will want to place the charger chip close to the charger input connector, and also close to the battery. The processor may need a lot of signals to be routed to it, and it may want to be closer to its own peripherals, which may be in a different location from where the battery connector and charger input connector want to be located. So in this case, it may not be a good idea to integrate the processor and the charger in the same chip since they want to be in a different location, and going back to the first point, the semiconductor process node for the processor may not be suitable for making a 5V tolerant charger chip. But sometimes the overall price/performance trade off is ok and you land on a node that integrates both low voltage circuits and battery tolerant circuits, so this is part of the fun of planning custom silicon for your specific needs.
Third, there is routability to be considered. It’s possible that you could integrate all your off the shelf components into one single chip, but you may still not want to do that depending on how congested your PCB routing gets. You need to also consider all the inductors and caps needed which may become a large congestion around your chip.
Fourth, there are some types of components such as sensors that are made in very specialized technologies such as MEMS, and these are not suitable for integration into a custom chip in standard silicon processes. You can of course, develop custom sensors which may provide benefits in cost and performance, and those usually either get co-package with a silicon companion chip for signal processing, or they may be connected to other chips in your system that can provide the signal processing needed.
2. Decide what makes sense to integrate and what doesn’t.
For some components on your board it’s usually better not to integrate them such as power capacitors and power inductors. Semiconductor processes don’t provide good enough cost and density to justify swapping an off the shelf power cap or power inductor for an integrated version. ESD protection and other diodes may be candidates for integration, and they can usually be absorbed into a custom silicon component with great benefits. Power FETs and other FETs can be easily absorbed into a custom silicon component, but you need to keep in mind that some power FET technologies are extremely superior for high power and high voltage applications and best kept as off the shelf components instead of integrating them. If all your FETs are in the Watts range, it’s usually better to integrate them. Every design is different and requires some engineering analysis to decide what makes sense.
Most off the shelf components can be integrated into one or a few chips, and this will provide you a BOM cost reduction and board size reduction typically in the 50% range for each, better reliability with the added features that you prefer which may be much more difficult and expensive to implement with off the shelf components. This needs to be analyzed taking into account your expected lifetime volume and the cost of the NRE to ensure you earn back the NRE with the savings over time and then profit further as your shipment volume increases.
3. Determine what are suitable existing components that could be used as the base IP to get to your desired Custom chip.
Developing custom silicon is usually done in parallel to the system development, as time to market is usually key for high volume consumer electronics. Therefore, it is desirable to find chips that are available off the shelf and try to base your custom chip project using those as base IP. Once you have a list of off the shelf components that look attractive, you can contact the Suppliers that make them to start a discussion about custom silicon.
4. Determine who are suitable Suppliers for your project.
First, you need to decide how comfortable your Company feels about the Suppliers you have listed as having suitable off the shelf components available in step 3. Does this Supplier have that component being fabricated in at least two foundry (i.e. semiconductor wafer fabs) sites, and two assembly and test sites? Does this Supplier have a good track record delivering shipments on time? What is this Supplier’s overall financial health?
5. Determine what is your current BOM cost with off the shelf components projected at the volume that you will ship.
It’s important to perform some basic ROI analysis to determine if custom silicon is a no brainer for your Company. Let’s explore an example. Acme electronics is shipping on average 6 million units per year, and product life is about 4 years. They pay about $2 for the electronics with their current discrete design, but they have determined by talking to some chip Suppliers (or determining estimated chip cost using commercially available calculators) that they can get one chip that can do all the things they need for $1. They engage with a Supplier that tells them they will design the custom chip for them for an NRE payment of $3 Million USD in three payments: $1 Million USD at kick off, $1.5 Million USD at tape out and $0.5 Million USD when the component is qualified for mass production. So Acme electronics needs to pay up front $3 Million USD, but they will save $1 in every unit they ship. So after they ship 3 Million systems they will recover their custom chip development NRE investment, and from then on out every system they ship will give Acme electronics $1 extra dollar of profit. Since they will sell 24 million systems during the product’s lifetime, after deducting the $3 Million USD spent on NRE, Acme electronics makes $21 Million USD of profit. So the ROI for them was = 21 Million USD/3 Million USD = 700%