Bare die are key for design miniaturisation

Using unpackaged die in embedded designs can have important benefits for designers offering new options for small-footprint designs, writes Shahram Tadayon
Embedded system designers have benefitted from procuring integrated circuits (ICs) such as microcontroller (MCUs) in die form for many years. Because the dimensions of die are much smaller than when the IC is packaged, the use of unpackaged die allows the product designer to significantly reduce the size of the electronic circuit.
High-volume products such as hearing aids, cell phones and RFID cards have become more comfortable, more portable and thinner due to the integration of die in the circuit design. Until recently, lower volume products have not benefitted from the same level of innovation as it has been historically difficult to procure reasonably priced die in low volumes.
The benefits of using unpackaged die in embedded designs can be substantial. The availability of IC products in die form gives customers a convenient option for small-footprint designs, enabling them to optimize their product designs for limited space and to implement innovative, proprietary packaging solutions.
Handling die, however, is not as straightforward as buying an IC in a conventional quad flat pack package and soldering it onto a printed circuit board (PCB). For this reason, a die sales industry has emerged to simplify the handling and utilization of unpackaged die.
For example, when an embedded designer procures a wafer containing thousands of die, the wafer is diced and placed on a leadframe or substrate that interconnects each die with other components (including other unpackaged die) in the embedded system. The resulting circuit is very small and may be placed in a module or a package that is used directly in a system or is then placed on a PCB.
Buying raw die from a semiconductor supplier is seldom a simple, straightforward process. High-volume semiconductor suppliers have fine-tuned the art of producing vast quantities of packaged ICs in a very efficient manufacturing flow and supply chain.
However, shipping wafers in small volumes is a non-standard process that can interrupt the efficiency of the manufacturing cycle.
Due to the recent demand by product designers to pursue product miniaturization and develop innovative electromechanical packaging, semiconductor suppliers are opening up to the market potential of shipping fully tested wafers. It is now possible to purchase die in wafer form (see Figure 1), and a single 200 mm wafer typically will contain a few thousand die.

There are additional benefits of using die in system designs rather than packaged ICs. Because the electronic circuit is contained in a smaller space, the length of the interconnections between the chips is reduced, which in turn reduces the effects of capacitance and inductance and thus minimizes the switching latency. Electrical noise is also minimized, which is particularly beneficial if there are radio frequency signals in the system.
Die utilization can be particularly effective for sensor-based products. MCUs often include an integrated on-chip temperature sensor that exploits the known characteristics of a bipolar junction transistor to make an accurate temperature measurement.
This integrated sensing technology can be useful for temperature compensation effects when, for example, implementing a temperature-compensated real-time clock. When there is an IC package around the die, additional thermal resistance is introduced into the system. A more accurate temperature measurement can be achieved when there is no package on the die.
Several technologies have emerged to accommodate the integration of unpackaged die in an embedded system. Multi-chip modules (MCM) have become quite popular in many product implementations. Variations on this theme also include multi-chip packages (MCP) and system-in-packages (SIP).
Numerous assembly house vendors can accept wafers, dice them and place them in these package types. Custom packages often are developed that take a mechatronics approach to optimizing both the electrical and mechanical properties of the system to provide a tiny footprint solution. A prime end-product example of this approach is a state-of-the-art hearing aid, which requires a very space-efficient design and high levels of circuit integration.
Product designers should consider using die if this approach will bring tangible benefits to an end product, such as shrinking form factors, reducing weight and making the product more ergonomically appealing to consumers. Close consideration should be given to the manufacturing technology.
There are a couple of common technologies used with die implementations today. For example, chip-on-board technology enables the die to be mounted on a substrate and electrically connected using conventional wire-attach techniques. The die is sometimes encapsulated to protect the fragile silicon during the manufacturing process.
Flip-chip technology is more elegant but requires some additional processing steps to the wafer to add solder balls underneath the die. The resulting implementation can be smaller and more robust but also more expensive to manufacture.
The die also must be sourced carefully. After a suitable die product is identified, it cannot simply be assumed that it can be procured in wafer form. It is necessary to check with the semiconductor supplier regarding product availability in wafer form. If the expected product volume is not very high (such as in the many thousands of units), most suppliers will not offer to sell wafers, or if they do, the wafer price will be prohibitively high.
Before procuring unpackaged die, the developer should identify ICs that will operate effectively in the system and that are guaranteed to be offered in wafer form. It is also prudent to ensure that the die are fully tested to the same levels that are applied to packaged products. It will be necessary to find an assembly house that can handle unpackaged die and that can perhaps offer additional services such as backgrinding (thinning the wafer to reduce the chip height, which is desirable for ultra-thin chips used in smartcards).
The good news for embedded developers is that all of the appropriate wafer handling technologies, IC products in die form and services are now becoming more commonplace and affordable. The availability of fully tested die in wafer form is enabling the development of innovative, tiny mechatronics devices that might not have been feasible using conventional packaged semiconductor products.