Where complete integration into an SoC (System on Chip) is not possible for practical or economic reasons, MCM technology often enables a convenient, low-footprint solution. The alternative for designers is to seek a discrete solution comprising two or more monolithic ICs and their associated external components. At first sight, the discrete solution may appear to offer a lower cost. But engineers need to look beyond the bill of materials to understand the true value in a MCM. Many of the costs associated with bringing up a discrete system are not obvious at the outset. In extreme cases, or where system integration presents particularly difficult challenges such as analogue signal-management issues, designers may pay the ultimate price: non-delivery of the end product.
High-performance multi-chip modules
There are many solutions and manifestations of MCM technology, but fundamentally a MCM combines two or more bare die or maybe chip-scale packages on a single substrate, as well as the additional passive or active components necessary to meet the MCM functional requirements. Alongside chip capacitors or resistors, these may also include magnetics or other specialist devices. The substrate supports the interconnections between devices; it may be a small laminated PCB or a ceramic material such as an LTCC (low-temperature co-fired ceramic). Automatic pick-and-place equipment is typically used to place the components on the substrate. A variety of mounting technologies may be used, including wire bonding, flip-chip or TAB bonding. MCM technology enables a miniaturised solution by using components in die form, eliminating most of the package overheads associated with a discrete solution. The dies can be closely spaced on the substrate, allowing a higher component count within a small footprint. Reliability is also improved over a discrete solution that comprise individually packaged ICs and their supporting components. The removal of IC packaging eliminates many of the joints and structures used to connect various pads on the die to the device pins, thereby removing many potential points of failure. The process variability of surface-mount assembly is also eliminated. MCM assembly processes are proven to be robust and reliable, and typically include thick-film on ceramic substrates, and die-attach, wire-bond and seam-sealing processes. In addition, military screening may be performed to further qualify the products. Reliability at the board level is also improved, because one MCM replaces multiple discrete, passive and active components that in other systems are used individually as separate parts. It is also important to note that significant costs are associated with designing separate discrete passive and active components onto a PC board as individual parts. For r instance, costs are incurred in procuring and managing components, dealing with lot-to-lot variability, and additional design complexity. The effects of assembly overheads - including the time to build each unit - and the extra challenges of optimising production equipment - for example feeder scheduling and line balancing - must also be added to the overall cost of a discrete solution.
Lower inventory costs
Since with an MCM there is only one component to procure, lot-to-lot variability is eliminated, as the system is fully tested and trimmed before delivery. The assembly process is also simplified. Moreover, the MCM solution eliminates many of the logistical issues associated with acquiring and managing large numbers of individual components. These include managing vendors; taking care of goods-in issues, including inspection and put-away; ensuring correct storage and adequate inventory; managing lead-times; and dealing with obsolescence of individual components.
Better signal-to-noise ratio
There are also valuable performance advantages. Eliminating package overheads not only leads to footprint savings and greater reliability but also eliminates parasitic effects, including lead-frame resistance and inductance for each included die. Close spacing of dies also reduces the signal-path lengths for inter-IC connections. As a result, an MCM solution achieves better signal-to-noise ratio than a functional equivalent implemented using discrete components. Upon completion of assembly, the MCM is also actively laser-trimmed for further noise-performance optimisation. Package-free assembly and close die spacing also give MCM integrators greater freedom to optimise the layout of the system. This, too, delivers a powerful performance boost compared to a discrete solution, but also solves some of the most serious challenges facing designers of high-performance systems.
Pre-engineered data-path
Consider the design of a high-speed, specialist data-acquisition system. Managing the layout to optimise signal-path lengths can present engineers with a multitude of parameters to manage simultaneously, and there are substantial issues with grounding that must be dealt with. Often, the black-artistry of the analogue engineer is the only effective tool to establish a route through the maze. With an MCM solution, these come already solved. Hence, MCM technology delivers perhaps its most significant advantage in specialist applications by providing an insurance against the costs of failure. This talk of project failure may sound excessive - but it can, and does, happen. Stepping back from the abyss, however, it is certainly true that a complex analogue design is not a trivial undertaking. It does require specialist-engineering skills that can be expensive, and it does take time to complete and fine-tune. Alternatively, if a specialist contractor has to be consulted, the overall cost of the project can quickly dwarf the extra price associated with an MCM. The MCM, on the other hand, represents a turnkey solution that comes with guarantees and technical support from the supplier. As an example, consider the engineering costs involved in developing and proving a high-speed analogue-to-digital converter, compared to using an off-the-shelf MCM solution such as the ADS-939, a 16-bit sampling ADC designed for operation up to 10MHz (Figure 1). The major analogue design challenges are dealt with internally to the package, as are the most sensitive system-layout decisions (Figure 2). For the customer's engineer, the remaining board-layout challenges are relatively easily solved. This ADC is delivered as a 40-pin ceramic TDIP component with a footprint of 53.3x27.9mm.
