MCM test trade-offs

The cost of design-for-test is a key factor when determining whether or not to use MCM technology. Higher-end MCMs must be designed-for-test (DFT) since not implementing DFT is more expensive in the long run. This may not be the case for lower-cost, lower performance MCMs. Among other advantages, MCM technology can offer smaller size, better performance, and lower weight. But the cost of poor DFT implementation can outweigh these advantages. For example, when remapping a PCB design into an MCM using off-the-shelf components without DFT built-in, implementing DFT into the MCM package may require the expensive redesign of chips or the addition of test chips. Then an MCM is designed with ASICs targeted for MCM packaging to begin with, implementing DFT can be relatively inexpensive if the proper DFT tools are available. These factors help determine the trade-off between the amount and cost of DFT against the advantages of using MCM technology. Structural test is the best solution for higher-end modules for bring-up, diagnostic and re-work reasons. For MCMs where a defective assembly still has high value and must be reworked, MCMs must be diagnosed.