Issue link: https://iconnect007.uberflip.com/i/1129312
JUNE 2019 I PCB007 MAGAZINE 13 1. PWB Layout The standard (and not so standard) metrics used before PWB layout include a packaging technology map. A simple technique exists to predict a printed wiring board's wiring factor (W F ) and its assembly complexity. The tech- nique is the packaging technology map [7] . By plotting parts per square inch against average leads per part on a log graph, the wiring factor (inches per square inch) and assembly com- plexity (leads per square inch) can be calcu- lated. 2. PWB Fabrication The metrics for PWB fabrication deal with trade-offs between the performance objectives and the PWB prices. This is where produc- ibility came in since prices need manufactur- ing yields before it can be estimated accurate- ly. Two key metrics were developed to bring about these price/performance trade-offs: 1. Complexity index (CI) to characterize the complexity of physical characteristics (size, layers, holes, traces, etc.) of this particular PWB so that first pass yield can be calculated come extremely complex. Many companies are working on this problem through DFMA. The focus is on six key design topics, including: 1. Optimization of PC design grids and layout [4] 2. Minimization of PC substrate costs [2] 3. Minimization of assembly costs [1] 4. Use of preferred parts [5] 5. Analysis of test coverage [6] 6. Partitioning of ASIC pinouts What they all have in common is metrics, but the design community is suspicious when the entire system is not being considered. They are afraid of suboptimization where the cost of a particular domain is lowered but the total system cost increases. The design community needs a software environment to integrate all of these separate programs. Common Metrics of DFMA The metrics that have been developed for DFMA occur in three domains: 1. PWB layout 2. PWB fabrication 3. SMT assembly and test Figure 4: Process flow for the design planning of a printed circuit.