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February 2014 • SMT Magazine 69 ESD or physical damage. Damage may also be caused by moisture contamination. All of these should also be declared as a materials loss, but often are not, since they happen regularly and it is not practical to force operators to enter the data manually into ERP. • Product Spoilage: If a PCB becomes damaged, the materials placed on it to the point of where the damage occurred will also need to be fed into ERP, one by one. This is quite a labo- rious process, which is almost never accurately performed. • End of Reel Losses: Toward the end of reels, operators may be left with a small quanti- ty of materials left on the reel, which they may decide to discard, but will rarely declare this to ERP. The second category of issue is related to the MRP back-flush methodology. This has been in use probably since the days of the Model T, as the way to account materials that have been consumed to cre- ate products, and it's still the most common method used today in electronics. The methodology is to simply multiply the number of the products made by their bill of materials. Significant issues occur with this approach: • Alternative Materi- als: The purchasing team will usually make sure that there is a choice of supplier for each material, and these choices can often change be- cause of quality issues, sup- plier relationships, and of course, the relative cost of materials between suppliers. Between each alternative, how- ever, there are small differences in the sizes of materials. SMT engineering will in- sist that these variations be kept within a tight tolerance, otherwise rejection of components by the SMT machines will increase dramatically. Where tolerance restrictions cannot be met, al- WHy are erP aND MeS SO LIMITeD IN eLeCTrONICS? continues ternative internal part numbers have to be cre- ated and assigned. The declaration back to ERP about which variant of each material, or combi- nation of variants, was used at run-time, across each of the several hundred materials used in a work-order, is rarely done consistently or ac- curately. • Time to Completion: The ERP system creates a high-level production plan, with the whole production process being split into a se- ries of stages. One of these stages will be the PCB assembly process. The back-flush opera- tion works only at the completion of all prod- ucts in the work-order through all of the stag- es. The reality is that PCB assembly processes consists of many operations, many individual steps. Each PCB may require two SMT opera- tions, one for each surface. Then there is AOI and reflow. Pin-through-hole and manual as- sembly also may need to be performed before the in-circuit test (ICT) operation and even functional test operations. With many products flowing through the PCB assembly stage con- currently, work-orders are split between processes and lines and may even be di- vided into sub-orders by the production team. The time taken to get all the PCBs of one work-order through to the end of the operation can be long, especially if failures at ICT take time to repair. Although some ERP systems may enhance the back-flush capability to some extent, a significant delay between physical materials being consumed and the usage ac- counted for within ERP al- ways occurs. During this time, ERP has no idea what count of materials may be available and so has no way to execute risk-free short term changes in the plan. No doubt ERP plays a fundamental role in electronics manufacturing but it cannot be ThE ESSENTIAl PIoNEEr'S SUrVIVAl gUIDE The time taken to get all the PCBs of one work- order through to the end of the operation can be long, especially if failures at ICT take time to repair. Although some ERP systems may enhance the back-flush capability to some extent, a significant delay between physical materials being consumed and the usage accounted for within ERP always occurs. " "