Jun 10, 2003
SPC Charting with NWA Software
Many electronics-assembly operations use process-capability analysis to select, optimise, and certify production equipment. Once the system is established, variable and defect tracking monitors the daily production. If control charting reveals serious problems, an engineering study can reveal the sources of the problems.
Manually produced control charts can track manufacturing processes acceptably, but collecting and charting the data disrupts the production process while putting extra burdens on assembly and inspection staff. Also, manual collection and charting do not directly integrate into vendor and in-house reporting systems; data retrieval over any length of time is at best awkward.
Spreadsheet-based charting improves on manual methods, but requires considerable up-front configuration and macro building. Maintaining macros requires an ongoing input of skilled labour and time. Adapting to changes in charting methods or requirements is expensive and time-consuming.
The alternative is to use SQC software such as NWA Quality Analyst and its companion plant-floor data-collection software, NWA Quality Monitor. For considerably less investment of time and money, the plant can collect data from the assembly operation and produce accurate control charts while minimising training time and interference with production.
Variable Measurement with X-Bar/Range Charting
In the following example, a Printed Circuit Board (PCB) manufacturer tracks solder height with NWA Quality Analyst. Keeping solder height within control limits is critical in PCB manufacturing because solder height determines how well components will sit on the board, and therefore, how well they will operate as designed. Also, if the boards are stored or stacked, the solder height determines whether copper in the components will migrate, causing the boards to become defective.
Inadequate solder height can indicate many problems including wear and tear on the solder stencil, a calibration problem with the machine applying the solder, inadequate stencil cleaning, or a raw material problem. Any of these problems can lead to higher production costs due to waste or rework.
In this example, the specification for the solder height is 0.25-0.55 mm. Periodically, samples of five units made at the same time are measured for solder height. This data is analysed with NWA Quality Analyst using X-Bar/ Range Charting to determine whether the height variation indicates an out-of-control condition. The Quality Control staff member knows that a certain amount of variation is natural to the process, but an out-of-control condition could indicate a problem that, if left unresolved, could become very costly to the plant downstream. Data from a full day's run is charted in an X-bar/Range chart.
Inadequate solder height can occur as a result of a range of external causes; the problem might be with the raw material, with the machine applying the solder, or even with the measuring device itself. Upon investigating the records, the process engineers found that the inadequate solder heights all came from lots using raw materials from a single vendor. Further analysis of lots from that vendor revealed the vendor's inability to produce consistent material. As a result, the manufacturer dropped the vendor as a supplier.
With the out-of-control condition removed, the process engineers analysed the process further. The process was run and charted for another day using only raw materials from reliable vendors. The process was in control but suffered from excessive variation and was not capable of producing output well within specifications. Such a situation indicates significant opportunities for process improvement because the in-control condition indicates that the source of the excessive variation is the process itself. In this particular case, the process engineering team discovered that the machine needed maintenance and recalibration. This was done and the results for the next day's production indicated a process in control and capable of producing output well within specifications.
Defect Tracking with Pareto Diagrams
After the board has been assembled, the PCB manufacturer uses Pareto Analysis to examine the relative contribution of different defects that lead to PCB rejects. One advantage of SQC software is that the user can easily examine the data from different perspectives with minimal time and effort. With NWA Quality Analyst, the user also has complete control over labelling, individual defect percent, cumulative frequency, and so on.
Pareto analysis offers an excellent example of how NWA Quality Analyst delivers improved capability for considerably less effort. In our example of the PCB manufacturer, the plant uses Pareto analysis to track defects arising from the following solder-related attributes:
Insufficient Solder
Excess Solder
Cold Solder
Shorts/Bridges
Solder splash/ball
Flags (peaks)
Open joints
Pinholes/voids
OtherMiscellaneous solder problems
Pareto diagrams are commonly used to rank the relative frequency of different categories of defects. With a Pareto diagram, the quality control staff can assess the relative contribution of different defects to PCB rejects and assign priorities for addressing their causes. The Pareto diagram is fundamental to defect tracking and analysis. NWA Quality Analyst lets you rank the charted defects by occurrences or relative cost. It is often useful to rerank defects by categories other than frequency of occurrence. One alternative ranking is cost. Reranking requires only a few mouse clicks in NWA Quality Analyst. The same data used to generate Pareto diagrams can also be plotted as percent defective control charts. NWA Quality Analyst's Chart Group feature lets users quickly display multiple charts for process comparison, simplifying process analysis and general quality reporting.
Application to Electronics Component Manufacturing
Although our example has highlighted PCB manufacturing, NWA Quality Analyst is used in a wide variety of electronics manufacturing. Since the electronics industry is highly competitive, engineers must continually change and improve their processes to reduce variability that could lead to waste and rework. NWA Quality Analyst is a very easy-to-use tool that allows them to identify where improvements could reduce costs.
NWA Quality Analyst is a tool for variable and attribute analysis. The critical measures vary from manufacturer to manufacturer, but setting up the files within NWA Quality Analyst is easy because the software is configurable and has a spreadsheet-like look. The NWA Quality Analyst Data Editor simplifies data setup and handling. All routine SQC charting functions are just a mouse click away.
Integration with Manufacturing Information Systems
Many electronics operations maintain production records in some type of manufacturing information system (for example, ERP, MRPII, MES, and so on). NWA Quality Analyst works well with these and has been integrated successfully with many database systems.
This success results from three NWA Quality Analyst capabilities:
1) Straightforward data transfer:
NWA Quality Analyst can readily accept data from external databases, spreadsheets, and manufacturing information systems. Data can be read in as delimited ASCII data files or as an ODBC (Microsoft's Open Database Connectivity standard) source. Once the ODBC source has been defined for a data set, its use is transparent to NWA Quality Analyst operation. The connection is automatic, and the user is always working with the most current data.
2) Automated charting functions:
In addition to ready data transfer by data file or ODBC, NWA Quality Analyst has an internal scripting system that allows the user to define standard sets of charting and reporting operations, then initiate a defined operation with a single mouse click. The package and these scripts can also be called by other software such as MES.
3) Parameterised charting and reporting properties:
Quality Analyst charts and reports can be easily configured to conform to virtually any analytical or reporting standards set by customers, regulatory agencies, or in-house quality-management staff. These parameters can be set using dialogue boxes in NWA Quality Analyst or transferred from the manufacturing information system.
Quality Analyst software is supplied and supported in the UK and Ireland by Adept Scientific plc, Amor Way, Letchworth, Herts. SG6 1ZA; telephone (01462) 480055, fax (01462) 480213, email quality@adeptscience.co.uk; or see Adept's World Wide Web site http://www.adeptscience.co.uk/. Adept Scientific is one of the world's leading suppliers of software and hardware products for research, scientific, engineering and technical applications on desktop computers.
With offices in the UK, Denmark, Germany and the USA, Adept Scientific is one of the world's leading suppliers of software and hardware products for research, scientific, engineering and technical applications on desktop computers.
This product can also be purchased on-line from Adept Scientific's WebStore at http://www.adeptstore.co.uk.
FOR MORE INFORMATION, PLEASE CONTACT:
Anna French,
Adept Scientific plc
Amor Way, Letchworth, Herts. SG6 1ZA
Tel: 01462 480055 ext.230
Fax: 01462 480213
Email: pressroom@adeptscience.co.uk
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