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Placement Accuracy Management: Key to Better Profits
December 31, 1969 |Estimated reading time: 4 minutes
The importance of quality control in PCB assembly operations has never been greater than it is now due to shrinking component and assembly sizes. Because of this, the ability to monitor and analyze trends in placement accuracy before they become a quality issue has never been more critical.
By Rick Flayler
Statistical analysis based on fundamentals of Motorola’s six sigma program is the most effective means to determine placement accuracy results. This principle of measuring quality in a quantifying manner lends itself to strides in machine performance, troubleshooting and analysis. The minimum target you want to achieve when applying six sigma principles to a chipshooter is sigma-level 4 (Cpk of 1.33 with a 1.5-sigma shift). This corresponds to a yield level of 99.38%. At this level, a user can expect to have 6,200 defects per million components placed (DPMO). These defects as they relate to SMT operations could be anything from misaligned, misplaced, tombstoned components, incorrect components, or dropped or missing components.
Figure 1. Chipshooter placement head.
These numbers seem easy enough; however, most chipshooters in the field do not operate at these levels. If they do, they unfortunately do not stay there for long. Many chipshooters out in the field are producing at levels of sigma 3 (Cpk of 1.00 with a 1.5-sigma shift). This corresponds to a yield level of 93.32%. At these levels, the defect rate per one million components placed jumps to 66,800 DPMO. Let’s look at a typical SMT line with the following assumptions:
Throughput per hour: 20,000 components Shifts per day: 1 Hours of work per shift: 7 Days per week: 5Weeks per year: 48
In this case, the total number of chip components placed by the chipshooter annually is about 34 million parts. At 4 sigma (1.33 Cpk), this results in 210,800 defects annually. However, at 3 sigma (1.00 Cpk), defect numbers increase to 2.3 million per year. The importance of monitoring and tracking the performance of your chipshooter is paramount to the success of your operation; and is a key ingredient to profitability in the manufacturing environment.
Until the acceptance of AOI in the SMT manufacturing sector, methods to determine the amount of defects off the line usually were left to the discretion of the operators or, in most cases, the final inspector at the end of the line. A chipshooter with a faulty nozzle would produce dozens of boards requiring rework before anyone would notice the problem. Often, this was the case because the person in charge of determining quality also was the last person on the line.
The AOI platform was created to combat this issue. However, this does nothing to help solve the chipshooter placement-accuracy problem. It simply catches the problem quicker, before too many boards have been affected at one time on the line, minimizing rework expenses when the process gets out of control. However, because the average in-line AOI unit can cost almost $125,000, this is an expensive solution. This seems like quite an expense for something that does nothing to solve the real issue - an out-of-control chipshooter.
In both cases, the solution (or perceived solution) is a reactionary one. The mindset is that we know we are going to have placement issues with our machines; therefore, let’s invest more money and technology to help detect the issues faster before occupying too much of company profits in rework expenses. The other perceived method to combat quality issues with SMT placement equipment is to adopt regimented maintenance schedules that may or may not have any bearing on the actual performance of the machine, at least in the short term. Not to say preventative maintenance (PM) is not important or vital to a machine’s well-being, but how do you really determine if the PM you are performing every week is achieving your goal? How do you know, for example, that your machine would not perform as well and for as long if you performed PM every two weeks instead of one week? Without a true method to analyze the placement accuracy of your machine, you may never know.
Predictive Analysis Control
This is a method in which a chipshooter owner can become more proactive in achieving 4-, 5- or 6-sigma levels on their machines, and keep the machines at those levels. Without a method to analyze and interpret placement accuracy, these methods cannot be achieved. This system:
- Analyzes trends in nozzle wear and determines nozzle life expectancy. The benefits of this are maximizing the life cycle of a nozzle, but replacing it before it becomes a quality liability in regard to misalignment issues and component waste.
- Verifies the accuracy and longevity of third-party nozzle manufacturers that allow users to take the guesswork out of determining which nozzle manufacturer is most suitable for their application.
- Removes the guesswork from trying to determine which head or nozzle is reducing company yields at any given time.
- Reduces the amount of time machines are down because of scheduled maintenance. Determine maintenance schedules based on when the machine needs them.
Issues Faced by Operations Managers
- Traceability/Process Control: External customers are directing you to provide cp and cpk levels internally for SMT production equipment.
- Quality and yields could be better; however, it cannot justify why adding more AOI equipment resolves this problem.
- Rework costs should be reduced and scrap rates are unacceptable.
- Every minute counts in any environment. Use the four hours on Saturday that normally would be slotted for weekly maintenance. What affect will this have on machine performance if you skipped a week?
Conclusion
By using a tool to measure placement accuracy, users become more proactive with regard to quality - instead of reactive. Instead of throwing money away on ways to “find” quality issues, users can focus on eliminating these quality issues before they reach the AOI machine.
Rick Flayler, president, TimeKey Corp., may be contacted at (925) 371-1600; e-mail: rick@timekeysolutions.com.