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Inspection Methodology
December 31, 1969 |Estimated reading time: 5 minutes
Part 1 of a 2-part series.
Robert Rowland
Frankly, I dislike the term inspection because it is a constant reminder that my assembly process still has too much variation. However, until my manufacturing process is capable of consistently producing zero defects, some form of inspection or monitoring is necessary to ensure the desired quality level. Surface mount assembly is a very complex sequence of events with a large number of individual activities. The trick is creating a balanced inspection and monitoring strategy without performing 100 percent inspection. This month`s column focuses on inspection methodology, techniques and manual inspection tools. Next month, I will review automated inspection tools and how to use inspection results (quantity and type of defect) to improve process and product quality.
Inspection vs. monitoring. Inspection is a product-focused activity, while monitoring is a process-focused activity. Both are needed for a quality plan; long term, however, the goal should be less product inspection and more process monitoring. Product inspection is reactive (the defects have already occurred) and process monitoring is proactive (the defects can be prevented) - obviously, prevention is considerably more valuable than reacting to a defect that already exists.
Inspection is really a screening process because it endeavors to find unacceptable product for repair. The evidence is pretty clear that mass inspection does not necessarily enhance or ensure product quality. Point number three from Deming`s 14 points is "cease dependence on mass inspection." Instead of mass inspection, Deming emphasized a strong process focus with the goal of creating stable and repeatable processes that are monitored statistically. Inspection is a subjective activity and, even with considerable training, it is a difficult task. In many cases, you can ask a group of inspectors to evaluate a solder joint and get several different opinions. The following test is a simple, yet effective, example of how difficult inspection can be. Read the following paragraph and quickly count the number of times the letter "a" occurs (the answer is at the end of this column):
Operator fatigue is a major reason why 100 percent inspection usually fails to find and isolate every manufacturing defect and, in addition, it is an expensive, non-value-added operation. It has rarely achieved the desired goal of higher product quality and customer satisfaction.
Several years ago, I started to use the term "process monitor" instead of inspector because I wanted to alter the production floor mindset from reactive to preventive. An inspector typically sits at the end of an assembly line and inspects product. In an ideal situation, the process monitor`s activities would be a balance between product inspection and process monitoring - e.g., verifying that the correct process parameters are being used, measuring machine performance, and creating and analyzing control charts. Process monitors assume a leadership role in regard to these activities; they help machine operators complete these tasks. Training is a key element. Process monitors and machine operators must understand workmanship standards (e.g., IPC-A-610), process monitoring concepts and related tools (e.g., control charts, Pareto charts, etc.). Process monitors also promote product quality and process monitoring. As a key part of the manufacturing team, monitors encourage a defect prevention approach rather than a find and repair approach.
Over-inspection is also a common problem. In many cases, over-inspection is simply caused by misinterpretation of the IPC-A-610 workmanship standard. For example, with regard to insertion-mounted components, many inspectors still expect perfect solder fillets on both sides with complete hole filling. Anything less is rejected. However, this is not what IPC-A-610 requires. Inspection quality will also rise and fall with the intensity and focus level of the inspector. For example, fear (management pressure) may increase the level of intensity and focus on the production floor and for a while, quality may improve. However, if mass inspection is the primary quality control method, then defective product will still be produced and may exit the factory.
Another term that I like to avoid is touch-up. Throughout the industry, many employees believe touch-up is a normal and accepted part of the assembly process. This is extremely unfortunate because any form of rework or repair should be viewed as undesirable. I prefer the term rework because it is typically viewed as undesirable, which is the proper message to instill throughout a manufacturing organization. It is very important to create a manufacturing environment where defects and rework are viewed as avoidable and highly undesirable.
Manual inspection is the first line of defense for most companies. Inspectors use an assortment of magnification tools to get a closer look at components and solder joints. IPC-A-610 has established some basic magnification guidelines based on the land width of the component being inspected. The main reason for these guidelines is to avoid over-inspection caused by over-magnification. For example, if the land width is 0.25 to 0.50 mm, then the desired magnification is 10X, with 20X to be used as a referee if necessary.
Every inspector has a favorite inspection tool; mine is a machinist`s three-lens folding pocket magnifier. It`s portable and the maximum magnification is 12X, which is just right for fine-pitch solder joints. Probably the most common inspection tool is the microscope, with magnification ranges from 10 to 40X. There are two things I don`t like about microscopes: they are fatiguing when used continuously and they usually lead to over-inspection because the magnification levels typically exceed the IPC-A-610 guidelines. They are useful when a potential defect has been detected and a more detailed examination is appropriate.
For general inspection, I prefer a video system equipped with a zoom lens (4 to 30X) and a high-resolution color monitor. These systems are easy to use and, more importantly, less fatiguing than microscopes. A high-quality video system can be purchased for less than $2,000, which is in the same price range as a good microscope. An added benefit of the video system is that more than one person can view the object, which helps when an inspector wants a second opinion or when training. Edmund Scientific (edmundscientific.com) has a large selection of magnification tools, ranging from hand-held magnifiers to microscopes to video systems.
The letter "a" appears in the paragraph 24 times.
ROBERT ROWLAND is an SMT Editorial Advisory Board member, instructor and coauthor of the book Applied Surface Mount Assembly. He is currently the process engineering manager at RadiSys Corp. in Hillsboro, Ore., and is also an active member of the SMTA and the Surface Mount Council. Contact him at (503) 615-1354; E-mail: rob.rowland@radisys.com.