SMT007 Magazine

SMT007-Jan2021

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60 SMT007 MAGAZINE I JANUARY 2021 Most AOI systems are very good at giving you data on simple defects like wrong com- ponents, wrong orientation, wrong polarity, and insufficient paste height or paste volume. Unfortunately, most AOI systems are placed after the reflow ovens and asked to make some difficult calls like cold solder, non-wet, de-wet, and insufficient joints. These are even difficult for well-trained human inspectors who would disagree with each other. It is no wonder that AOI systems are wrong quite often. Therefore, when trying to assess the qual- ity approach of a company, you need to focus on whether the company's focus is on qual- ity assurance or quality control. There is a big difference between these two approaches. For example, quality control implies activities after product is built such as inspection, test and repair. On the other hand, quality assurance involves activities before product is built such as DFM, use of only acceptable parts (copla- narity, solderability, moisture and temperature sensitivity), and use of a documented process recipe for all the process steps (printing, place- ment and reflow profiles). Quality control may not add value, but quality assurance does. When you place the AOI system before the reflow oven in the SMT line, your focus is on preventing the defects before they hap- pen—a quality assurance approach where AOI machines provide more reliable answers. On the other hand, when you place the AOI system after the reflow oven, your focus is on qual- ity control so that defective products don't get shipped to the customer. I am not suggesting that you ship the products without inspection or test which are non-value-added steps but necessary and also more difficult for most AOI machines. If you plan to place AOI machines after the reflow oven, you should lower your expectations from the AOI machines. Managing your expectation is key to happiness whether you are dealing with people or machines. With the above background in mind, when auditing a site that uses AOI machines, you could ask questions to determine if enough thought has gone into selecting the location of the AOI machine in the SMT line. For example: Why was that location of the AOI machine in the SMT line chosen? The answer to this ques- tion will give you an idea whether the com- pany's focus is on quality control or quality assurance and whether the company knows the difference. Here are some other questions related to quality worth asking during your audit: • Which flux are you using for touch touch-up and rework? • Have you implemented no-clean board processes? • Do you clean the boards anyway even though the flux is no clean? Why? • How clean is and how do you measure board cleanliness? • Do you perform any type of accelerated stress screening? If so, what are the range, temperature ramp and length of the test? What percent of boards? • Do you perform any type of functional tests? What percent of boards? • Which debugging analysis tools do you use for troubleshooting defective boards? Do you prefer a specific set of equipment and why? To get an idea of the overall quality focus of the company you could ask them to describe their overall quality organization and reporting structure and question such as: • Is there a formal Quality Manual and procedures? • Are there different manuals for tin-lead and lead-free? If so, are those manuals under controlled distribution? • What type of indicators do you use to monitor overall quality? Quality control may not add value, but quality assurance does.

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