As users begin to realize a clear relationship between solder paste deposition quality and soldering process at the component level, solder paste inspection will play an increasingly important role in test strategies.
For years, many process engineers and quality managers have questioned the benefits of solder paste inspectors. Although the process flow is often accompanied by high defect levels, many production lines are never actually inspected. Some users question its cost-benefit analysis results, while others believe that, in particular, it is only useful in the new product introduction stage or product trial production period, but is unprofitable for mature product processes. For them, the information provided neither brings about any quality improvement of the relevant products, nor does it link the need for such improvement to insufficient equipment configuration.
In any case, as today's advanced processes tend to use smaller components, such as, or, real-time inspection in the production process is particularly important and will become a standard process to ensure quality. Therefore, people are aware of the functions that need to be rediscovered, and also realize that just calculating the volume is no longer enough to ensure the quality of the process.
Solder paste printing process
Compared with other processes, the solder paste printing process has potential instability because there are more variables. According to research results from numerous companies and universities, the printing process has greater variability. The reason why there is such a large variability is that the printing process contains a large number of uncertain process parameters. It is generally believed that the sum of these parameter variables is close to 1, including (but not just limited to) the type of solder paste, formula, environmental conditions, Type of stencil, stencil thickness, aspect ratio and area ratio of apertures, type of press, squeegee, print head technology, printing speed, etc.
Typical printer performance quantification parameters are percent transfer efficiency and standard deviation of solder paste deposition, meaning that the solder paste profile exactly matches the calculated volume of the aperture. Interestingly, the transfer efficiency of a typical plate can vary between ~.
Although the transfer efficiency of rectangular apertures is generally better than that of square and round apertures, there will still be a volume difference in the horizontal or vertical direction when printing. Vertical apertures print better with slightly larger aperture volumes, while the worst printing occurs with round or square apertures less than an inch in diameter. It should be noted that when transfer efficiency decreases, the standard deviation will also increase. At the same time, a decrease in transfer efficiency will also lead to a decrease in volume repeatability.
Another interesting phenomenon is that research shows that for an array type package, the volume of solder paste is not as important as the consistency of all array pads, that is, as long as there is an equal amount of solder paste on all pads, All will be accepted. However, if some pads have less solder paste than others in the array, this can result in bad solder joints.
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