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As users begin to recognize the clear relationship between solder paste deposition quality and soldering process at the component level, 3D solder paste inspection will play an increasingly important role in testing strategies.
For many years, many process engineers and quality managers have had doubts about the benefits brought by solder paste inspection equipment (SPI). Although there are often high defect levels in the SMT process flow, many SMT production lines have never truly performed SPI testing. Some users question the cost-benefit analysis results, while others believe that SPI, especially 3D SPI, is only useful during the New Product Introduction (NPI) stage or product trial period, and is not profitable for mature product processes. For them, the information provided by SPI will not bring any quality improvement to the related products, nor will it hook the demand for such improvement with insufficient SPI device configuration.
Anyway, as today's advanced processes tend to use smaller components such as 01005, μ BGA, or PoP, the use of real-time inspection in SMT production processes becomes particularly important and will become a standard process to ensure quality. So, people realized the need to rediscover the functionality of 3D SPI and also realized that simply calculating volume is no longer sufficient to ensure process quality.
Solder paste printing process
Compared with other SMT processes, solder paste printing has potential instability due to the presence of more variables. According to research from numerous companies and universities, printing processes have a variability of over 60%. The reason for such great variability is that the printing process contains a large number of uncertain process parameters, and it is generally believed that the sum of these parameter variables is close to 40, including (rather than limited to) the type of solder paste, formula, environmental conditions, template type, template thickness, width to thickness ratio and area ratio of openings, printing machine type, scraper, printing head technology, printing speed, and so on.
Typical performance quantification parameters for printing machines include transfer efficiency (TE) percentage and standard deviation of solder paste deposition. 100% means that the solder paste contour on the PCB matches exactly with the calculated volume of the opening. Interestingly, the transfer efficiency of a typical SMT board can vary between 20% and 130%.
Although the transfer efficiency of rectangular openings is usually better than that of square and circular openings, there may still be volume differences in the horizontal or vertical direction during printing. Vertical openings have better printing effects when there is a slightly larger hole capacity, while the worst printing effect occurs on circular or square openings with a diameter less than 0.012 inches. It should be noted that when the transfer efficiency decreases, the standard deviation will also increase, and the decrease in transfer efficiency will also lead to a decrease in volume repeatability.
Another interesting phenomenon is that research has shown that for an array type package, the importance of solder paste volume is not as significant as consistency across all array pads, meaning that as long as there is an equal amount of solder paste on all pads, it will be accepted. If the solder paste on some pads is less than on other pads in the array, it may lead to the occurrence of defective solder joints.
