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May 2015 • SMT Magazine 31 SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature A. Securing Solder Print Inspection Measurements The goal is to control the result of the sol- der paste printing process (i.e., provide the best conditions to obtain high-quality solder joints). However, waiting with inspection until after reflow is performed is a slow and quite expen- sive way, in terms of time and rework, to verify the quality of the solder deposits. Instead, it is more useful to have inspection directly after the printing process in order to have immediate feedback and the ability to control the printing process within an acceptable timeframe. There is currently only one effective way to control a large number of solder deposits within an elec- tronic manufacturing and that is to use a solder paste inspection machine (SPI). Of course, using an SPI as the sole instrument to ensure one of the most important process parameters within electronic manufacturing makes it important that one really can rely on the machine to give correct information. The method therefore started by ensuring that the inspection machine in itself was reliable and had the correct accuracy. This was done by performing a gauge repeatability and reproduc- ibility (GR&R) analysis of the SPI to ensure that it gave the same results each time. There are two important aspects of a GR&R analysis: • Repeatability: The variation in measurements taken by a single person or instrument on the same or replicated item and under the same conditions • Reproducibility: The variation induced when different operators, instruments, or laboratories measure the same or replicate specimen These two aspects were addressed by hav- ing two different operators perform the inspec- tion operation at different times in production. Since a GR&R only addresses the precision of a measurement system and not its accuracy, it was also necessary to measure a deposit that is already well known. For such a purpose, it is not preferable to use a board that has been solder paste printed since the paste in itself will change form and characteristics with time, temperature, humidity, etc. Instead, a reference was designed that replicates as many features of a solder paste printed board as possible. The choice fell on a reference that was created from a brass metal sheet that had been etched in two steps. This was made in order to recreate the pattern of a solder mask and copper traces, which would be present on a regular printed circuit board. The pads were then plated with copper to resemble solder paste. To protect the surface treatment from corrosion, the copper was plated with pure tin. This creates a reference board that re- sembles a printed product and can be used to verify the capability of a solder paste inspection machine. Consequently, the reference board has the following characteristics: • Resistant: It is made from metal that will not degenerate due to time, normal temperature and humidity levels or other environmental effects • Reflects an actual printed circuit board: Since the reference is etched in different layers, it simulates shadowing effects and has PCB traces, vias, etc. Thus it resembles the top layer of an actual product • Entire SPI measurement area is tested: Due to the large size of the reference board it is possible to measure the SPI's performance not only in the center of the machine but also in the outer areas figure 1: reference board for solder paste inspection.