Issue link: https://iconnect007.uberflip.com/i/686649
48 SMT Magazine • June 2016 control. Right now, the trend is to have an SPI to inspect the solder volume. Las Marias: How does the different type of the components on the board impact the solder paste inspection process? Chong: For example, if you have a component and you have a volume of deposit on one side and the other side has lesser volume, there might be a pulling effect on the other side more, when you go through the reflow process. Most likely this would result in a tombstone effect. It is important to have a co-planarity check on the volume to know whether the ratio between the pad 1 and pad 2 volumes are the same, so that they have the same pulling force. Because if there is a volume imbalance, there will most likely be a tombstoning effect. Peallat: That is the same for the BGA. For ex- ample, when you have a 256-pad BGA, it is im- portant to compare the volume in between the pads because if will have less volume just on one pad, it may create a void. It is very difficult to find out afterwards. You have to use X-ray, which is a very expensive technology. If you are able to take that approach at the printing process, then you reduce the cost of re- pair or rework. With a big field of view, you see the whole BGA at the same time. All other SPIs are cropping the image around the unique pad and you never have the whole picture. That is the key advantage of our SPI. Chong: Earlier, we were talking about the large field of view that we have, the more region of interest that we can locate for this referencing. If the pad field of view is smaller, it can only cal- culate a smaller region. If there is some differ- ence or some disturbance in the surroundings, you may have a miscalculation. Las Marias: What do you think are the factors to consider when selecting an SPI solution? Peallat: From a measurement point of view, I will say volume measurement accuracy. If you are placing very small components, it's really the measurement of the volume. I will say the with the pad size. When pads are very small, the deposit is only a fraction of the volume of the aperture. With our next generation of SPI, we are using a parameter called area aperture ratio, which is the key for small pads. We set up the tolerances with this area ratio. For example, the 01005 is not the same as the 0402. So our toler- ances for very small pads are adapted to these pads. Moreover, process variability is greater for smaller pads, or with smaller area aperture ratio. Chong Choon Hee: To complete this, the tradi- tional way is that the region of interest is around the small pads with dedicated z-references. If you use an ultra-large field of view, the system is able to take more references for measurement on a very large portion of the PCB. The com- pilation of all these references gives the exact z-plan of the PCB, including the warpage calcu- lation. Therefore, when it comes down to small pad measurement, the accuracy of the system is enhanced. Las Marias: What about the volume of the solder? Peallat: When the deposits are becoming small- er, volume is key, because the shape of the de- posits induces high variability of the process for very small pads. With poor transfer efficiency, shape is not consistent. Volume measurement depends on z-reference. That is where most of the SPI makers are struggling. With our tech- nology with angled cameras, global z-reference- ment, the measurement of the volume is a lot more accurate than others. Chong: As the specs get smaller and smaller, the transfer efficiency is getting more difficult to SPI PARAMETER CONSIDERATIONS FOR TIGHTER TOLERANCES " With our next generation of SPI, we are using a parameter called area aperture ratio, which is the key for small pads. "