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SMT007-Apr2020

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76 SMT007 MAGAZINE I APRIL 2020 would maybe fill a thimble, so this will be time better spent for both of us. One thing I have learned after 20 years is that while there are endless combinations of materials that could be used for an assembly, the number of actual assembly process steps is fairly low and consistent. There are a few exotic processes that won't be covered today, but if you have one, I would love to see it. I will address the basics that seem to be consis- tent with 99% of all manufacturing I see on a regular basis. As with all good stories, let's start at the beginning. Receiving materials is a step of the process that is not always thought to be directly related to reliability, but it's just as important as any other step. I was in a facility just last week that had one of the biggest issues I see with receiv- ing where the dock doors open directly to the warehouse racks. I was in a normally arid part of the world, and the issue is if the doors are open to the outside environment, and a wind kicks up, there is a real opportunity for dust that might be laden with ionics to come right in and deposit on the racks or even to the pro- duction floor. This could pose a threat to sol- derability if there are any open packs of bare boards or parts. If the process is truly no-clean, there may also be an issue with the adhesion of confor- mal coating if that is to be used. Further, there is an issue with spikes in the amount of avail- able atmospheric moisture when it rains. This can cause the same issues as dust, but you can also add in problems like an under-cured sol- der mask absorbing moisture into the board and causing more problems down the road when that moisture comes into contact with other process residues. The one thing that is normally covered and exempt from the dock doors being open is parts that have a moisture sensitivity level. The impact would be on the dry cabinet and how hard it works to maintain a low percent- age of relative humidity. In general, you want to segregate the receiving area and the storage areas to be sure that the outside environment has as little impact as possible. Normally, in the same part of the facility, you will find solder paste and epoxy storage. You want to be sure that your refrigerator tem- perature and relative humidity are being mon- itored with a calibrated device with alarms for exceeding the upper and lower control limits. It is always recommended to have a system for monitoring the temperature and humidity in all parts of the facility, and receiving is no exception to that. Now, let's move on to the surface-mount solder process as it is normally next in line, and this is where the real action starts. The first part of this process is, of course, the sol- der paste printing process. When it comes to paste printing, I can only recommend a few tips, as we don't see a lot of problems stem- ming from this process. What we see on a regular basis are poorly cleaned stencils that collect paste in the corners of the apertures that can cause skips in paste deposition. A good paste process will include automated solder paste inspection (SPI) to find anom- alies. If a misprint is discovered, the board should only be cleaned with an aqueous wash process of some sort if cleaning of mis- prints is allowed at all. The same goes for the stencils. We now have properly pasted boards, and parts are placed properly I assume, it's time to get them into the oven for reflow. The ther- mal profile is pretty much the most important part of this process, so it's imperative to do it right. Doing it right includes taking a look at the paste manufacturer's recommendation, and then realizing they have no idea what you are building. Receiving materials is a step of the process that is not always thought to be directly related to reliability, but it's just as important as any other step.

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