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Improving Efficiency of Through-Hole PCBAs
Board Talk
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TranscriptPhil And welcome to Board Talk. This is Jim Hall and Phil Zarrow of ITM Consulting. In this guise we are known as The Assembly Brothers on Board Talk. We’re here to help you muddle through your process problems, dealing with electronics assembly, and soldering, and all that. Jim, today’s question is from LC. We are trying to improve the efficiency of the through-hole of our mixed tech, surface mount through-hole PCBAs. What are our options regarding soldering through-hole components? Jim I commend LC for thinking about the efficiency of his assembly process. This is something we would all do well to pursue for all of our assembly processes. Phil Well spoken from our lean six sigma black belt master, Jim. Generally, when it comes to soldering through-hole, we feel there is a hierarchy of what you want to get into. At the top of the hieracrchy is your first choice, pin in paste otherwise known as reflow through-hole or intrusive soldering. Here of course, understanding there is always a but, right? You have to have components that are thermally compatible. The component height has to be able to clear the tunnel on your oven. Hole geometries have to be accommodated. That is something that is doable. Leap also has to be taken into consideration. But if you do that, Jim and I feel very strongly that pin in paste in the way to go because of the way you are streamlining your process. Jim The first criteria for pin in paste that you identify was the components have to be compatible with the temperature. This has traditionally been one of the limitations for pin in paste, particularly when we got into SAC lead-free, where our reflow temperature dropped to 240 to 250 degree range. That eliminated the possibility of reflowing a whole lot of through-hole parts because they simply could not take the temperatures. Even tin-lead at 200, 210 there were a lot of parts that couldn’t take it. One of the additional benefits that is emerging is interest in low-temperature solders using the Bismuth alloys where peak reflux temperatures are now in the neighborhood of 180. This now makes pin in paste a technically acceptable option for a wider range of components. It is one of these secondary benefits coming off of the investigation of low temperature soldering, using Bismuth alloys and having reflow peak temperatures below 180. Of course, it was originally investigated to reduce high temperature warping of advanced BGA packages and to avoid head in pillow defects, which are probably the most insidious defect you can create and the reason that most people were looking at low temperature alloys. But a whole bunch of secondary or tertiary benefits come up. The first one that people jumped on is reduced electrical consumption of their ovens, going from peak temperatures. Where in a SAC environment you may have the hottest zone is 260, 270 degrees. Now your hottest zone in your entire oven may only be 200 or 210. So that is bound to be a significant energy savings along with ecological issues too. Also, pursuant to that, if it becomes a reality the third level would be the ability to use lower cost materials in electronic components since they would not have to be subjected to the higher temperatures. Phil Now if one of these gotchas say that you can’t use pin in paste for soldering your entire through-hole, or any of the through-hole, the next step on the hierarchy would be looking at selective soldering. Specifically a machine, a selective soldering automatic machine. The reason we prefer this over the rest are the fact that it is automatic, there is no tooling involved, its not operator dependent. There are caveats with this too, in terms of being able to design for the selective soldering machine. There are still a lot of requirements. We feel it is still better than wave soldering and it is more accurate than wave soldering. After selective soldering, moving down would be robotic soldering. There are a number of machines out there. This is using either robotically driven soldering irons or even laser, which is something that I would like to see more of personally. But again, you have a programmable methodology. There is wear of the tips if you are using the soldering iron approach to take into consideration. But of all these automatic processes, one of the main drivers would be pin count. In some cases, you may have a very high pin count in terms of through-hole. We’re looking back again at wave soldering, number four on the hierarchy. Again, you have surface mount components on the bottom using the aperture fixtures for those. And finally, would be hand soldering. Hand soldering why, because it is operator dependent. That is a major variable there. That would be the hierarchy, pin in paste being the first one to go to. Beyond that selective soldering, an automatic machine, robotic soldering, wave soldering, and finally hand soldering. Anything you want to add to that Jim? Jim The ideal one, pin in paste, it eliminates the entire secondary through-hole line, other than insertion. You go through your whole process, then you take it off that line and you go through another line where you have insertion and your wave soldering, your selective soldering and whatever. All of that can go away. I want to comment, pin in paste is not for the faint of heart. It is not a trivial process to implement. Typically we think of pin in paste and low temperature solder take a step back and look at the efficiency and the economics of assembling a mixed technology board with surface mount and through-hole. Eliminating all separate through-hole soldering operations has tremendous benefits to overall efficiency of our assembly operations. Phil Well, we hope we answered the question. On some of our other Board Talks we have discussed pin in paste pretty heavily, and probably will be doing it even more so in the future. It is one of our favorite processes. We have worked with it quite a bit. Give us a little bit of a soapbox here. It makes good sense. Beyond that, this is Phil Zarrow and Jim Hall, the Assembly Brothers, thanking you again for tuning in. However you are soldering your boards, be it pin in paste or using a cold fired soldering iron whatever you do, please don’t solder like my brother. Jim And don’t solder like my brother. |
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