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Common PCB Design Decisions that Cause Manufacturing Problems
Board Talk
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TranscriptPhil And welcome to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers, who by day go as ITM Consulting. We are here to answer your questions on assembly processes, methodologies, equipment, materials, and all the good stuff. Today, we have a question from G.B. What are the most common design decisions that can have a negative impact on the PCB assembly manufacture, and how could we correct them during the design? Jim, I don’t know where to begin. We could do a whole workshop on this. In fact, I believe we have. Since we are limited to a five-minute sound bite here, let’s talk about the top two. How does that sound, Jim? Jim And far away is the spacing of the SMD components on the board relative to through-hole joints. You want to have as much clearance space between any surface-mounted part and any through-hole joint to make the through-hole soldering as less problematic as possible. Unfortunately, on some boards, surface mount parts get really close to through-hole joints and it becomes more and more difficult to do the through-hole soldering. Where you are not just heating the board up like you are with reflow, but you are having to add solder and flux and heat. Now, obviously, if you can do pin-in-paste and you do the whole thing with reflow, then it goes away. There are some issues there too, but most of us don’t. Most of us are going to have to do some separate through-hole soldering step. They get too close immediately; you have to eliminate wave soldering. Now, you are moving to selective soldering. And even there, you can only make the nozzle so small on a selective soldering machine. As surface mount parts get closer and closer to the through-hole lead, it just gets more and more problematic. Even good hand soldering, with a good skilled operator, can be difficult if the location of the surface mount parts is too close to the through-holes. So that is my number one Phil. Phil I definitely concur with you. I think that is definitely one of the big nasties. I would say second to that, the next thing that comes up is making sure the design uses the correct pad size and shape. As we all know, if you deviate from those, there are a lot of other things that could go wrong. The design probably has a major contribution to the tombstone defect that we see. There are a lot of other things. By misdoing that, we could have components shifting, which you might make electrical contact, but you may not have optimum physical contact with a component. The same thing with when using solder mask-defined pads versus non-solder mask-defined pads with regard to the pitch of the components you are using. There is a lot. They should really be doing their due diligence. Unfortunately, you have to keep on them with regard to the actual pad size and shapes. Again, like we said, there are a whole lot of other things that designers do to screw us up. As we like to say, forgive them, Father they know not what they are doing. They know, but the fact is that most designers are not exposed to the assembly area, what we are trying to do every day. Things used to be a lot more tolerant back in the through-hole days, but with surface mount, all bets are off. I am happy to see that from what we have seen, most of the CAD programs are getting better and better in bridging the gap, but it still isn’t there. In future episodes, we will talk about some of the other problems. Things we like to see and do, like please pick the lowest MSD level of components. There are a whole number of things. Watching the component count. Most designers are not familiar with how many feeders you can have on a given pass. I think that is it, Jim. Anything you want to add to that? Jim No, I think you have covered it all, Phil. Phil Well good, you have been listening to Jim Hall and Phil Zarrow, The Assembly Brothers, who by day go as ITM Consulting. Whatever that designer has done to you, please don’t solder that board like my brother does. Jim Don’t solder like my brother, either. |
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