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Insulation Between Overhanging Component Lead and Circuit Conductor
Board Talk
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TranscriptPhil And welcome to Board Talk with Jim Hall and Phil Zarrow. The Assembly Brothers, who by day work as ITM Consulting, also some long nights too. Today we have a question that has to do with solder masks. This is from F.A. F.A. writes, we have an unusual component situation, a long lead SMT component is overhanging the pad. Can solder mask in this area provide an acceptable level of insulation between the overhanging component lead and the circuit conductor running below? Jim I am the assembly engineer out on the floor. Why do I have to deal with this problem? Don’t I have better things to do? This is not something that should have happened. Your pads on the board should be the size of your leads. You shouldn’t have leads overhanging the pad. He asked a question, will it work? Yeah, probably. But do you want to put your name on a board that is built this way? That is up to you. Let’s think about some of the issues. First off, if you have a lead hanging over the pad you aren’t going to get any fillet on the end of that lead. Virtually all lead configurations, according to IPC 610, you want an edge fillet or a toe fillet. If the end of the lead is hanging off, lets assume it is a gull wing lead, it is just too long for the pad. A typical gull wing lead, you are going to look for a toe fillet. Well, you are not going to have a toe fillet. Question right there, in terms of is the solder joint acceptable. In terms of the mask, yeah solder mask is an electrical insulator and it may provide an acceptable level of insulation. But let’s think about this. If it is right near the pad, the lead is hanging over the pad it is going out over the solder. You are right at the edge where the solder mask was deposited. Is that a sharp? Is it uniform? Is it repeatable from one PCB to the next? Is the thickness repeatable? Are you going to get a reliable configuration on every board that you assembly through this process? Again, we could go on and on with all of these questions about it probably should work but there are questions in reliability. As a process engineer, you shouldn’t have to deal with this. The board should be designed properly, with the proper size pads or the proper components should be purchased. I could envision another issue where some overzealous purchasing agent got a great deal on this replacement gull wing leaded part that is identical except that the leads are a little bit longer. If they overhang, they aren’t going to bother anything. We saved a lot of money buying the components and now you have to deal with it. The company has to take the risk of whether you want to build a configuration like that and supply it to your customers. Design for manufacturability. Design your boards properly. Get the right components. Quit putting process engineers under this will this work, maybe. s it reliable enough? Do I trust it and so forth? Phil I guess we could sum it up, we always like to say we see a situation is it a symptom or a defect. In this case a defect, maybe. You are certainly not getting the right mechanical strength there and you are looking at using solder mask as duct tape, for lack of a better word. The symptoms? A slacker in design or a slacker in procurement, as Jim mentioned also. There is no reason you, on the floor, should be suffering from this. I know we are going to get our IPC specification scholars writing in to tell us that this is absolutely verboten within the various specifications of IPC 610, 600. Jim Certainly, if you are building a class 3 board you would never do this. Phil You would be banished from the garden of Eden. I don’t even want to think about it if you are building class 3. But even class 2, this is just not right. We look forward to your comments, hopefully in support of what we are saying. There might be somebody out there that says no, put the solder mask on. Thank you for wasting the last five minutes or so with Phil and Jim at Board Talk. Thank you for joining us. And remember whatever you do, whether you are going to use solder mask or not, don’t solder like my brother. Jim And don’t solder like my brother. |
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