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Intrusive Soldering vs. Wave Solder
Board Talk
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TranscriptPhil And welcome to Board Talk with Phil Zarrow and Jim Hall, the Assembly Brothers, who by day go as ITM Consulting. We are here at Board Talk to answer your process questions regarding through-hole and surface mount. Today’s question, Jim, is from J.W. As a contract manufacturer, we are faced with various challenges of board design. In this instance, we have a six-layer, five-ounce board with extensive ground planes and minimal thermal relief. There is a surface mount connector on the edge of the board which also has two through-hole mounting pins. These pins are difficult to achieve proper vertical fill. We are currently wave soldering with top and bottom side preheat using a Durastone wave solder pallet. Also, there appears to be sufficient hole diameter pins, not a tight fit. Would a pin in paste, or intrusive soldering, be a better option than wave soldering? Is there any harm in running both processes to ensure barrel fill? I always prefer intrusive reflow as your first option. If possible, when you are dealing with through-hole components, otherwise surface mount board, in so many ways it streamlines the process. It can be a concurrent process for the most part. You are using the same reflow machine and getting away from the fun and joy of wave soldering. But there are some pre-requisites there. Whether all of the components on the board can survive the thermal excursions that it is going to see during reflow. Then beyond that, there are some design considerations, all of which are very manageable in terms of hole diameter vs the pin diameter of the through-hole components. I am a little concerned about that one that you say has a large fit but if design can be done to accommodate that, this may be a very viable option, and in my opinion if you go to intrusive you won’t need that wave solder. The idea is to replace it. Jim I tend to agree with you Phil. He is talking about running wave soldering with a Durastone pallet. That to me is describing selective soldering using a wave soldering machine. He has surface mount parts on the bottom side. He has this pallet made of Durastone which covers up the surface mount parts and has apertures in it to expose the through-hole joints. Those are always problematic in terms of getting enough heat through the aperture up into the through-hole joints to get good hole fill. By going to intrusive soldering, you eliminate that mask because you are doing it at the same time as the surface mount parts. You have full heating access to both sides of the board. You should have a much better chance of getting complete heating of those holes so that you get proper hole fill. Wave soldering pre-heat, surface mount parts have been pre re-flowed on top and bottom so you can’t reflow them again. You are limited to how much pre-heat you can dump into the board. If you are doing this as part of a reflow cycle, that whole board is getting heated up to reflow temperature and you get a much better chance of getting complete heating of the barrel of the hole and getting good hole fill. Phil We would basically recommend to see if you can accomplish intrusive reflow in your design, including the components, your hole diameters and everything else can accommodate it or if some of the parts have problems, if there is a higher temp substitute for it. That would be great, if money is no object. But also, if you can’t do intrusive, I would seriously consider a selective machine as opposed to wave soldering. Jim Yeah, where you can control the heating to that individual joint with your immersion. Phil, if someone wanted to get more details about pin in paste, how would they do that? Phil Well, you can certainly go to the ITM Consulting web site. We have papers that were done by myself and Joe Belmonte some time ago. Another good resource is Bob Willis over in the UK. I think everyone knows Bob anyway. Bob has done some pretty good work on this too There are a number of resources out there. We have been doing it for quite some time, it is a pretty established process. But again, just like everything else it is application specific. In this case, we are looking at thermal excursions and whether your parts are compatible or not. Jim Phil, there is one final line here. Is there are harm in running both processes to ensure barrel fill? I would assume that they would try reflow a through-hole, pin in paste during the reflow and then run it again through the wave soldering machine with a pallet to re-melt those joints. I would be very concerned about trying to re-melt that through-hole. When you go through a wave soldering machine, you have the hole at least partially filled from the intrusive soldering or the pin in paste. Now you are going to try to wave solder that hole. I don’t think that would work personally. You would never get the previous solder to melt and you would never get the previous flux to evaporate. You would just get a lot of blow holes in it. I don’t think that is possible. Phil I would be downright afraid of it. First of all, remembering that the IPC 610 spec is for 75% hole fill. Bearing in mind that a lot of times you don’t always get a positive fillet on either side with intrusive reflowing. In order to achieve that minimum 75%, the methodology that we generally use for intrusive soldering is an overprint of the annular ring so you are getting sufficient volume. Sometimes you can aid getting even more volume, if it is required. Don’t look for that positive fillet for cosmetic effects, but if you need more solder sometimes the design will lend itself to step stenciling. You can step up where this connector is. Finally, there is supplementing with preforms. Jim I like preforms. Phil Yeah, they work well. Again, application specific what you can do and what you can’t do. But remember, you might but don’t count on getting the positive solder fill on either side of the board that you get with hand soldering and wave soldering. It is not what intrusive is about. As we always teach, avoid things for cosmetic reasons. Obviously, get as much hole fill as you can but minimum spec of 75%, whether it is class 3 or class 2, is more than sufficient in terms of sheer pull strength. Don’t get spooked by cosmetics, I guess you could say. I hope we answered J.W.’s questions and other people out there wondering about it, in our little discussion about intrusive. Regardless if you are wave soldering, intrusive soldering, hand soldering, wave soldering intrusive soldering together, I hope not, but whatever you do please don’t solder like my brother. Jim And don’t solder like my brother. |
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