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Problems With Starved "J" Lead Joints
Board Talk
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TranscriptPhil Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, Pick and Place coming to you from Board Talk headquarters high above the New Hampshire skyline. What kind of question do you have there today, Jim? Jim This question comes from M.M. "We have insufficient solder joint heel fillets with a PLCC J lead component. The aperture on the solder paste stencil is one to one or 30 by 100 mils." Pretty big aperture. "Visually, the solder fillet appears okay, but testing indicates that we need additional solder at the heel joint. We cannot increase the stencil thickness due to the presence of TSOPs and QFP packages. What do you advise?" Phil We don't know what the stencil thickness is, but apparently it's a little on the shy side for those poor little hungry little J leads. Jim The world has come to fine pitch components dominating everything because of hand-held devices. You know, even in cell phones they still have the same conflict because they have all those fine pitch components and then they solder an RF shield down which needs huge volumes of paste. So what we have here ladies and gentlemen is a classic case of different solder volume requirements sometimes called mixed mode printing, broadband printing and so forth. And the answer is a step stencil. Phil Yes, everything old is new again. Jim The most robust solution, is a step stencil. Where the questioner here indicates, "I'd like to make the stencil thicker, but I have fine pitch component so I can't." So you step up the stencil at your J lead locations, or you step down the other ones using a thicker stencil, whichever is more economical based upon the distribution and the number of the different styles of components. Phil And we're assuming you have ample clearance between where you have to step down to accommodate step down. If not, another alternative would be double printing. Jim Yes, double printing, where you print a thin stencil first, then print with a thick stencil which is etched out, relieved for the other ones, but you really need about three mils of difference in order to do that. Phil And another printer, too. Jim The next option would be staying with a thinner stencil and over printing. And people go, "Oh, but won't I get solder balls and stuff off the pad?" and of course that's a risk, but I'd like to point out that overprinting beyond the pads has been a very common practice in intrusive soldering or pin and paste for many years and it can be done with a minimal risk of solder balls and bridges. Particularly, remember, you're dealing with a 50 ml pitch part so the chances of bridging are pretty small. And of course we have to add the option of using a preform, adding a preform in your placement system or dispensing additional solder paste. You print your paste and then during your placement operation, one of your feeders has a reel of these solder preforms and your placement machines simply picks it up and places it in the wet paste out beyond the edge of your J lead heel where you want that extra solder and then when you reflow, it melts and combines with the solder paste and gives you the extra solder you need. Phil So show some compassion and feed those J leads and feed them often. Beyond that, this is Jim Hall and Phil Zarrow, the Assembly Brothers saying that whatever you're trying to re-flow soldering, whether j leads, gull wings, whatever ... Jim Don't solder like my brother. Phil Don't solder like my brother. |
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