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your , please


your , please | 17 February, 2000

We appear to have solved a major reflow process problem (poor quality joints on a 20 mil QFP100...excessive solder left on the lead leg, not much left for a joint) with a design change to the board. We gave ourselves considerably more room for a heel fillet and everything seems hunky dory now.

My question is, is it just due to the mechanics of the additional room for a heel fillet (which I assume is a function of the viscosity of the liquid solder and the radius it can achieve) or is it more likely due to the improved thermal characteristics, i.e., more balanced thermal mass below the joint and an increase of the affect of surface tension there? Please feel free to shoot this down, correct my terminology, etc. I am a dry sponge with respect to reflow behavior, but am anxious to get wet.

Thanks, folks.


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Re: your COMMENTS, please...duh. nm | 17 February, 2000


Re: your , please | 17 February, 2000

Steve: Wadja mean "I fixed it, now tell me what I did to fix it." Steve, you�re one scary guy. ;-)

"Steve�s Longer Pad Theory" - Stubby pads will not make solder flow-up the leads of a QFP100, leaving sparse solder on the pads. Given a choice, solder would prefer not to defy gravity and climb the lead. It would prefer to form big ugly blobs and then if forced to, form a kinda spread-out blob, shorting the leads on the QFP together.

"Steve�s More Balanced Thermal Mass Below The Joint And An Increase Of The Affect Of Surface Tension Theory" � I like the "More Balanced Thermal Mass Below The Joint" part, because that balance keeps all the contact surfaces the same temperature and provides for a nice even solder flow. � But I�m not sure what the "An Increase Of The Affect Of Surface Tension" part means. The flux reduces the surface tension of the solder, but it should be doing that ... ?

"Steve�s Unspoken Theory Of Corrosion On The Pads That Limited Solderability" - We assume this was hypothesized, but never postulated, because pads other than the QFP100 soldered well.

Go get 'em

Dave F

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Re: your , please | 18 February, 2000

I've got a process run amok, really thick skin, and a little bit of knowledge. Now you REALLY oughta be shakin' in your boots.

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