Tony

Thanks for this report. It makes me feeling a bit better. Some months ago I
had to compile layout rules for SMT and I based them on calculations Dieter
Bergman presented back in 1992 ( now in IPC-782 ? ). However I didn't trust
the spec ( that's the way we scientists are ). I recalculated the land
pattern for a tantalum capacitor type "B" with the formula for the elastica
of eutectic solder in air. The result was astonishing and I nearly didn't
dare to show it to our industry partners. Unfortunately there was no more
money in the project to verify the results. Anyway here are the results:

Pad size reflow: Z = 5.6mm; Y = 0.99mm; W = 2.61mm
Pad size wave: Z = 12.2mm; Y = 0.99mm; W = 2.61mm

Z = Overall outside distance of pads
Y = Inside distance between pads
X = With of pads

which results in  4mm exposed land. Of course, the elastica applies only to
a laminar wave. I wish I had the founds to evaluate a correction factor for
turbulent waves.

Best regards

Guenter Grossmann


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