Dewetting is caused when a metallic surface has such impurities that the
wetted surface, when the high surface tension of the molten solder
causes it to form a natural sessile surface, retreats back into discrete
drops. There are numerous causes but the most common one is when you
have used any form of abrasive (e.g. pumice, abrasive brushes etc.) on
copper. The abrasive particles get implanted into the copper (often
actually swageing copper over some of the particles). The copper is
thereafter wetted, but each particle provides a non-wetted area over
which there is no wetting, causing the retreat. Abrasives are fatal to
good wetting.

Another observed cause is bleed-through of epoxy. The copper crystalline
formation in the ED foil is epitaxial, meaning that pores in the range
of 0,1 - 1 um diameter can traverse the foil, even 70 um thick. During
pressing, the residual solvent in the prepreg, especially in 76xx styles
(multifilament glass), evaporates and the vapours escape into the pores,
taking with them minute amounts of epoxy which reach the surface. These
may be sufficient to cause dewetting. Brushing can smear this epoxy
further over the copper.

Another cause is with badly-maintained plating (electro- or electroless)
baths which permit organics to be co-deposited with the metal. Again,
these give sub-microscopic to microscopic non-wettable areas peppered
across the surface.

Hope this helps.

Brian

Rajkumar wrote:

> Hello Does anyone know what is the actually cause of dewetting of
> solder paste? As anyone out there are doing any work in understanding
> 'dewetting mechanism'? I hope someone can provide me with some
> information. Thank you in advance RegardsRajkumar

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