Mechanical bonding relies on an uneven, rough surface which, when covered by a planar liquid, will experience a decrease in surface energy, thus promoting adhesion. On the other hand, diffusional and chemical bonding rely on intimate contact between the two interacting surfaces and thus are enhanced by smooth surfaces.
On balance, for soldering, a smooth surface is preferred: practical
experience shows that roughened surfaces are often associated with
oxide and contamination, both of which work against wetting.
Now return to the main text.