Thin solder coating is not necessarily a result of dewetting. It can
also be caused by incorrect settings of the airknives of the
HASL-machine.

Daan Terstegge
Unclassified mail
Personal Website: http://www.smtinfo.net

>>> [log in to unmask] 11/17/03 10:45pm >>>
Recently we have had some disagreements with suppliers over what
constitutes dewetting.  I would like you opinion.

On one recent batch of boards, solder thickness measurements on a
single
quad pack IC pad ranged from 30 to 600 micro inches.  The solder was
deposited so that it resembled a volcano top.  The edges of the
rectangular pad were in the 500-600 micro inch range with a circular
like
area in the center at 38 micro inches.  This type of irregular coating
was
seen throughout the board.

It is my opinion that a solder coating thickness on a  pad less than
50
micro inches indicates dewetting.  It is also my opinion that lower
thicknesses (and hence poor soldrability from dewettted pads) leads to
long term degradation problems in storage and little Intermetallic
formation. There is some data from HUG *Hot Air Levelling User Group"
that
the cutoff number might be closer to 25 microinches (Circuit Tree Oct
20
artcicle by Furrow and Fellman)

I am not considering ground planes.  The most likely cause would be
inadequate cleaning of the copper prior to HASL and/or a HASL flux
that
contains a high (greater than 5%) concentration of acid (hydrochloric
or
hydrobromic).

we accep to IPC A610, Class 3 requirements for space launch system
use.

Any feedback?

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