Prasad,

I haven't seen the HASL solder deposits you referred to as resembling
"volcano top", but from your description and thickness measurements it
does sound like there is "dewetting".  I'll forward to you separately
three images to confirm why I feel there is dewetting.  I'll also send
these image to Steve Zeva and see if he can post them on his web site.

Regards,
George
George M. Wenger Andrew Corporation
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]


-----Original Message-----
From: Prasad Godavarti [mailto:[log in to unmask]] 
Sent: Monday, November 17, 2003 4:45 PM
To: [log in to unmask]
Subject: [TN] Mimimum HASLThickness for HASL Boards

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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