As long as we're talking about this process, I was wondering what could be
done to prevent the via holes on pcb's from retaining excess solder during
HASL.
With a double reflow process, the solder will form a ball on the bottom of
the via during the first reflow due to gravity.
When the assembly is flipped over for the second pass, the ball is now on
the top of the pcb and will interfere with the screen print operation,
causing severe flooding due to the poor gasketing around the ball. Could
lead to a ruined stencil in some cases.
Thanks! Bill C
-----Original Message-----
From: mark simmons [mailto:[log in to unmask]]
Sent: Tuesday, May 01, 2001 6:04 PM
To: [log in to unmask]
Subject: Re: [TN] HASL can the height be controlled?
Hi Jim...well...yes it can be controlled in a "range". The HASL process is
"geometry dependent" meaning that it is repeatable and controllable on like
pad sizes and shapes. Typically, the operator will select the smallest land
or pad area as a target site (these small sites tend to resist the leveling
efforts of the air knives most, and are the first place to lose control (too
thick, improper hole clearing, etc). Once control has been established in
the critical areas ( adjustment in thickness is possible) in a range
generally within a couple hundred micro inches). This air pressure setting
and knife angle will now be leveling all sites. In surface mount and some
mixed technology designs, the larger land areas can get pretty skinny. (
maybe a little less than 100 micro inches) this has long been a point of
controversy with regards to shelf life and solderability. History has shown
us however, that while the deposits can be very thin, (in some areas) shelf
life of a least a year under normal storage conditions is common.
I hope this isn't more than you wanted to know. I have been involved in
this process from its inception and tend to over simplify sometimes.
hoping this helps,
Mark Simmons, V-Score Central, Inc.
West, Jim wrote:
Hi,
Can the Height of HASL be controlled? I have seen the process but don't
remember exactly where there are any control points.
Thanks,
Jim West
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