Thank you for the explanation Richard, whenever I have been shown this kind
of problem I have found the solder coating to have been out of control, but
as we have limited contact with assemblers perhaps you can clear up some
confusion for me.
You say that solder on via's and other surface features that do not have an
associated aperture in the stencil will prevent gasketing. Given that it is
unusual to have the solder coating thicker than the solder mask, is this
because the copper is generally electroplated thicker on small areas and
towards the outer edges of multiple image panels? And could this problem be
eliminated by etching part way into the gasket? You then say the aperture is
smaller than the pad; are you suggesting that the stencil can be held away
from the surface by the thickness of solder this close to the edge of a pad?
-----Original Message-----
From: Stadem, Richard [mailto:[log in to unmask]]
Sent: 11 October 2005 07:03 PM
To: [log in to unmask]
Subject: Re: [TN] Hasl
The idea of the flat surface finish is for both stencil gasketing to the
board and for placement accuracy. To form a "perfect brick" of solder paste,
there has to be a certain amount of pressure (a lot less than most people
think). To prevent squeezout of paste between stencil and board, there has
to be good, really good, gasketing or sealing between the stencil and the
board surface. The problem with HASL, as Dr. Parker stated below, is that
the round bump (meniscus) of HASL sticking up above the surface of the pad
on vias and on surfaces where there is no accomodating hole in the stencil
prevents this tight seal, and allows the paste to squeeze out. (The aperture
design is typically slightly smaller than the pad). It is also a problem
when you go back and perform pick-n-place, as now you want to place the SMT
components onto your perfectly formed bricks of paste, but with about 50-100
grams of pressure. If the paste is not stable to a flat pad, the parts slide
all over the place. While some of this is forgiven by the surface tension of
the solder in liquidus, bridges do form, components misalign, etc. This is
especially critical in a no-clean process. If large (relatively) amounts of
the solder paste are not "tight to the brick" and fully registered on the
pad, the paste does not fully agglomerate into a single solder joint.
Separated solder paste does not usually conglomerate back into the central
solder joint during reflow. Extraneous solder balls form. This is very bad
news if you did not plan on washing them away.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker
Sent: Tuesday, October 11, 2005 12:03 PM
To: [log in to unmask]
Subject: Re: [TN] Hasl
Ramon
After reflowing a soldered surface, you will form a meniscus which will
interfere with SMT component placement. To avoid the problem I suggest a
flat metal finish such as immersion silver or flash gold (not ENIG)
Best regards
Lee
J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
----- Original Message -----
From: Dehoyos, Ramon<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Tuesday, October 11, 2005 12:52 PM
Subject: [TN] Hasl
Hi technetters:
Is there a fine pitch limit to the use of HASL board plating
finish due to its variation in thickness? Does selective solder strip
and reflow plating, SSSR, have a significant advantage over HASL? Any
comments relating to platting finish would be greatly appreciated. I
thank you for your comments.
Ramon
ME
410.552.2210
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