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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