From: NAME: WILLIAM G KENYON
FUNC: Chemicals/Electronics
TEL: 302-652-4272 <KENYONWG@A1@ESVAX>
To: NAME: [log in to unmask] <"[log in to unmask]"@ESDS01@MRGATE@ESVAX>
There is a simple way to eliminate solder balls from the assembly
process...by making a change in the PWB fabrication process.
The fab process to use is called SSD or Solid Solder Deposits.
To provide a thumbnail sketch- solder is applied during or after
fabrication in sufficient volume to provide the volume of solder
needed to make a good solder joint. The "solder" can be applied
as plate, paste or liquid solder- depending on the process
variation used. If not liquid solder, the "solder" is reflowed.
The next step is to flatten the solder deposits to make solder
"bricks" on the board. The board is then washed (most of the
solder pastes are inexpensive water soluble types) to remove all
the residues and ALL SOLDER BALLS. Since no components are
present, a simple water washer can be used. Again, no components
so ultrasonics can be used without anyone getting worried. A
quick visual inspection to make sure all the solder bricks are
there and you are done. Screen on a sticky no clean flux, follow
with a Mylar cover sheet and you can store the boards for up to
one year. Pull off the cover sheet, stack, punch up the right
programs on the pick/place and reflow machines and start zero
defect ZERO solder ball assembly.
No test prints, no solder paste, no stencil cleaning, minimal
turn around time on the shop floor when changing part numbers.
Don't really need inspection or cleaning either. This process is
running now- if may have been used to make your laptop, since it
is used routinely to do 10 mil pitch TAB attach for laptop CPUs.
There are about four versions of the generic SSD process
available, so it is not single source.
The SSD concept was first used to make double sided VHSIC boards
with 20 mil pitch QPFs in one pass through vapor phase soldering
in 1984 with 17 mil pitch the following year....so like many
ideas in electronics manufacturing, it has been around long
enough that it is safe to adopt it.
For global assemblers, the SSD concept is in the documentation
publication plan of IEC TC91: Surface Mount & Related Processes.
IPC actively participates in the work of this group.
Bottom Line: If you want a no clean no solder ball process that
will run on fine pitch parts, this might just be the answer for
you. While there is an additional cost for the PWBs, the savings
at the assembly step more than make up for it, generating a net
cost savings.
--Bill Kenyon
Global Centre for Process Change, Inc.
302 652-4272/-5701 Tel/Fax
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