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February 1997

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Subject:
From:
"esvax::mrgate::a1::kenyonwg"@esvax.dnet.dupont.com
Date:
Fri, 7 Feb 97 10:49:22 EST
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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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