Hi, Toby,
Mini stencils are expensive, awkward to use in confined spaces and quite
unnecessary if you're using plastic BGA's with eutectic solder balls.
Typically what I use is a block of material (usually aluminium, but it need
not be) with an area machined down to about half the depth of the balls.
Into this I screen print flux past - tacky flux, as you call it. The BGA is
then sat in the flux to coat the balls before it is placed on the board and
reflowed.
This method minimises the risk of mis-printing and as the PBGA's don't
actually need extra solder, there is a reduced risk of solder bridging as
well. Just be sure, if you have untented or unfilled vias that the vias are
clean of old flux from the site clean-up operation before you put the new
component onto the board, adn that you have profiled the rework station
correctly.
Peter
"Tobias.Chang" <[log in to unmask]> 20/11/2002 12:41 AM
Sent by: TechNet <[log in to unmask]>
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] BGA Installation
Hello to everyone,
IPC 7711, #5.7.1 and #5.7.2 is about using wire solder, or solder paste to
prefill lands for BGA/CSP Installation.
I was just wondering if its standard practice to use a soldering
iron/solder
wire to melt little blobs of solder onto the PCB before placing a BGA for
rework...maybe I'm not understanding the concept of pre-fillnig lands. And
I was also wondering if this "prefill" procedure is used only when placing
NEW BGA chips, or if it would apply if you take a chip off, and try and
place that same one back one without fully reballing it.
And I was also wondering if there are any comments about installing BGA
chips during rework, whether its better to use stencils of some sort to put
paste between the chip and the board, or if just flux will do. And if
stencils are the way to go, if you would recommend a stencil that puts
paste
onto the chip, or if the paste goes on the board.
Personally from what I understand, replacing a BGA works just fine if you
use tacky flux and then put the BGA in the rework station. Not sure why
there is even a market for BGA rework stencils.
Thanks to all those that reply... =)
Toby
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