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June 2009

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 3 Jun 2009 14:00:53 -0500
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Hi, Paul,
I have personally reflowed (literally, hands-on) thousands and thousands
of BGAs, CSPs, and flip chips, all sizes and types, soldermask-defined
and non-soldermask defined, without using solder paste. I have done this
as part of developing rework processes, as part of a "normal" production
assembly and reflow process, and as part of a laser ball attachment
process using tacky flux only. I have done this on flex, on rigid PWBs,
and rigid-flex. I have done it on medical products, military products,
disc drive preamps, communications products, and automotive products.
However, this should not be done with liquid flux, only a flux in paste
form (commonly called tacky flux). 

I know also that there are tacky fluxes, and then there are tacky
fluxes. Some are much, much better than others. 

I have never experienced a single premature failure. Not one.

I know from firsthand experience that this is considered by many
companies to be a standard, preferred process. 

Can you expand on why you had to rework a lot of BGAs soldered with this
method? Or can you tell us why more solder paste would be required? More
volume does not equal increased reliability, in fact the opposite is
usually true. With the additional (unneeded) volume that the paste
provides, there is also an increased tendency for bridging. Also,
increasing the volume of solder simply gives you a larger and more rigid
set of "cheese loaves" between your BGA substrate and the PWB. This does
not improve your modulus of elasticity of the BGA solder joints; it
reduces their ability to accommodate the delta CTE of the BGA substrate
(interposer) and the PWB itself. 

I find using a thin veneer of tacky flux on the PWB pads improves ball
wetting to the pads tremendously as opposed to using a brick of solder
paste. Likewise when performing ball attachment to the pads on the BGA.

I also find it much easier to clean away the tacky flux residues (at
least the ones that I use) than it is to clean the flux residues left
behind from the solder paste. The tacky flux can even spill over and be
on the bare PWB, and I have firsthand experience that certain tacky
fluxes actually protect the PWB from excessive heat, even from laser
damage.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Wednesday, June 03, 2009 11:43 AM
To: [log in to unmask]
Subject: Re: [TN] BGA-113 package

Steve,

I would not recommend you only use flux to reflow the BGA...

I have had to rework a lot of BGAs because someone thought that that
would be a reliable process for rigid PCBs...

Paul

Paul Edwards

Surface Art Engineering


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Kelly
Sent: Wednesday, June 03, 2009 9:10 AM
To: [log in to unmask]
Subject: [TN] BGA-113 package

Hi All,

We have a rigid flex that has a BGA-113 (microstar BGA) package to be
mounted to it. The routing in this area on the top layer leaves me with
.0005 of soldermask clearance from the edge of the BGA pad and .003
distance
from the edge of the soldermask to the nearest track- in other words I
have
.0035 of clearance from pad edge to next track assuming a perfect etch
and
perfect soldermask registration.

Having rooted around Texas Instruments web site I believe that it may be
possible to reflow this BGA without adding solder paste to the pad -
just
letting the ball collapse do the attachment. If my scenario is correct I
could open up the soldermask and not have too many concerns about
bridging
between the BGA pads and the tracks. Any thoughts or comments would be
appreciated.

Thanks Steve Kelly



Steve Kelly (PLEASE NOTE NEW E-MAIL ADDRESS)

(416) 750-8433 (work)

(416) 750-0016 (fax)

(416) 577-8433 (cell)




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