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August 2001

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 24 Aug 2001 12:40:23 +0300
Content-Type:
text/plain
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Guy,

I would not recommend straight or co-solvent vapour phase, with or
without ultrasound: my experience in this field has been rather
negative. It can clean the first few peripheral millimetres great, but
it is **extremely** difficult to get right into the centre, even with
strong solvents. In particular, we found that the HFC or HFE co-solvent
process was not brilliant. The HC solvent got to the middle, fine, but
it got stuck there and the subsequent HFC/E cleaning was not good enough
to remove this contaminated solvent, retained partially by the balls.
Subsequent mechanical removal of the component clearly showed flux
residues. Also, the HFC/E losses are greater with the co-solvent process
than with a straight VP process: I hypothesise that some solvent mixes
with the residual gunge under the components and does not evaporate so
readily in the vapour phase/freeboard stages of the process. As these
losses are very global-warming, they are environmentally as well as
economically undesirable (except for the solvent manufacturers!).

I have no ready-made solution to this problem: the best results I've
obtained have been experimental in aqueous machines with high volume
vertical flooding of the assemblies held at an angle between 15° and 30°
from the vertical during the wash stage and then rinsing with vertical
fine sprays, which have to be carefully designed that the individual
water droplets, arriving at the component-board interface, have an
average diameter of under the gap size and sufficient velocity to force
its way in and displace the dirty gunge left over from the wash. I found
it requires about ten such rinses to be effective, each lasting a second
or two, at two or three second intervals, assuming no other components
shadow the critical spacing. However, I do not deny that even this is
not foolproof. If the real estate permits it, assuming the usual BGA
configuration of no balls in the centre, a square cut-out in the board
is ideal, but designers like to use this space for fan-outs. HDIS
design, allowing in-pad microvias, can overcome this problem.

Brian

Guy Ramsey wrote:
>
> We have done some underfill here, very limited quantities will no clean
> fluxs. These were 5 mil parts with 2 mil balls. It depends who you talk to
> there are advantages to cleaning but there are also problems. We had
> reasonable success with the no-clean process.
>
> IMHO, Even if you get the semi-aqueous solvent under the part, how can you
> rinse it out? We use Vigon A200 in out inline cleaner and Kyzen Ionox I3330
> in our batch cleaner. I have seen residue under large BGAs after cleaning.
> Both are excellent but the water rinse just won't go into tight places.
>
> If I was going to try this I think I would experiment with the HFE in a
> co-solvent process. But it's expensive, requires modern equipment and good
> process monitoring.
>
> Guy Ramsey
> Senior Lab Technician / Instructor
>
> E-Mail: [log in to unmask] <mailto:[log in to unmask]>
> Ph: (610) 362-1200 x107
> Fax: (610) 362-1290
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Lambert, Katherine A.
> Sent: Thursday, August 23, 2001 12:00 PM
> To: [log in to unmask]
> Subject: [TN] Cleaning under BGAs
>
> Hi folks,
>
> We're embarking on something new.  We will be assembling boards with BGAs.
> Some are small and fine pitch, others are regular pitch but quite large
> (40mm square).  We will be underfilling them with adhesive.  Because we're
> underfilling, we have to clean the flux from underneath the parts.  We're
> using an RMA flux based solder paste.
>
> Now for the question - how should I clean the flux from under these parts?
> Any clues as to which solvent to use?  Any ideas on what type of equipment
> to use?  I have at my disposal an in-line semi-aqueous cleaner and a vapor
> degreaser.
>
> Thanks for any insight.
>
> Kathie Lambert
> Process Engineer
> Northrop Grumman
> Baltimore, MD
> 410-765-9746
>
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