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August 2005

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Subject:
From:
"Stadem, Richard" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard
Date:
Mon, 22 Aug 2005 09:58:09 -0500
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Eliminating the most obvious, which is the nitrogen curtain brushing
over the components at the end of the tunnel, and the board is massive
enough to retain heat such that the components are still in liquidus,
and there are no "dust bunnies" hanging down near the exhaust stack,
then:

1. Is the copper layout balanced for the BGA pads?
If several ground and power planes are connected to one side of the BGA,
they sometimes cause such a major delay in the solder paste obtaining
liquidus between one side of the pad pattern and the other that they can
literally pull the BGA off of it pads.

2. Is the solder mask below the lateral line of the bottom of the
component body?
With .020" ball diameter, the part can literally bottom out on top of
the mask if the mask is even .010" above the pad surface. If there is
flux or solder fines as a result of slightly misregistered paste print,
the component will move around as the solder fines are agglomerating
together, like a board on top of marbles.

3. Is the print properly registered on the pads?
If not, again the surface tension may be enough to shift the part to one
side during initial liquidus.

4. This package is very small in terms of mass. If steps are not taken
to keep the solder paste under control, it can absorb a certain amount
of moisture that leads to voiding. The violent boiling out of this
moisture can cause very small components to literally blow around during
the reflow process. Check the X-rays for presence of voids. If they are
present, then it is a good bet that some of the problems you are seeing
is due to the volatility of the solder paste. This is more prevalent
when using a water wash paste, but can also happen with alcohol-based
no-clean paste as well.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Le Thanh Tung
Sent: Sunday, August 21, 2005 8:16 PM
To: [log in to unmask]
Subject: [TN] Sifted part at BGA location

Hello everybody.
At present, we are facing to one problem as below:
BGA component is shifted after go through reflow machine.
We have checked 100% mounting condition at BGA location before reflow,
we sure the BGA is not shifed but after reflow the shifted part defect
still happened.
although we lost much time to investigate but the problem is still not
be overcome.
The detail of BGA is as below:
    - BGA size: 15 x 15
    - Pitch: 1 mm
    - Ball dia.: 0.49 mm
    - Product: RoHS PCBA
Anyone who have this experience?
Thanks

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