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November 2005

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Subject:
From:
Ivanoe Pedruzzi <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Ivanoe Pedruzzi <[log in to unmask]>
Date:
Tue, 22 Nov 2005 09:55:49 +0100
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Jeff,
a grid in the aperture of a stencil of that sort is, in my
experience, a good advice.

This is the only way to keep  the part seated in its position when
reflowing.

If not divided in smaller parts, the surface tension of the liquid in
that area is strong enough to re-align the part on the center of the
stenciled paste.
Besides shifting, you can't even prevent the component from rotating.

I haven't ever experienced any problem with flux entrapped nor with
boiling.
A sort of "tic-tac-toe" grid is what I was used to have in the aperture.

have a good one,
Ivan

Il giorno 22/nov/05, alle ore 06:46, Jeff Brown ha scritto:

> Hi Everyone,
>
> We have a problem with a fairly unique component that requires
> soldering
> to a PCB. I'm really hoping that someone has seen this before as it
> looking really awkward.
>
> The component is mounted on a piece of fiberglass about 25mm x
> 16mm. The
> footprint consists of a ground pad which takes up 95% of the area, and
> four signal pads with 0.5mm clearance from the ground pad.
>
> The manufacturer recommends a grid of stenciled paste for the
> ground pad
> due to the large area, but we are concerned that if the paste melts
> from
> the outside edges in, then it will at a minimum result in flux
> entrapment, and at worst, the trapped flux will boil and lift the
> component, or move the molten solder causing shorts to the signal
> pads.
>
> I imagine that even if we used solid paste pattern that this would
> probably only make the problem worse.
>
> Or are we just being overly pessimistic?
>
> Any thoughts on options would be gratefully received...  Currently
> wondering about the possibility of applying BGA balls to the
> package so
> that there is some clearance between the board and the component.
>
> If anyone is interested, I can send the component datasheet to them
> directly.
>
> Thank you all in advance
>
> Jeff Brown and Josh Vear
> EOS Space Systems
>
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