Peter,
I like this concept very much, as it is easier than manually dispensing flux
on every ball. Do you know of any commercial sources for gadgets as you
describe to immerse BGA's a known depth into paste flux? I think I once saw
a simple device like this demonstrated as part of the rework process for a
very expensive rework station (can't remember the brand). Jeff, does your
company make anything like this for applying flux to the part instead of the
board?
Regards,
Seth Goodman
Goodman Associates, LLC
tel 608.833.9933
fax 608.833.9966
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of
> [log in to unmask]
> Sent: Tuesday, May 14, 2002 7:24 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA replacement
>
>
> The most straightforward and practical advice I've had about replacing
> BGA's is not at all fancy, and works.
>
> There is always a problem with trying to put solder paste down on an
> already-assembled board. There's too little space, microstencils are
> expensive and fiddly, with problems of how to anchor them, etc, etc.
>
> There is also no need, really, to apply extra solder paste to
> replace BGA's
> either - their balls are made entirely of solder. Not using extra solder
> results in a lower stand-off height, and if this is a problem, then by all
> means use solder paste, but, what works well on a properly cleaned site is
> flux paste.
>
> Small, simple stencils can be made with a screen window that accommodates
> the ball pattern up to a depth of about half the ball height, or whatever
> you prefer. Into this window, you screen flux paste or solderpaste with a
> hand squeegee, then sit the BGA into the paste to coat the balls to the
> predetermined height (set by the screen thickness). Then, simply
> remove the
> BGA from the screen, align it over the site and place it on the board.
>
> In practice, you don't necessarily need very fancy alignment equipment
> either - simple copper features on the board like a pair of small pads on
> each side of the device boundary can be used for visual alignment that is
> accurate enough. The BGA's self-centring abilities will pull it completely
> into line if your reflow profile is correct.
>
> I confess I was very sceptical when I was first told about this
> method, but
> it does work, even with micro BGA's. Try it sometime. We're getting very
> good at making expensive machines to overcome a relatively simple problem,
> when what we have to do is understand the mechanics of the process to find
> something much less costly, if a bit less idiot-proof. Maybe we
> should stop
> employing idiots, or put them somewhere else where they can't do any harm.
>
> (Now ducking behind the parapet)
>
> Peter
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