Good morning, all.

We've been utilizing that area since mid-'93 and have not run in to any
problems that have not been brought to light.  We make a fair effort at
the footprint generation step to create channels through the BGAs that
were initially used for routability but, are just as often used for
secondary side component placement.  We have essentially stuck to the
caps and resistors for those areas for reasons of package size and the
necessity of having these components located in the proper proximity to
the impacted nets.  Component cost, in case of problems during rework,
becomes an added bonus.

A minor process difference from what you stated, we do not send our
cards over the wave.  We stick to reflow ovens.

Best of luck.

Scott E. Lee
Manufacturing Engineer
Mercury Computer Systems
978-256-1300 x162
[log in to unmask]

> -----Original Message-----
> From: Stephen R. Gregory [SMTP:[log in to unmask]]
> Sent: Thursday, October 01, 1998 10:23 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] BGA Reflow question
>
> In a message dated 10/1/98 6:30:39 AM Pacific Daylight Time,
> [log in to unmask] writes:
>
> << Technetter's
>
>  On a multi-layer board with parts on both sides, is it OK to use the
>  real-estate on the opposite side from a BGA to place resistors and
> caps?  I
>  know this would affect BGA rework, but I'm running out of board space
> here.
> :P
>
>  Advice, suggestions?
>
>  Thanks
>  Scott Holthasen>>
>
> Mornin' Scott!
>
> First, just let me say that if I could, I'd give you a big hand shake,
> and a
> pat on the back! It's so refreshing to see someone actually think
> about how
> their designs affect us worker bees on the production floor!    B^)
>
> Well, if I had my 'druthers, I 'druther there not be anything on the
> bottomside, but we all know that's not being realistic with what we
> want these
> boards to do nowdays. But if you're running out of room to the point
> that you
> need to put things beneath a BGA, I'd try to put only resistors on the
> bottom,
> that's coming from a perspective that they will stand up to the
> thermal
> cycling of wave solder better than caps do. But it really doesn't make
> a
> difference whether you put them beneath a QFP, PLCC, or a BGA, there's
> always
> going to be the same risk that you'll also have to rework the chip
> components
> beneath a large processor type device if that needs rework.
>
> Have you utilized chip resistor networks in your design yet? I work at
> a
> contract assembly company and I'm kinda' curious why I don't see them
> used
> more often in board designs. They're widely available as far as I
> know, save
> quite a bit of space, and really increase throughput during SMT
> placement.
> Instead of placing one resistor per placement cycle, you're throwing
> down
> four. I always see at least 2 or 3 resistor values on a board that are
> use
> quite a few times in an assembly...(I think those values are called
> pull-up
> and pull-down resistors aren't they?). They make capacitor arrays now
> as well,
> I see them advertised in all the magazines, but I don't see many of
> them on
> boards...how come?
>
> -Steve Gregory-
>
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