Eric,

There is usually a mix of fine pitch leaded components and a BGA/LGA.
This puts us to a preferred 5 mil stencil.  
That's not a lot of solder to fill the gap of a LGA.  This closes the
paste application process window to a peep hole not a window. 

Also there is virtually no standoff height when finished.  Don't even
consider getting under a LGA clean.

If you need to replace a LGA, it's difficult to get a consistent level
of solder on the board and/or component to make every connection.

With a BGA you have the solder ball volume to make up for any
differences in paste volume.

BGA gives better standoff height so you can actually clean under the BGA
if required.

Replacing a BGA is easy compared to the LGA.  Solder height on the board
does not need to be as consistent as the LGA to get good results.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
Sent: Tuesday, April 19, 2011 12:04 PM
To: [log in to unmask]
Subject: Re: [TN] LGA geometry advice

Thanks Jerry,

Could you tell me why? I'm aware that BGA is favoured but can never come

up with any killer reasons for not using LGA.

We use LGA because it results in a cheaper product which is more likely 
to result in a sale....and yes I expect you'll point out it's a false 
economy but people worry more about the $$s on a purchase order than the

cost of rework (within reason).

Regards,



On 19/04/2011 16:52, Jerry Dengler wrote:
> Eric,
>
> I would like to comment on the choice of LGA's.  These devices usually
cause an undue amount of trouble to manufacturing.  They are easily 3
times the trouble of a BGA.  I would rather do a .5 mm pitch BGA than a
1.0mm pitch LGA.
>
> Regards,
>
> Jerry Dengler
> Production Manager
> Pergamon Corporation
> 380 Crooked Lane, Unit# 3
> King of Prussia, PA  19406-2567
> U.S.A.
> (610) 239-0721 Phone
> (610) 239-0720 Fax
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
> Sent: Tuesday, April 19, 2011 11:36 AM
> To: [log in to unmask]
> Subject: [TN] LGA geometry advice
>
> Rights folks!
>
> I'm designing a new surface mount device which originally started off
as
> 5 X 5 square.
>
> It has 25 I/Os.
>
> It's LGA is a 5 X 5 array of 0.60 diameter pads on a 1 X 1 pitch.
>
> The device has now increased in size to 7.5 X 7.5.
>
> Now, do I increase the LGA spacing to cover the increased area again
or
> do I stay with what fitted on the 5 X 5 device? I'm inclined to stick
> with the original LGA as I believe that the stress analysis will tell
me
> that the smaller grid will have lower stresses in the SJs than the
> bigger grid.
>
> Are there any practical reasons for maximising grid size?
>
> Regards,
>

-- 
Eric Christison
Consumer&  Micro group
Imaging Division

STMicroelectronics (R&D) Ltd
33 Pinkhill
Edinburgh EH12 7BF
United Kingdom

Tel:	+44 (0)131 336 6165
Fax:	+ 44 (0)131 336 6001

The contents of the email are ST confidential.




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