Yes. Correct. Soldermask defined pads cause the crack during thermal cycling, and the crack propagates through the SJ, just like with BGAs. And yes to the voiding issue.
The voiding comes about because if you print a 1:1 paste deposit, during reflow the solder on an LGA has nowhere to go. It is trapped between the LGA pad and the PWB pad. There is no place for solder to flow out like there is on an endcap on a chip or on an SOIC lead, for example. In addition, the solder volume is much smaller than the same component in BGA form. Less solder volume holding the same mass puts more surface tension on the small molten solder joints during reflow. With less SJ surface area, the flux impurities, etc, have less room to outgas, and some spattering occurs which is why there are so many solder balls seen. Some of the trapped gases remain inside the SJ, which is why there are so many voids. Increasing and locking the component height and thus controlling the SJ height (with the same amount of solder) ;-) provides some room to form small solder columns rather than compressed cheese loafs, and this takes the pressure off of the molten SJs, fixes the voiding, fixes the extraneous solder balls, fixes the accessibility issues for water or other cleaning solvents and improves the reliability due to the increase in modulus of elasticity a slightly taller solder column provides. A 5% aperture reduction is part of the solution.
With LGAs, imagine the PWB expanding and contracting at a higher rate (greater CTE) than the component body, and think about the stresses this puts on the very small squat SJs during the service cycling, along with the soldermask compressing the solder joint at the junction with the pad. Something's gotta give.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Wednesday, April 20, 2011 7:59 AM
To: [log in to unmask]
Subject: Re: [TN] LGA geometry advice
Also, once again, land pattern design should not be overlooked. For instance Linear's guidelines do not talk about thermal relief, recommending soldermask defined pads in big copper planes. This is wrong and will adversely affect soldering.
Richard,
Will higher standoff reduce the normally massive voiding?
Thanks,
Ioan Tempea, ing.
Ingénieur Principal de Fabrication / Senior Manufacturing Engineer
T | 450.967.7100 ext.244
E | [log in to unmask]
W | www.digico.cc
N'imprimer que si nécessaire - Print only if you must
-----Message d'origine-----
De : Stadem, Richard D. [mailto:[log in to unmask]]
Envoyé : April-20-11 8:46 AM
À : [log in to unmask]
Objet : Re: [TN] LGA geometry advice
Well, the problems with LGAs extend past cleaning issues, but those are certainly there. The other issues are related to solder joint height, or more precisely, the lack thereof. Yesterday I wrote almost a two page response that I intended to post, including methods of reliability improvements, but then I thought to myself "All the hard work I did to develop these process methods, why should I give them away to competitors?" So just let me say that because of the short solder joints associated with LGAs, there are reliability issues that result due to the SJs very low modulus of elasticity. There are also problems associated with extraneous solder balls caused by not enough solder volume to support the mass of the body during reflow. The key to improving the reliability is to increase the height. Once that is done, you fix the SJ reliability, you fix the solder fines, and you fix the cleanability.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
Sent: Wednesday, April 20, 2011 3:19 AM
To: [log in to unmask]
Subject: Re: [TN] LGA geometry advice
Thanks very much Jerry.
Thanks Richard, I don't see your statement on reliability as necessarily
contradictory. What I take from the posts is that if you have both BGA
and LGA on the same board then LGA reliability can suffer. On boards
destined for aggressive environments LGA can be an issue because of the
difficulty in cleaning under the device but in consumer applications
where boards are not cleaned then the problem doesn't exist.
Regards,
On 19/04/2011 17:42, Jerry Dengler wrote:
> 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
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