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From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Werner Engelmaier <[log in to unmask]>
Date:
Mon, 28 Mar 2011 13:44:27 -0400
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Hi Wayne,
The main reason for the increased reliability is the increased SJ height; indeed, the compressive stresses will relax fully due to creep in relatively short time [faster for SnPb, slower for SAC].
Werner

 

 


 

 

-----Original Message-----
From: Thayer, Wayne - IIW <[log in to unmask]>
To: Werner Engelmaier <[log in to unmask]>; [log in to unmask] <[log in to unmask]>
Sent: Mon, Mar 28, 2011 1:28 pm
Subject: RE: [TN] Solder Joint Height Control



Hi Werner-
 
Thanks for setting this straight.  One small question remains in my mind:  Because of the high rate of solder creep, I have seen solder fail when used as a mechanical attachment—trying to hold things in tension.  It’ll hold the load for quite awhile, then fail years later, depending on stress, temperature profile, etc.
 
Does this principal just apply to putting solder in tension?  Because if it applies to compression, then the staking approach you explain below should only work until the solder has enough time to de-stress through creep.
 
Thanks,

Wayne
 

From: Werner Engelmaier [mailto:[log in to unmask]] 
Sent: Monday, March 28, 2011 1:11 PM
To: [log in to unmask]; Thayer, Wayne - IIW
Subject: Re: [TN] Solder Joint Height Control

 

Wayne,
You got it wrong—you do not want the same CTE as solder and it should not act like an underfill. The way "SMT glue" works is as follows: (1) on heating the glue's high CTE raises the component away from the PCB and increases soldering gap, (2) on cooling the solder solidifies 'freezing' the larger soldering gap before the glue shrinks substantially, (3) the glue shrinks on cooling putting the solder joints under compression [which in itself increases reliability], (4) the SJ has a significantly increased SJ height increasing creep-fatigue life.
Watch for my reliability column in the May issue of Global SMT & Packaging magazine.
Werner

 

 

 

-----Original Message-----
From: Thayer, Wayne - IIW <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, Mar 28, 2011 10:00 am
Subject: Re: [TN] Solder Joint Height Control

Hi Mike-




 





 
"SMT glue" is not a good option unless you can verify that it will protect the 




 
solder joints like underfill does.  If the temperature coefficient of expansion 




 
is different from that of solder within the expected service temperature range, 




 
then it will degrade the reliability of the system.  I know that various 




 
participants in this forum have used "staking compounds" at the corners of BGAs.  




 
Perhaps one of these would do the job.  But a process engineer will have to 




 
design a reasonable process and then validate the results.




 





 
Wayne




 





 
-----Original Message-----




 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner




 
Sent: Monday, March 28, 2011 9:51 AM




 
To: [log in to unmask]




 
Subject: Re: [TN] Solder Joint Height Control




 





 
I am not a process engineer so I give you the below pending their responses.




 





 
Placing the component at different heights won't do anything as that will




 
not change the component and pad wetting when the solder joined is formed




 
and that is what primarily determines overall joint volume and shape. You




 
could try reducing pad size so that the overall height of solder is greater




 
by having a smaller footprint, but this runs risk of solder balling/beading




 
off the pad. (Or printing extra solder to existing pad to give same effect).




 
An alternate might be to use a blob of SMT adhesive (of the type used for




 
bonding components prior to wave soldering to give a stand off. 




 
What is the height you are achieving now and why do you think extra height




 
is necessary might be better questions to address before doing any of this




 
though. Also I am sure other Technetters will want to know about actual pad




 
sizes, aperture reduction and stuff like that, so you might want to start




 
doing a little researching.




 





 
Regards




 
 




 
 




 
Mike-----Original Message-----




 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW




 
Sent: Monday, March 28, 2011 1:53 PM




 
To: [log in to unmask]




 
Subject: Re: [TN] Solder Joint Height Control




 





 
Hi Dale-




 





 
I've responded with a different subject name because that is how TechNet




 
keeps track of the threads.




 





 
I think you're doomed without a process engineer, because I don't believe




 
there is a fixed answer to your query.  The simple approach is to "try" a




 
thicker stencil, but frequently that makes no difference with a big heavy




 
part having only 2 leads.  So you may have to put something underneath to




 
get a required joint height.  That's really complex because it would be




 
simple to put something beneath there that destroys reliability.  Someone




 
used to make a spacer for apps like that which dissolved in the




 
post-soldering cleaning process, but I haven't used them in the last 20




 
years, so I can't tell you where to find them.




 





 
Wayne Thayer




 





 
-----Original Message-----




 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dale Hart




 
Sent: Monday, March 28, 2011 8:36 AM




 
To: [log in to unmask]




 
Subject: [TN]




 





 
Technet Folks




 





 
 I'm not a process engineer, sooo how would one go about increasing the




 
solder




 
joint height (G dimension) under a component.  The company's components




 
generally have a height of 10 -20 um.  The screen thickness is 6 mils.  The




 
solder is 62Sn 36Pb 2Ag not sure about the flux, particle size, storage time




 
between printing, pick and place, and convection reflow, if that is




 
important.




 
The chip resistors are about 2012, caps 3225, with SOP16 (copper lead




 
frames).




 
Didn't actually measure the pad size but visually they appear properly sized




 
for




 
the various components.  The pick n place machine is an older type that uses




 
a




 
spring loaded head and they program each component height into the machine.




 





 
Dale Hart




 
Universal Technology Corporation




 





 
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