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]<mailto:[log in to unmask]>] On Behalf Of Mike Fenner
Sent: Monday, March 28, 2011 9:51 AM
To: [log in to unmask]<mailto:[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]<mailto:[log in to unmask]>] On Behalf Of Thayer, Wayne - IIW
Sent: Monday, March 28, 2011 1:53 PM
To: [log in to unmask]<mailto:[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]<mailto:[log in to unmask]>] On Behalf Of Dale Hart
Sent: Monday, March 28, 2011 8:36 AM
To: [log in to unmask]<mailto:[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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