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Subject:
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 13:11:22 -0400
Content-Type:
text/plain
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text/plain (420 lines)
 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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