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April 2010

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Subject:
From:
Paul Edwards <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Edwards <[log in to unmask]>
Date:
Wed, 21 Apr 2010 14:22:31 -0700
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Cedric,

In our experience, HiP/HoP has been a mixed bag of profiles, paste flux and metallurgy issues as in the current discussion...

But the biggest contributor to the problem we found on parts is contamination by an organic substance...

When an organic material coats the balls(or contact surfaces) and gets baked onto the surface in the solder paste pre-soak... The typical fluxes will not break down this baked-on (and sometimes carbonized) material leaving a micro-thin layer trapped between the melted solder paste and the BGA ball... When the joint cools you get the HiP/HoP effect...

The only way we have found to eliminate the problem is to pre-clean the devices or run the profiles 2X to 3X longer and 30+C hotter to get the organics brittle enough that the molten solder can crack the carbonized shell and get bulk wetting and absorption into the paste generated solder mass...Once the two solder masses are free to transport mass then the material viscosity (internal surface tension) and gravity will transport the solder so that the shell is moved to the outside of the solder joint forming a typically good joint.

Paul

Paul Edwards
Surface Art Engineering


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Cedric ORAIN
Sent: Wednesday, April 21, 2010 11:39 AM
To: [log in to unmask]
Subject: [TN] Head in Pillow and grapping effects mitigation without Nitrogen

Hello all, 

Very fine pitch BGAs (typically 0.5mm) and 0201 chips are becoming common in
our company for some months. 

We recently have to face with new defects type such as grapping effect and
head in pillow defect. 

The solder paste we use has been qualified for even smaller component (WLP,
0.4mm pitch) but in Nitrogen atmosphere.

I'm now wondering if this solder paste is suitable with Air reflow. I'm also
wondering if a solder paste suitable in that case even exists. 

Having a look to this study from intel (link hereunder, page 31), I found
that the people from Intel are recommending a solder paste volume larger
than 2000 mil^3 as a minimum.
 
http://edc.intel.com/Download.aspx?id=3044&returnurl=/default.aspx

That is, off-course, impossible for a VFBGA (which solder paste deposit is
closer to 900mil^3) but this reading makes me feel uncomfortable. 

So the question is: is it possible to find out a solder paste which is
suitable for VFBGA assembly in Air reflow? 

If it is not against TechNet rules, have you some solder paste references
that you could advise?

Here are some additional details: ENIG PCB finition, SAC305 Alloy.

Cedric ORAIN 
Technical Director
Electronic Manufacturing Services
www.inovelec-groupe.com 

PS: I'm very new and I'm also French. I wish it will not be too much for
you! Anyway, thank for being patient if I have difficulties to understand.




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